When biologists started classifying life forms, everything was divided between animals and plants. Then single celled organisms were found that crossed boundaries, they could both move and photosynthesize, so microbes got their own group. Then biologists realized that fungi also had a mix of plant and animal like features, so they also got their own group. Microbes were further split.

Today, scientists use phylogenetic classification: a group includes everything with a particular common ancestor. This ancestor isn't usually known, but by figuring out which life forms are more closely or distantly related, groups can be defined based on an implied ancestor. Most commonly, groups are defined as everything descended from the common ancestor of two known species, or as all descendants of the last ancestor of one group that isn't also the ancestor of a second group. An example of the first type is the "archosaur" group, which includes all descendants of the most recent common ancestor of birds and crocodiles. As an example of the second, the "Avemetatarsalia" group includes all descendents of the first ancestor of birds which was not a crocodile ancestor. The closest non animal relative to animals is a group called choanoflagellates: anything more closely related to your favorite animal then to Chaonoflagellates is an animal.

However, the group was originally based on physical features, and all animals have certain ones in common.

• Animals are Eukaryotes. All earth life is made of cells (apart from viruses, if you include them as life), some cells have lots of mini-organs including a nucleus that stores genes, these are Eukaryotes. Eukaryotes also include fungi, algae, plants, many kinds of microbes, but not bacteria, or a group called Archea that resembles bacteria with some chemical differences.
• Animals are muticellular. Like plants, fungi, and some algae, animal bodies are made of lots of cells. These cells specialize, changing shape and other properties to fill specialized roles in the body. Most animals have distinct tissues, which combine similar cells into a larger structure. Most with tissues further organize these into organs, and the organs for noticeable organ systems which perform particular functions for the animal.
• Animals need food: The word for needing to eat other living organisms, or artificial imitations, is heterotroph. Plants, algae, and some bacteria and Archea are autrophs, they can use nonliving sources of energy such as the sun to power themselves.
• Animals need oxygen: Aerobic is the word for this. Some organisms can break down material with no oxygen to get enough energy, such as yeast that rises bread and makes alcohol. Some animals can do this for short bursts, like your muscles when sprinting, but outside those short periods animals need oxygen.
• Animal cells lack cell walls: Plants, fungi, and many bacteria have a rigid structure surrounding their cells. This provides support in plants and fungi, helping them stand up. Animals lack this, having flexible cells instead. This allows most animals to move quickly, which almost no other multicellular organisms can do. Movement is probably the most obvious animal quality for most people.

Fungi are close relatives of animals, shown by mushroom’s close to meat like taste and texture. Fungi share a number of chemicals in common including pigments like melanin, and chitin in their cell structure, a material found in arthropods in particular. The closest relatives, choanoflagellates, resemble some specialized cells in some animals. Plants, slime molds, and algae are not that closely related to animals.

See Evolution for more.

Officially, biologists use a system called phylogenetic classification: a group includes all descendants of some common ancestor. To classify animals, biologists need to know how animals are related. In practice, the exact ancestor is almost never known, but a tree of relationships between different types of animals can be mostly reconstructed, as described previously animal groups are defined by having a common ancestor implied by relationships between known animals.

Thanks to how evolution works, this system mostly matches common names of animals, groups and/or species with an ancestor in common tend to physically resemble each other. One exception is when one group evolves from within another, such as land vertebrates evolving from fish or bees evolving from wasps, technically “fish” isn’t a formal group and wasps include bees, but a scientists won’t assume a bee stung you while you ate chicken if you say that a wasp stung you while you ate fish. Another possibility is unrelated groups evolving similar characteristics and being lumped together. Outside of animals, algae are like this, three separate groups have evolved but they are similar enough to be called the same thing. The closest formal concept for something like fish and wasps is "Evolutionary grade" representing a group of life forms with a common ancestor that all share certain physical features, descendants of the group that lack these features are not included. Dinosaurs and a lot of other writing about dinosaurs shows the distinction in action, in everyday speech Dinosaur excludes birds, but the scientific term Dinosauria includes them, writing about Dinosaurs often includes bird information due to scientific use but focuses more on the big extinct nonflying animals that people are interested in. The result of all this is that., if you are describing or reading about a group of animals, keep in mind which system is being used, and whether a formal, phylogenetic/cladistic system (common ancestry) is being talked about, or a meaning more related to physical characteristics. In the final part of this useful notes, most groups listed have a common ancestor, but a few are more like an evolutionary grade if it makes organization easier.

To figure out how life is related, including animals, scientists look for certain types of similarities. Evolution works by a combination of random mutations plus which changes end up surviving, whether by random chance or because they help organisms survive and reproduce more, so similar qualities that are hard to change through small adjustments, or don't effect survival much, more likely mean the life forms with those similarities are closely related. Repurposing or adjustment of existing structures is a common pathway of evolution, so body parts with a similar underlying structure even with different overall functions or shapes suggest close relations. Animals that use a similar physical method of doing something when multiple structures can do the job also suggest close relations. Genetic studies use mutations, often in noncoding DNA or genes with minor effects, to measure the time between splits and similarity between animals; these assume that mutations are more or less random, and when groups split a different collection of mutations will build up over time in each separated group. Animal embryo development can be used as a guide, it often resembles animal evolutionary history, likely because changes in growth and development are a major pathway for physical changes to evolve.

Characteristics that do affect survival a lot and evolve easily will likely adapt quickly to a role or environment, meaning distantly related animals living in a similar way will have them in common while closely related animals that will live differently will not. The similar general shape of whales, most fish, and prehistoric Ichthyosaurs is an example, other features such as bone structure, whether the creature has gills or lungs, and evolutionary intermediates point to whales and Ichthyosaurs having separate groups of land based ancestors, which all evolved a similar shape separately due to how useful it is for moving quickly through water.

Prehistoric animals obviously can’t have genes sampled, but physical characteristics from fossils can be compared. If there are enough fossils of the right animals, ancestry can be directly traced. Fossilization is a chancy process, a dead animal needs to be under very exact conditions and most don't fossilize, but some animals are still much more likely then others. Mineralized body parts like mollusc shells, coral structure, or vertebrate bones and teeth for some examples fossilize relatively well and the evolution of these groups can be followed relative closely, but some soft bodied or small animals groups have no or next to no fossils and their prehistory can only be guessed at.
The first animal fossils are disputed, possible candidates range from around 570 to around 760 million years ago. Some studies based on other evidence suggest they existed around a billion years ago, some point to a more recent origin, evidence is not clear enough to narrow this down. Multicellular life in general was developing at this time, algae and fungi were also evolving. The first likely animal fossils look like fixed in place creatures that fed on mats of bacteria on the ocean bottom, possibly with some filter feeding, though whether these "Ediacaran Biota" fossils were actually animals is still not fully settled. Clear animals fossils related to modern groups show up from several million to tens of millions of years later. Sometime around 550-600 million years ago, some animals developed muscular movement. The first fossils of burrows developed around 540 million years ago, around the same time animal diversity increased enormously in something called the Cambrian Explosion. Most modern animal groups came into existence around this time. UsefulNotes.Prehistoric Life has you covered from here.

Dinosaurs kept mammals in hiding. The Lion is the King of the Jungle. Culture often assumes that predators and large animals dominate over prey and smaller animals, and are overall more powerful and successful. Concepts such as higher and lower animals, or the great chain of being, are also common, where larger, more complex animals and vertebrates are considered better, more advanced, etc. than simpler and smaller ones.

This is not how scientists see it, as Evolution describes. While predators and large animals are individually threatening, collectively prey and smaller animals do quite well for themselves. Animals are usually spread out enough that prey and smaller ones aren’t constantly under attack, as you’ll notice if you go hiking or spending time in a wild area. As a result, most smaller animals and prey spend most of their time going about their business and sometimes running or hiding from something dangerous. Lions, Tigers, and Bears might be the big predators in modern environments, but plenty of smaller cats, lizards, birds, spiders, worms, etc. are living out their lives quite successfully. Likely this is true of dinosaurs and mammals, mesozoic mammals were filling similar roles to racoons, rats, squirrels, etc. that live in equivalent areas today.

As for higher and lower animals, The evolution link describes opinion well. All animals have had to face a similar set of challenges in living in an environment, finding food, etc., so anyone survives that is well evolved and adapted to their role: Sponges and jellyfish have adapted just as much as cuttlefish, bees, or whales. Complexity involves tradeoffs, more energy, more difficult growth and development are the big ones, so a simpler and less active animal may well work better in certain ways of living. Vertebrates scientifically are seen as just one group of animals, so invertebrate makes as much sense in classification as nonarthropod or isn'tmollusc or other-than-cnidarian, though on sites like this one invertebrate gets some use as vertebrates are more represented in pop culture and society in general. If a type of animals has lasted a long time, it means they were very well adapted for their environment, and that said environment has also lasted a long time, so “primitive” animals are those which ended up with a long lasting, useful shape in an environment that hasn't changed too much over that time. "Backward" evolution, where an animal ancestor gained something and lost it again has happened often: Flightless ants and ostriches had ancestors who evolved and lost flying, whale and sea turtle ancestors moved from sea to land and back, slug and squid ancestors gained and lost shells, flatworm ancestors gained and lost many internal organs.

Animals are just living their lives, not pursuing any larger goals, but population, diversity in species or in roles a group fills, geographic spread, amount of time a species exists before going extinct, are all used as measures of success.

Deep Sea, Desert, Deciduous trees, …uh..dirt? …running out of D’s. Where an animal lives affects everything else about it. Life lives in a very narrow band near the earth’s surface,from the dirt/seafloor below ocean trenches and some deep mines, to flying high in the atmosphere, with much of it in the oceans or close to ground level. This narrow band has very diverse environments, however. Where to live is also affected by size, a microscopic animal will find plenty of space in someone else's body or a tiny hole, for example, which to a large animal would just be a piece of the environment.

Animals are usually well adapted to one environment that they live in long term, but short trips to others can be common, such as land animals swimming or ground dwelling animals climbing. These can be a useful part of an animal’s lifestyle if they regularly lives near a border area, where animals might use different environments for different purposes (such as seals mating on land while catching food in water). How strongly the boundary restricts an animal depends on the animal’s adaptations, and how similar the environments are.\\
Some qualities of environments, and how animals might need to adapt:

* Water vs. Land: Water and air are very different materials, so very few animals can live in both. The differences affect almost everything an animal does: breathing water and air are very different, movement is different, sensory information transmits differently, temperatures change less in water, are among many major effects.

* Saltwater vs Freshwater: You know how you shouldn’t drink too much seawater, because you’ll lose more water getting rid of the salt than you took in? Animals living in water must handle this all the time. A concentration difference between water inside and outside an animal means water will either diffuse in or out, and an animal will need to either expel extra water, or extra salts or other material to keep their internal fluids the right amount of concentrated. Animals usually are adapted with appropriate systems for a certain salinity range, go outside and the animal dies from too much or too little water.

* Richer vs. Poorer environments; All animals need to eat something, and eventually that eating leads to other life forms that get energy and nutrients from the external environment. To have more life in an area, an environment needs mineral nutrients, a source of energy (almost always sunlight), water, and avoid extremes that can kill most life. Environments with all these will be very productive and support a lot of life, environments missing something a lot less.

Shallow water, rainforests, and weird deep sea hydrothermal systems are examples of highly productive areas. All have nutrients from soil/the ground or the sea floor, the first two have lots of sunlight for energy while hydrothermal vents have heat and heat driven chemistry, rainforests have lots of rain while the other areas are in water. Deserts and most of the ocean are examples of poorer environments. Deserts are missing water, the deep ocean has little sunlight and relies on falling material from above for energy (apart from hydrothermal vents and some other seeps), and the open ocean surface has few mineral nutrients, being far away from any rocks that they come from.

Richer environments can support more of all kinds of life, including animals. This greater number allows for lots of different species to maintain good sized breeding populations, meaning more species variety. However, this also means more competition from greater amounts of animals nearby. Poorer environments are often slower paced, animals need to conserve limited resources and adapt to low populations, or be able to quickly move between resources that do exist.

* Complex terrain: Grasslands, open ocean, deserts are easy to navigate, wide open spaces. Forests, reefs, and rugged terrain like mountains are quite a bit more complicated, with spaces to hide in, roundabout travel routes with obstacles, and changes from location to location. Animals in more complex environments may need unique travel methods, and some way to navigate or otherwise find what they need. More open environments allow for faster movement, and better use of some senses (longer sight lines, easier hearing, and similar).

* Temperature, Wind, Moisture, Dissolved sediments, other environmental properties: Obviously, animals will need to adapt to these. Variability in these conditions much also be adapted to if they change a lot (such as variable weather).

* Larger living things: Some animals live on other animals or plants. Some live Inside other animals, or burrow into plants. Living things for obvious reasons don’t want to be burrowed into, thus will likely have defenses that the smaller animals must work around. They will also need a way to move from host to host, which might require some venturing outside.

Living inside an animal gives good temperature control and a good source of food, almost all animals living in others are parasites. Living on the outside of animals might just be a good attachment point, it also may offer feeding opportunities on blood, skin flakes, other parasites, or such. Living on or in plants may be useful for parasites, or the plant is just an easy surface to travel on or burrow into as with some creatures living in wood.

* Burrowing: Burrowing through sediments takes more effort than swimming or moving through air, and there is little light, plus sound and smells don’t travel that well. In return, underground provides protection from things at the surface, such as temperature changes, bad weather, or other dangerous animals. Sediments contain plant roots and other accumulated debris for nutrients, plus other burrowers, but many burrowers will venture outside to get food. Cave dwelling animals let something else do the burrowing, but still have to handle the lack of light and other underground conditions.

* When to Live: Environments change over time. This could be somewhat random, as in weather, or predictable as in day night cycles or seasons. These changes can create very different conditions to deal with, a cold winter, dark night, or watery high tide is quite different then hot summer, bright daytime, or dry low tide. Other life reacting to these changes means animals must further adapt. This could be behavior changes (being active during one part of a cycle and resting in another part, chasing certain times a year for lots of eating, mating, etc.), moving around to follow favorable conditions (seasonal migrations, or ocean creatures moving deeper and shallower over a day/night cycle.), flexibility (changing diet but otherwise acting similarly), or body changes. Irregular changes like weather can be trickier to adapt to, animals might move or change behavior, but might also have preparations for a range of conditions that could occur, even taking advantage of them.

All animals get energy from other living beings, which means all animals need food. A good meal for an animal gives more nutrition than it costs to find, catch, and digest, enough extra to fuel the animal’s other activities, and doesn’t poison the animal.

Animals can’t use large particles of food directly, so it must be digested down to component chemicals. All animals to do this use chemical methods to break down food, and most use physical methods to break food into smaller pieces that are more easily attacked by the chemicals. Once digested, the results are absorbed. Catalytic proteins, enzymes, do much of the work, acidic or basic conditions also help break down a lot of nutrients. Most chemicals life uses are made into long chains of smaller units, digestion breaks down bigger chemicals into these small pieces that can be rearranged to suit the animal, or used for other purposes. Vitamins, minerals, and some other smaller chemicals are absorbable directly once dissolved. Physical methods of breaking food down include biting off small pieces, chewing, or stomach churning, among other methods. Some animals can do some chemical digestion outside themselves, by injecting of spitting digestive liquid into their good, or by pushing their stomaches outside the body to envelope food and doing the usual job from there. Once absorbed, nutrients can be stored for later use in a variety of ways: circulated in body fluid, turned to body fat for later use as energy, absorbed into cells to wait for use, minerals might be absorbed for later released by mineralized body parts.

Animals need a few nutrients in common. Proteins perform most functions in a body. Many types of molecules can act as energy sources, though carbohydrates and fats are more common. Animals need all required elements (Nitrogen, Calcium, etc.) supplied in some way, the major elements are common to most, but a few minor ones may vary. Water makes up body fluids, and is needed ot ensure they are properly diluted/concentrated. Interconversions are possible between many different types of molecules, but animals may be limited in what can be converted, and conversions do take energy, so some specific chemicals, like vitamins in humans, are needed. Animal’s ability to process nutrients and their diets will tend to evolve together to a point where harder to get nutrients are conserved, or methods evolve to make them from other inputs, while easier to get nutrients can be used more wastefully.

Very few foods are completely absorbed, not all chemicals in them are required or usable by the body, or possibly harmful. Leftover material that isn’t absorbed becomes dung (poop, feces, scat….pick your favorite of the many, many toilet words out there), and is expelled in some way. In most animals, the digestive system is a tube with two ends, one for taking in food and one for dumping the waste, sections this tube can specialize for particular stages of digestion as a result, giving things like stomachs, intestines, chewing parts, and similar. Some early diverging animals have one entrance that both eats and drops, with a main stomache equivalent that does all the work of digesting and absorbing. Some animals lack any particular organs, these eat small particles that can be processed by individual cells.

You’ve probably heard of carnivores, omnivores, herbivores, and maybe scavengers, but there are quite a lot of specialized names for other types of animal diets. Piscivores eat fish, insectivores…guess, detritivores eat random particles or debris, grazers eat plants low to the ground, frugivores go for fruit….It’s a long list. An animal that can use only one or a few types of food is called a specialist, an animal that eats lots of types of food a generalist. Generalist carnivores eat lots of types of meat, generalized herbivores lots of types of plants, generalist with no other word attached implies an Extreme Omnivore.

The interactions of lifeforms eating each other form something called a food web, understanding these interactions is very important for understanding any particular environment. At the start of these food webs will be something that captures energy from the environment. In our world, these are almost always photosynthetic plants or algae capturing energy from sunlight, but decomposing bodies or particles from elsewhere, or chemicals released from hydrothermal vents or seeps can also supply the initial energy to appropriate organisms. As these organism are eaten, and the eaters are eaten, and those are eaten, etc., energy is lost at each step through inefficiencies in energy use and the fact that a lot of food energy is used for day to day activities instead of growing a body. This means that the original energy capturers, like plants, will tend to be most common and/or fast growing, herbivores or equivalent less common and/or faster growing, then predators (or parasites), then predators of predators, etc., with more generalist eater abundance depending on their exact diet. The exact decrease depending on how the organisms in question use energy: greater fraction used for growth vs. other things means less energy loss. A lot of energy goes through decomposition, scavenging, and detritus, with the same loss at different steps occurring.

Here are some types of diets, and what they imply for the animal. Remember that within these descriptions, animals may be more or less specialized (A predator may focus on lots of kinds of prey or just a few or one type, plants eaters may just eat leaves from one type of plant, lots of leaves, any green stuff, etc.).

* Predators of similar sized prey: These are the most famous predators, probably. T. rex attacking a Triceratops, crocodiles dragging large animals into the water, sharks attacking seals or helpless movie swimmers. On a smaller scale, cats catching mice, squid catching fish, spiders or a Slaying Mantis catching larger insects, some flatworms preying on earthworms also fit this category, among many others. While successful catches are most commonly shown, in practice most attempts fail, or few attempts are made, by most of tis type of predator. A similar sized prey animal is a ‘’lot’’of food, just imagine a person dealing with about 100-150 ib/75 kg of meat. A person needs about 2-4 pounds of meat a day if that’s the only food item, so a carnivorous version of a human could catch something like a deer or large fish roughly every couple weeks, and accounting for inedible parts, still have have enough food for themselves plus feeding a family, or group, and people need more food for our body size than most other animals. With this much food, a few missed hunting attempts per catch are plenty survivable, while prey facing death is under much more pressure to escape.

Eating similar sized prey requires weapons and often speed to catch and kill it, this speed is often a short burst for an ambush. If living alone, such predators need enough storage (a stomach or otherwise) to actually store a good amount of food from their catches. Digestion is meat is relatively easy, but there is often a lot at once, these predators may rest after kills to allow their digestive systems to go to work. Meat is heavy on protein and light on carbohydrates, so animals adapt energy requirements accordingly. Some vitamins or similar chemical found in plants are also often missing, so predators will adapt to make or not require as much of them, while minerals such as calcium or sodium are more available with the opposite effect.

* Predators of smaller prey, or other prey that can’t fight back: Think anteaters or chimps eating an insect hive, starfish or octopuses attacking clams, the early bird catching the worm, egg eaters, or similar. Obviously, such prey can’t physically fight back effectively, but it may still have other defenses such as shells or poison that the predator has to work around. A big challenge is finding enough food, smaller prey can often hide, and requires much total individuals eaten, possibly meaning more energy spent searching for food. Colonies or clusters are good targets for predators. Nutritionally, smaller prey and larger prey are similar.

* Scavenging: Dead meat will start to decompose, which means bacteria and fungi which may cause diseases or be poisonous to the predator if the body has been dead too long. Deaths from other causes are also random, so a scavenger needs to compensate for less stability in food sources.

In practice, much scavenging is done by meat eaters taking advantage of a dead body. Meat is meat, and dead bodies don’t fight back or escape, so meat eaters will take advantage of an easily meal if available and not too decomposed, their digestive systems are often somewhat adapted to handle microbes already. Detritivores below also act as scavengers, dead bodies are just another type of particle or debris they can eat, especially if they can chew or separate smaller parts of a bigger corpse. The most well known pure scavengers are probably vultures and some maggots. Flying allows vultures and adult flies to cover more area and see more, allowing them to find dead bodies more easily. This also reduces the randomness of scavenging, over a wide range some sort of corpse will be available. A highly acidic stomach, and secretions by maggots, deal with microbes.

* Detritivores: Earthworms are no doubt the most well known of these, eating soil microbes, leaves, and other soil litter. Other well known examples are dust mites eating skin flakes and dung beetles eating you know what. Detritus consists of dropped bits of other organisms, waste products, random particles, and microbes, various types of small stuff and debris which still has some nutrients in it. Many of these animals appear to literally eat dirt, though they are eating for nutritious particles in the dirt, not the rocks or sand, which are passed through. These have to handle lots of different types of food, since edible debris comes in all forms, plus deal with bacteria doing the same thing. Detritus as a food source won’t try to fight, and lacks defenses compared to living beings.

* Water based filter feeding: Sponges, Mussels, Basking and Whale Sharks, Baleen whales are well known. Water can carry particles, whether detritus, microbes, or small animals or other organisms, some animals can feed on these. Many small living things also float in water currents, such as single celled algae or some very small animals. Some larger animals take advantage of this food source. These animals need a way to filter water to capture the food particles, plus some way to move lots of water through said filter. Filter feeding is a common food source for sessile animals, meaning animals that don’t move. Water currents carry particles to these animals, who as a result don't need to do much to capture said particles, allowing very simple body forms and low metabolism, leading to low food requirements which these particles can supply. Swimming animals need more energy, but the act of moving through water allows more food to be captured, these animals will go after particular rich clusters of their preferred thing to filter.
* Symbiotic Algae or Bacteria: The closest thing to a Planimal, some animals allow single celled algae into their bodies, these use photosynthesis to produce nutrients which the host animal absorbs. A few animals (like tubeworms in hydrothermal vents) so similar with bacteria that use non light equivalents to photosynthesis using surrounding chemicals or heat. Fungi and Algae doing something similar create lichens. The host protects the algae and supplies it with minerals it needs. Animals that get nutrition this way are typically attached or otherwise use little energy, the amount of sunlight striking more active animals doesn't come close to supplying the power they need.

* Roughage eaters: Eaters of general plant material, like leaves, stems, wood, or similar. Horses and cows eating grass, leaf eating caterpillars,….there’s a lot of these. By biomass, plants are the most abundant organisms on earth, so these organisms have plenty of food to eat. However, outside some specific parts, plants are poorly nutritious, and are hard to digest. The main component of plants, cellulose, can’t be digested by many animals, in human nutrition it is one of the main dietary fiber chemicals that pass through. Herbivores therefore depend on certain types of gut bacteria and fungi to do the hard work of digesting cellulose for them. A lot of herbivores do practice coprophagy (eating their own feces) to reabsorb nutrients otherwise expelled as dung. Plants can’t fight or escape, but can have poisons, and some are physically tough or have spines, needles, or other physical defenses.

Animals that eat roughage might spend more time digesting their meal, to better exploit what is there, or use more intense digestion such as chewing. Some animals can digest cellulose, others rely on bacteria to do the job. Termites and cos are the most well known with such bacteria, since these bacteria produce methane and contribute to greenhouse warming. Large digestive systems will be common, the animal will eat more total food than a similar sized meat eater.

* Fruit, seed, tuber, and other eaters of nutritious plant parts: Think bees and hummingbirds eating nectar, fruit flies, Some parts of plants are much more nutritious than stems and leaves. Places where plants store nutrients, such as tubers, are obviously something an animal would like. Spores and seeds need to carry some starter nutrients to get the new organism going, and need to be light to spread easily, so also contain concentrated nutrients. Flowering plants have evolved to take advantage of animal eating to spread seeds, producing sugar rich nectar and fruit that animals can eat, which also spreads pollen and seeds for the plants producing them, often further than wind can do so, and in a more efficient way. Pollinators moving from flower to flower means more pollen actually fertilizes something, and seeds are spread out further, meaning less self competition and a supply of fertilizing dung to get things started.

An animal specializing in these food sources needs less total mass of food than a roughage eater, but less total food is available and the animal must search for it. These food sources are also limited in certain nutrients, which the animal must compensate for. Flowers and seeds are typically brightly colored and give up unique smells, so good senses for these things are useful.

* Fungi: Fungi aren’t common as a main food source, most fungal growth is spread out filaments called hyphae that are hard to separate and eat and most animals for the the mushroom and mold growth instead, but a lot of animals will eat fungi as part of a larger diet. Instead of cellulose, fungi use something called chitin, which is different to digest. Fungal chemistry is otherwise close to animals and not a lot have hard parts, so digestion is relatively simple to do. Fungi obviously can’t run away, but have a range of toxins, most of you have probably heard of hallucinogenic mushrooms or poisonous mushrooms.

* Body fluids, parasitism: Bloodsuckers, tapeworms and other parasitic worms, aphids eating plant juices, botflies growing in skin. When you think of parasites, this is the food source of most of them. Body fluids are an excellent nutrition source, carrying lots of ready to use nutrients for the victim organism, and packed with cells ready to be broken down for more. However, while parasites don’t kill their hosts outright, they do weaken it, so the host organism will fight back and attempt to remove the parasite. These organisms as a result need to avoid defenses, such as an immune system attack from the host, or attempts to physically remove, maybe kill the parasite. Plant fluid eaters don’t deal with as strong defenses, but may have to handle poisons. Parasites also need a way to get into the animal, piercing skin and other defenses for bloodsuckers and plant sap eaters, or getting completely inside the other organism, which means burrowing in, or entering through an existing orifice like a digestive system or wound, digestive systems in particular are obviously hostile and an animal will need protection to move through it.

Halfway between parasite and predator is something called a parasatoid, where an animal lives inside another one for a long time, but is guaranteed to eventually eats up enough of the host to kill it. Many insects use this method of feeding when larvae.

* Extreme Omnivore: Rats, Crows, household cockroaches, and most famously the species reading this page, whose wide range of possible foods makes the culinary industry and lots of Food Tropes possible. Being a generalist eater has many advantages. From the same territory, more potential food sources means more food available. There is less risk, since if one type of food runs out, others are available. More environments can be lived in, since the animal is likely to find something it can eat no matter where it goes. However, less specialization might mean less ability to handle one particular type of food, though many generalists do just fine anyway. Animals eating lots of different foods will seem to have mix and match body parts from several types of more specialized eaters, plus lots of general purpose adaptations that can be used multiple ways.

Many animals are actually less specialized than you might think. Fruit eaters might pick infested fruit with insects, nutritionally this gives some protein. Large herbivores occasionally will eat small animals. In between full on Omnivore and full on specialist, most animals will eat food from several categories described here: predators will scavenge when they can as described in that bullet point, many plant eaters will happily grab leaves, stem, fruit, or anything else soft enough and available on a plant.

In human culture, breathing, heartbeats, and blood are strongly associated with life, and for good reason. We talk about the bloody cost of war, the breath of life, the beating heart of something as its most important, central part. In medicine, airway, breathing, circulation are the first priorities for emergencies, the loss of any of these is immediately threatening. The same, more or less, is true of most animals, though the organs are different, hypothetical cephalopod CPR or Defibrillators would need to handle three hearts, Insect medics would never use mouth to mouth as they get oxygen from tubes in their body instead of lungs, and water creatures would pump oxygenated water over the gills instead of ventilating lungs.

In addition to food, animals need oxygen for energy. The combination of food molecules and oxygen releases a lot more energy for the same amount of material as breaking down the molecules without oxygen, making more efficient use of food. However, oxygen does not dissolve well in body fluids and if generally hard to store, meaning it has to be continuously absorbed for most animals (even ones that hold their breath have to breath somewhat often ad take in a lot of air.). Body processes also produce a lot of waste, and life in general will often need to absorb or release chemicals to keep things balanced inside their bodies (such as removing water or salt to keep concentrations in a certain range). Toxin removal is also important.
Small life forms in water can intake oxygen and nutrients and expel waste through diffusion. In diffusion, random movement of molecules means concentrations tend to even over time, chemicals will tend to move from areas of high concentration to low concentration. An organism producing waste products will build them up, by consuming nutrients will lower their concentration, the higher waste concentration and lower nutrient concentrations within an organism vs. outside causes material to diffuse the desired direction. Organisms with membranes can use active transport, pulling material across using energy, but within internal fluid and in water outside, diffusion controls movement.

Diffusion is faster with higher concentration differences, over smaller distances, and across wider surface areas, so smaller, flatter, animals can get a lot of what they need this way, animals with lower metabolism will have an easier time meeting their needs, quite a number or smaller, flatter, less active animals use skin breathing to supply their needs with no trouble. In larger, plumper, and more active animals diffusion would move nutrients to them and within them too slowly, armor, thick skins, or other outer structures also interfere with exchange. Many waste products also don’t diffuse away in air, so land animals of all sizes need other ways to remove them. In more demanding animals, specialized structures are used t more quickly absorb oxygen, remove waste, and move chemicals through the body,

You’ve probably heard that your lungs or small intestine has the areas of some number of football fields/tennis courts/large sports field of choice. This high surface area comes from lots of folds, the large surface area allows a large amount of material to quickly diffuse or be actively transported into your body. Breathing organs and intestines in most animals are similar, a structure with lots of surface area, thin membranes to allow a lot of material to transfer, and good contact with body fluids or blood to take in the desired materials. High surface area structures include folded tubes like in vertebrate absorption organs, stacks of flat plates as in fish gills and some intestines, or branching treelike structures where lots of dividing stalks supply the area.

Breathing in air vs. water presents different challenges, and very few animals can do both. Oxygen’s low solubility in water means that a large amount must be passed over gills, and gills must extract as much oxygen as possible. For some animals, water must be moved over gills to get enough, the trope of sharks needing to swim is an example, though some sharks use their mouths and throats to pump water instead. Oxygen is much more available in air, to the point where some water living animals like whales and some fish can breathe occasionally and hold enough air for long underwater periods, but breathing structures can be sensitive to drying out, and a lack of buoyancy means the structures must be supported some other way. Bring a water breather to air, and gills or other water based structures collapse or dry out, bring an air breather into water, and it cannot absorb enough oxygen and breathing structures are possibly damaged by the water. Creatures that can breathe in both either have two sets of structures, such as some fish with gills and something to absorb from air, or gill or tube like structures made of strong materials and protected against drying out. A few fish, and many crabs, use the second method.

Gills have a range of forms, from the fish gill style slabs of feathery material, to what look like antlers or tentacles in some species. The baglike lungs you have are a vertebrate and snail thing among a few other animal groups, air is circulated in them through body movements or specialized muscles such as your diaphragm. Insects, millipedes, andsome other arthropods have passive exchange, through body tubes in insects or structures near the legs centipedes and millipedes, oxygen diffuses into branching tubes that get very small and provide high surface area for absorption. Some lungs are flat plates or feathery structures similar to gills, with air diffusing instead of being actively pumped.

Active pumping moves chemicals faster then diffusion would, with the variety of chemicals they need to move, most animals use some sort of pumping. Chemicals are carried in body fluids, If fluid is circulated inside a body, it can either be extracellular fluid, which fills up general body spaces, it is what fills blisters, or blood separated into a separated system, or something in between where fluid cycles between tubes and extracellular fluid (Humans and many other vertebrates actually have one of these in addition to blood, called the lymphatic system.). Circulation can be accomplished by body movements, a good fraction of blood in your vein is pumped this way, but specialized hearts are common which can more powerful and consistently pump material. The complexity of hearts depends on the animals, from fish with a single chamber or insects with a simple tube, to the two gill hearts and one body heart in cephalopods, or the four chambered bird, mammal, or crocodilian heart, both the last two systems have separated blood that cycles between breathing organs and the rest of the body, with hearts between pressurizing the blood at each transition. Hearts pumping extracellular fluid is possible, insects as an example have a system that pumps fluid through a tube to one end of the body, it then slowly moves to the other end as extracellular fluid.

Some chemicals, such as sugars, salts, or carbon dioxide, can dissolve in water and are carried this way. Insoluble molecules can carried by binding to carrier molecules, or emulsified to better mix with water. The most well known, important, and common carrier molecules are oxygen carriers, hemoglobin is the carrier molecule in vertebrates which makes blood red, other types of animals have different molecules, causing different colors of blood or body fluid.

A few animals simply build up or store waste in their bodies, they are short lived enough to not be weighed down by this buildup. For the vast majority that must expel waste, some waste products can be removed by the same structures that intake oxygen and nutrients: breathing organs in animals remove carbon dioxide, and in water animals gills can remove some waste that easily dissolves and diffuses in water. However, animals may need to actively remove wastes faster than such diffusion would allow, or remove material absorbed from the environment, such as saltwater living animals concentrating and removing extra salt. Land animals in addition need to actively expel solid and liquid waste which does not diffuse through air. Thus the kidney or equivalent structure. In many animals, this type of organ filters blood or body fluid, either removing particular chemicals, or sorting out only what the animal needs and excluding the rest. In humans among other animals, this waste is mixed with lots of water and leaves as a liquid, animals in drier environments or ancestors in the same often exclude paste of thick liquid or goop instead. This waste can be separate or mixed with digestive leftovers. A few specialized glands can exist for particular wastes: Salt glands are a common thing in ocean animals to remove excess salt to compensate for body fluids getting absorbing salt from seawater.

You’ve probably heard of Warm blooded and Cold blooded. Warm blooded animals make their own heat, are energetic, and have a higher temperature that shows up on infrared cameras Cold blooded animals are the opposite. Even if you haven’t, on this site you’ve probably seen related tropes: The Cold Blooded Killer who shows little emotion and calmly goes about their business, vs. the Hot-Blooded one who is energetic, emotional, constantly active. These types aren’t too far off from the animal qualities they are based on, though the world being the complicated place it is many animals blur the warm blooded/cold blooded distinction, metabolism, how active animal are, and how they handle temperature and heat have lots of variation.

The word “Metabolism” shows up a number of times in these useful notes. It technically means “chemical reactions occurring in an organism”, but mostly you’ve likely heard it to mean “how much energy a person/organism has”. These end up meaning about the same thing: just about everything life does involves chemical reactions, with energy producing ones used to power energy consuming ones. More energy production means more of these take place and the organism can do more of just about everything (Think not just physical activity, but also thinking, growing, basic body functions, etc.) but also needs to eat more, less energy production means the opposite. Closely associated with metabolism is temperature, heat is generated by energy use, and affects how quickly metabolic reactions occur, in addition to worries about freezing or burning. All lifeforms including animals work best at particular temperature ranges, animals have a number of ways of ensuring their bodies are at a correct temperature.

Most lifeforms use a huge variety of chemical reactions too many to describe here, biology often studies “metabolic pathways” or “metabolic systems” to see how materials are reacted, what reactions are connected, how the presence or certain chemical effects others, etc. They do mostly fall into a few categories. Energy producing reactions react or break down molecules to power everything else. In animals, oxygen reacting with nutrient molecules to carbon dioxide and water (also nitrogen or sulfur containing waste products if proteins are broken down) are the main reactions of this type, generally these happen in a series of small reactions where the nutrient molecules go through several steps and are converted to carbon dioxide and hydrogen ions, the hydrogen ions are then reacted with oxygen to produce most of the energy from the reaction plus water, vaguely similar to what fuel cells do in human technology. Sometimes, some animal tissues might break down larger molecules to smaller ones in a way that releases energy. This produces far less energy per mass of fuel molecule, but can be useful for short term power if oxygen can’t reach the tissue quickly enough for what the tissue is demanding. The resulting energy is used to produce a chemical called ATP. This stores energy in small packets, when energy is needed for something, ATP chemically reacts in ways that use its breakdown to power the process in question.

Some energy goes to nonchemical processes, like muscle contraction, nerve impulses, or pumping molecules that move material across cell membranes. Some is used in the second main type of chemical reaction, building large molecules: small building blocks are put together in various sequences to make proteins, fats, and various special purpose molecules. In animals, the smaller building blocks for such reactions come from digesting food, sometimes with some chemical processing (If you read about essential amino acids, the nonessential ones can be made by your body using such processing of essential ones if not enough comes from food) Some reactions get rid of waste products or dangerous chemicals, or at least process them into something easier to remove and/or less toxic. Such waste products could be ingested, or come from unwanted reactions within an organism. Some nutrients are used in a similar way, turning something an animal eats into a similar molecule the animal actually will use. Digestion breaks down large molecules into smaller ones an animal can absorb more easily and process into what it needs. This usually releases a little energy, but not in a way capturable by the animal.

Many toxins affect animals by interfering with some of these chemical reactions, an animal’s metabolic rate also affects how quickly toxins start to take effect, and how quickly they can be chemically processed if an animal does this.

Almost all of these reactions are performed by types of proteins called enzymes acting as catalysts. Catalysts by definition speed up chemical reactions, they can also change what types of reactions occur. The second property in life forms and human industry, laboratory work, and other chemistry, is used to ensure fewer side reactions occur, which avoids wasting material to produce useless or even harmful products instead of the desired one. Of the remaining reactions, some use different catalysts (such as protein production, which uses structures formed from something called RNA to transfer genetic information and perform the actual protein assembly), some occur freely. Proteins can change structure under different conditions, enzymes included. You’ll have seen this happen if you cook eggs or curdle milk, high temperature for eggs and acidity for milk change structure of certain proteins, which cause hardening of eggs or clumping in milk. These changes are part of the reason life forms have to control internal conditions: get too hot, acidic, alkaline, etc. and some proteins won’t do their job properly.

Most animals don't need to stay continually active, and this would pointlessly use up energy, instead peaks of activity and rest are done at different times. Often, rest is useful for longer term, "housecleaning" tasks like waste removal, digestion, healing, or some forms of learning, short periods of high activity can then use short term resources (fast sources of energy, producing more waste then can be removed at the moment, and similar) that are replenished over rest periods. Sleep in animals that do it is this sort of rest periods, it eats up about as much total energy as activity while awake, but uses it for those previously mentioned maintenance tasks, particularly in the brain. When conditions are poor (little food, long winters, and similar) many animals can go into hibernation or equivalent, greatly reducing energy use until better times for activity come.\\
Temperature control in animals has a couple components: whether an animal keeps a constant temperature or can have a wider range, and whether an animal matches the environment or is hotter. All of these come with tradeoffs: a constant temperature means an animal can optimize to function at that temperature and be more active then an equivalent flexible temperature one, but maintaining constant temperature takes energy, behavior, and/or organs that a flexible temperature animal does not need to have. Higher then environmental temperatures mean a more active animal, but higher temperatures mean more heat loss which has to be compensated for in some way. What makes sense for any particular animal depends on how active its life is/how much energy it needs to produce, how much food is available to generate that energy, and what temperatures it must adapt to in its environment.

The temperature an animal settles at depends on how heat production and heat transfer to or from the environment balance, animals can control their temperature by adjusting these things. All animals generate heat internally, metabolic reactions are never perfectly energy efficient and the losses turn into heat. Internal friction, electrical impulses, and other energy transmission that isn’t stored also ends up as heat. The amount of heat generated internally depends how active an animal is, some animals can directly produce heat by not using produced energy for anything else. Absorbing sunlight also heats an animal, the energy in the light converts to heat when absorbed. Turning liquid water (any liquid, actually, but water is what animals use) to vapor absorbs heat. Animals as a result can heat themselves through activity (physical movement, direct heat generation, shivering, or otherwise) or spending time in sunlight, and land animals can cool themselves by coating in water, panting, sweating, or other ways of getting water to evaporate.

Heat transfer to or from surroundings occurs through radiation (of infrared, at temperatures of interest) and through direct contact (conduction and convection, if you’ve studied your physics). It happens faster the more area is transferring heat, the greater the temperature difference, heat transfer through direct contact happens faster if the materials the heat flows through are more thermally conductive, and if any surrounding fluid is flowing (Meaning windchill and water currents.) Water is a far better heat conductor than air, but tends to maintain a more even temperature, so animals in water will lose or gain heat faster but not have to deal with as large of temperature changes. Underground doesn’t transfer heat as quickly and temperature stays more constant, so caves or deep enough burrows can be used to avoid temperature extremes. Body tissues that slow wind and/or transfer heat slower, such as fat or fuzz, add weight and tissue to grow but greatly reduce heat flow. Shapes with more surface area per size increase heat flow, shapes with less reduce it (So wide and flat transfers heat easily, long a thin a bit worse, rounded the least), individual body parts can be shaped this way and moved to allow faster or slower heat flow.

Internally, unless an animals is heating up from the outside, core areas will be warmer than the surface or farther out parts like appendages. This is because, if temperature is to stay constant, heat generation in each part of a body must be balanced by heat transfer somewhere else, and an animal can only lose heat from its surface, so there must be a change from higher in the middle to lower on the surface for heat to flow. This temperature difference will be undetectable for low metabolism animals producing very little heat, but higher metabolism animals will by noticeably warmer internally then the surface, particularly in thicker parts of the body, Body fluid (including blood) circulation helps to move heat, animals might send more heat to an area that needs to be heated up, circulate a lot through a high temperature area to cool it, keep heat in the middle instead of surface to reduce losses, or make many other adjustments for proper heat control. Animals with separate blood systems might have incoming and outgoing vessels arranged next to each other, this allows outgoing slightly hotter blood to continuously transfer heat to slightly cooler incoming blood nearby. The overall effect is that The outgoing blood is continuously cooled to close to the incoming blood’s temperature, and little heat is lost. (similar arrangements, called countercurrent heat exchangers, are used in industry to transfer heat well and make the most use of it.)

Animals range in size from microscopic to several tends or aone to two hundred tons. You’ve probably noticed that small animals are rarely just copies of large animals or the other way around, they tend to have different shapes, ways of living, and structures. Individuals within a particular species also can have different sizes, sometimes this is random from individual to individual, sometimes it follows a pattern, such as males and females in species with them being different sizes. (And young animals are never born or hatched full size).

There’s a few general ways size affects what animals are like.

First off, larger animals are physically longer, heavier, and have more stuff. Yes, this is obvious. This means they need to eat more, have more internal organs, can’t fit into smaller spaces but can avoid some obstacles smaller animals can’t, and will be physically stronger, including in fights. Also obvious. Not as obvious is that, combined with food availability, smaller animals can be more specialized and more diverse. Animals need to maintain a large enough breeding population to avoid unlucky population collapses/extinction from disasters and maintain some genetic variety, larger animal’s greater food needs means that they must rely on very abundant food sources, or be flexible enough to eat a wide variety of foods, while smaller animals can specialize more and still have enough food to support a good population. Larger animals will also tend to go for larger food items, collecting lots and lots of small food items would take too much effort to get enough food unless a concentrated source could be found. The greater variety of roles available means that smaller sizes have more species of animals then larger sizes, generally, until the smallest of animal sizes are reached.

Male/Female size differences come down to these obvious factors as well. If fighting for mating opportunities is common, whichever sex does more fighting will tend to be larger, typically males. Eggs require more work to produce then sperm, requiring larger organs to do so and more energy, this can push females to be larger in some species to support these organs.

Another major effect is the Square-Cube Law, there are lots of situations where something that depends on surface area or cross section interacts with something depending on volume. Strength of any tissue (muscle, bone, shell, soft tissue, etc.) depends on cross section, while the mass it has to move or hold up scales by volume. As a result, while larger animals are overall stronger, smaller animals are stronger for their size, take less injury from falls or stretching, and do not need as good a support structure for their body on land (Water buoyancy supports animals much better making this less a concern, but large animals may still need support structures for muscle movements) . Air or water drag scales by surface area, while the energy an animal can produce to move scales by volume, so larger animals tend to move faster, flying animals, meanwhile, will tend to be smaller as lift will scale by surface area while mass scales by volume. (Land animal movement is more complicated, though larger animals tend to be able to move faster also, until opposite scaling effects for injuries and strength come into play) Diffusion and heat transfer occur based on surface area, so larger animals will tend to change temperature more slowly, and won’t absorb nutrients or oxygen or remove wastes or lose heat as quickly through skin, this means larger animals tend to have more specialized organs to fill these functions. Flow through a wider tube produces less friction, but takes longer through a longer tube. This means all else equal, food takes longer to move through a large animal digestive system. Blood circulation or equivalent will be different, larger animals will have separate blood circulation as cellular fluid would lose its nutrients and oxygen over a longer distance, and separate tubes reduce obstacles and friction the blood experiences. Greater pressure differences in different body parts mean large animals need stronger hearts to compensate.

Reaction times can be faster for smaller animals, as nerve impulses have a shorter distance to travel.

The largest animals in the world are mostly filter feeders, predators, plant eaters, or some mix of the previous, these food sources are the only ones that can supply enough food easily. Almost all are in the vertebrate group, a hard skeleton is needed to support land animals and very useful for supporting water animals, all the big invertebrates live in water. Other groups with skeletons need to molt, which slows growth and is dangerous if an animal can’t avoid attackers, or are stationary animals that don’t have the lifestyle to grow large. The largest of these, baleen whales and sauropods if you include prehistoric life, have food sources that benefit from large size: Sauropods eat roughage where a long trip through a digestive system is useful for digesting it, Bealeen whales filter feed, which is more efficient with a big mouth to grab big growths of plankton or schools of fish. Other qualities combine to encourage or allow large size: Sauropods have air filled bones which make them light for their size, and reproduce by laying lots of eggs, allowing fast population growth if they suffer a temporary die off and requiring a lower population. Baleen whales live in the ocean which can support them, and the wide open ocean allows most of the planet to act as a single breeding population for any species.

At the smallest scales are microscopic animals, including mites, various worms like many nematodes, Tardigrades, or small crustaceans. The smallest animals known are very simple parasites called Myxozoa. These include detritivores mainly, including some bacteria eaters, and some parasites: being small makes it easier to live in a host without sucking food dry (unless a major infection takes place), and detritus often consists of small particles that a small animal is well suited to gather. Many of these animals have lost organs, small bodies mean less need for a circulation system or lungs to move important chemicals around.

Multicellular organisms need some way to hold their cells together in larger structures. Much of the time, networks of long molecules form this structures, in animals, these are proteins such as collagen. If you’ve used gelatin, it comes from partly broken down structural proteins, plant based gelling agents (like agar or pectin) come from equivalent molecules in plants.

The resulting gel like texture works for most animal tissue, some animals use larger water filled structures as well to maintain a shape or provide something for muscles to work against, like a water balloon or other full water bag. There are limits to what water and gel can do, however, so many animals use rigid, hard materials for some body parts, such as skeletons for support, shells, scales, scutes, or similar for protection, and/or teeth and claws and similar for weaponry, eating, and other environment manipulation. These structures could be made of hard biological molecules (like your fingernails, made of something called keratin, or insect shells from chitin) or from minerals that make a rockline structure (Bones or snail shells are examples.) These structures may or may not have some living cells mixed in. If living cells are not in the hard part, it will have a larger portion of whatever material gives it strength, but also can’t grow, heal, or change in any way (Why insects and other arthropods must molt, their shells are pure chitin and have to be replaced completely to allow growth). Including more living cells means the opposite (Your bones are an example of this.) Nonliving hard parts are produced by nearby living body part secreting material, while more alive hard parts can self regenerate and grow. Materials with more minerals in them are usually stronger for their size and weight, but also heavier for a certain size, require lots of specific nutrients (calcium based minerals are most common), and genetics to produce them can be tricky to evolve. Even the most mineralized hard materials usually have a little bit of protein, which helps bind the structure and make it less brittle.

If an animal has a jointed skeleton, joint material needs to be both strong and flexible to allow movement but still support the animal. Densely packed structural proteins usually fill this role. Joints create weaknesses, whether in a fight, in accidents, or from wear over time, but a mix of hard body parts of flexibility is extremely useful for animals that have them. Jointed skeletons have evolved only a single digit number of times, but include most animal species, including all flyers and walkers.

The outer layers of an animal have to interact with its environment, whether protecting from dangers (attackers, sharp objects, chemicals, temperature, etc.), using it for resources (breathing, waste disposal, and similar.), and some senses. Some of these of course involve tradeoffs, thinner skin can breathe more easily and have better senses, but won’t provide much protection or heat insulation, skin that can do more of everything will be more complex with the disadvantages that creates. Coverings built more for protection can use any of the structural pieces described above, from more dense protein networks for thick, tough skin, to hard molecules to mineralized shells or plates.

Skin or equivalent can contain lots of sub organs for various roles. Some cells produce secretions. These may come from the skin as a whole, or specialized glands can push chemicals out through a smaller channel while leaving the remaining skin less permeable. Hard body parts like scales, shells or similar usually use specialized structures for production, this allows for better control of their shape as well. Structures like hair or feathers are similar, these are formed from structural proteins just like harder parts. Controlling heat transfer is often important, insulating material like fluffy feathers or fur, or underlying fat layers slows it down, increased blood flow and structures that increase area speed it up. Pigments are typically produced by special cells and stored in the skin, different colors might be used for camouflage or communication. For bare skinned land animals, pigments also help protect from ultraviolet light damage by absorbing it instead of molecules vulnerable to damage. Color changes (as in the chameleon trope) might involve changing the type of pigments an animal has, or changing the shape or size of the structure containing them, affecting how light interacts with the pigment.

Ask most people what is unique about animals, and movement will be high on the list if not first. Almost all animals can move, even attached in place oceanic ones usually have swimming larvae and/or can move individual body parts for various things.

A few early splitting animals mainly use cilia when moving. These are small hairs sticking out of the main body, by vibrating/wriggling they push against water. Lots of cilia doing this in a coordinated way can cause decently large movement. However, most animals use muscles or similar. In muscle cells, fibers of protein in long bundles are attached to each other. Using energy, the fibers can be made to crawl along each other, causing a pull and creating a force. Attaching these fibers to something else allows the muscle to pull things. No equivalent exists to push, but in animals the body parts muscles work against will be arranged so that a good range of motions is possible. Muscles/groups of muscles typically come in pairs, arranged in a way where one group causes the opposite actions as what it is paired with. A couple common pairings are muscles on opposite sides or a limb, tentacle, or body that bend it in opposite directions, or lengthwise and circular muscles around a cylinder. In the second one, the lengthwise muscles pull to contract the tube, whole the circular ones push on material inside the tube, which squeezes out and extends it. Muscles often work with elastic tissue which can store energy, and return to a starting position if muscles relax.

You’ve probably seen red and white meat, these are examples of how different types of muscle fibers can be adapted for their roles. Relatedly, you may know of strength vs. aerobic exercises, or sprinting vs. long distance running. The first requiring more immediate power that doesn’t have to last long, the second requiring more endurance. Muscles can adapt in various ways for these different roles, such as storing energy producing molecules for a quick boost, or having the ability to process lots of food and oxygen to allow for longer periods of activity,. Muscles contract when stimulated some way, what causes contractions and how they are stimulated changes from muscle to muscle. One example is insect wings that beat several times per nerve signal, which allows faster flapping, another his how your heart generates the signal to beat by itself with the rest of the body controlling only how fast it does so, this keeps the heart beating in as many situations as possible even if something happens elsewhere. Other adaptations include the density of contracting proteins or what sources of energy it can use.

Muscles and cilia get used internally for moving things through a body, to move an entire body they have to push against the outside environment. Whether and how fast to move depends on an animal’s lifestyle, faster movement obviously is helpful for a lot of things, but it uses more energy and/or requires particular body shapes to reduce drag and push against the environment properly. Slow moving or attached animals such as coral or starfish can have a variety of shapes, which makes them great for background scenery, these shapes can be adapted for capturing particles coming from all directions or protection in ways that faster animals can’t do.

The exact shapes needed for fast movement depend heavily on environment, but there are a few general principles. How fast anything (animals, vehicles, etc.) moves depends on how much energy it can generate to propel itself, how efficiently that energy goes into propulsion, and how much energy is lost to friction with the environment, direction control/balance/and stability also need to be accounted for. For animals, the energy is generated by muscles or cilia, so larger, faster metabolism muscles or cilia cells better supplied with nutrients and and oxygen (Unless using quick burst non oxygen energy sources) can move faster. Propelling an animal means pushing against something: water while swimming, air while flying, the ground while burrowing or walking/running/crawling/climbing. The exact details of how hard an animal can push depend on the material. Energy losses come from various types of friction, reducing this allows faster movement. Internal friction can be reduced by reducing extraneous body movements, holding rigid body parts that aren't needed for moving. Muscles need energy to stay contracted and don't store energy well, so elastic body parts and hard body parts might be used for support roles, or to store energy that would otherwise get lost, which is used for more movements, External friction is reduced by using streamlimed body shapes, keeping the body more rigid (more movements produces more friction), and having less contact with materials that produce more friction. Balance, stability, and direction control involve pushing on the environment in various ways, exactly how this is done depends on the environment. Coordinating these movements is an important role of a nervous system, if an animal has one. Animals with larger brains will have more flexibility in body shapes and how they move.
Some of the above requirements work against each other (more control and balance structures mean more body parts to use energy and create friction, more pushing against an environment implies more movements creating more friction, etc.) Exactly how the above requirements are balanced out depends on what kind of movement an animal uses. Short bursts of speed (such as for ambushes or quick escapes) can use a lot of energy to move as fast as possible, so instant energy production and pushing hard on the environment for fast acceleration are most important, reducing friction not as much. Long distance constant movement (for chases, migration, searching wide areas, and the like.) will not need as much acceleration but conserving energy is important, so this movement demands streamlined shapes, good breathing and blood flow to supply lots of energy over a longer time, and powerful enough propulsion to overcome friction but energy efficiency is also important. Agile movement demands lots of ways to change direction quickly, straight line movement allows for other qualities to be optimized.

Water is dense and viscous. The density is slightly less then soft tissue, which means buoyancy supports animals quite well, animals can overall get less dense then water using a number of substances, such as air/gas bladders, or lots of oil. However, the viscosity and density mean moving through water creates a lot of drag, though this drag and related forces also make water a good material to push off of. For moving slowly, almost any shape will work, the animal just needs to move some body part against water in a way that uses the water backwards. For moving quickly and efficiently, streamlining to reduce water drag is valuable. The most streamlined shape is a long, thin, tapered shape of some kind, this results in the typical fish or squid body, or the shape of a submarine for human vehicles. Pushing against water creates lift and drag forces, directly dragging a flat surface against water creates a lot of drag and a lot of force for acceleration, but also uses a lot of energy. Some types of angled motions, on the other hand, produce more lift, resulting in a lower force but less energy loss for the same amount of movement. Jets of water are a specialized technique used by cephalopods, these can achieve high speed and high acceleration but are not as energy efficient. Keeping a body as stiff as possibly reduces water drag from body parts moving through it, reducing energy loss. Direction control comes from directing water in different directions while moving, or pushing differently with different body parts, the same lift and drag forces used to propel an animal can allow it to change direction and orientation, or keep the body stable when moving in a particular way. Fins are a common body part animals use for this, the wide flat surfaces produce strong forces to either change direction or keep an animal in the same orientation while swimming.

For animals on a solid surface, ground produces more friction then water or air, but also provides more support, movement is about reducing friction with the ground, but still having enough contact to push off the surface and support the body. Crawling animals might push off objects or bumps on the surface, or use sections of their stomach to push while raising or at least reducing pressure on other sections that move forward, alternating sections pushing and sliding or moving allows the whole body to move. Some crawling animals have slime, smooth skin parts, or other methods to reduce friction. Legs or similar organs allow animals to move faster, they reduce the contact area with the ground compared to resting on a stomach. However, legs that cover less area need stronger structure, and legs that are flexible with a wide movement range often need complex systems of muscles and skeleton that not all types of animals will have. Animals with small numbers of legs can have balance issues, 2 legs is unstable and needs constant adjustments to stay upright, 4 legs often have 2 or fewer legs on the ground when moving, mire legs allows more to be on the ground when moving, allowing more stability. Balance is particularly important for large animals, falls can cause serious injuries, and even smaller ones need to stay upright to be able to move properly. Animals with awkward body shapes might have unusual techniques for righting themselves.

Moving on legs involve cycles where different legs are on the ground pushing on it, while others are lifted and moving forward. Most of you can instinctively tell “running” and “walking” gaits apart, but defining these in a clear way isn’t exact. Running tends to have legs on the ground less, also tends to have more energy stored as elastic energy in body tissue, rather than energy of motion of the legs, or gravitational energy as parts of a body rise. Different gates tend to be more efficient at different speeds, partly because legs tend to naturally move at particular frequencies (they act something like pendulums) based on leg length, weight, and other characteristics. Energy is lost mainly through internal friction, elastic body tissue not perfectly storing energy, and when feet or equivalent hit the ground, so efficient movement will reduce these things. Longer legs in many cases allow faster movement. Longer strides mean legs change direction less, which reduces some energy loss, and they can make some muscle motions easier, but longer legs of course require more nutrients to maintain and grow. Motion of lots of individual legs requires a good neural system to coordinate. The less smooth motion also creates vibrations in the main body, and other body functions (like breathing) will adapt around these movement, you’ve probably noticed yourself when running that breathing follows a matching rhythm, other animals with longs will do similar.

Land animals can obviously swim, some water animals walk or crawl on the sea/lake/river bottom, and some water animals can crawl on land. The big limits are generally breathing and/or support. Body shapes for one environment aren’t generally optimized for another: legs sticking out create friction in water, though they can push against it for slower speed swimming, fins can drag on the ground without providing support or speed. Animals adapted for both environments generally have body parts that can handle both sets of requirements, like legs with webbed feet or hands that fold flat against the body. Many animals of course adapt to mostly move in one environment wi=hile being slower but passable in another. Water bottom crawling and walking are similar to land based versions of travel. Water drag means these travel methods are slower, but some buoyancy is provided, and a bottom animal doesn’t need to control it as finely as a swimmer.

Flying has evolved very few times in animals (insects, birds, bats, pterosaurs, plus some very close prehistoric bird relatives are the only ones known), gliding has evolved a few more times but is still rare. Air obviously can’t support a heavy animal by itself, but generates a lot less friction that ground or water and allows open movement. Almost all flyers and gliders use airfoils of some kind, as with airplane wings, when air moves over the foil it is deflected. The air pushes back on the foil, deflect it downward and the animal is pushed upward, fast enough and enough air deflection and the flyer can stay at whatever height it has or even climb. Airfoils produce the most lift per weight and drag when thin and flat, which you are probably familiar with on flying animals, or kites and airplane wings. Deflecting air in this way does automatically create drag, limited the distance a flyer can travel unless it has a way to increase speed or height again. Most gliding animals just use gliding for limited distances (as a shortcut/climbing alternative, for escape, or similar) where this isn’t a problem, soaring animals (and many human built gliders) can use rising air currents to gain height, powered flyers (animal or human) push against the air to speed themselves up or climb.

All flying animals use flapping as their source of propulsion. Wings in flapping move up and down, on the downstroke, the wing is held close to horizontal and extended, this catches as much air as possible and shoves it downward and usually backward, sometimes in other directions on some animals. On the upstroke, the wing is rotated to cut through the air, this generates less drag. The large amount of air pushed on the downstroke then upstroke means overall a flapping wing is pushing air down and in the opposite direction to whichever way the animals is moving. Wings or other gliding surfaces, sometimes other flat body parts like bird tails, also act as control surfaces. By angling/twisting or similar the surface, air is directed various directions to push the animal where it wants to go. Since staying up in the air requires energy, which increases proportionally to weight, flying animals need to produce a lot of power for their size.

In the opposite direction, burrowing/tunneling animals have to move through tougher materials than any other kind of movement. Soil, sand, or other sedim…uh…powders don’t flow unless some minimum force is applied, and are quite dense. To move it out of the way, animals need a lot of strength, since the burrow needs to be larger to fit a bigger animal, high strength per size is needed (so large animals won’t burrow much partly due to Square-Cube Law.) A long thin shape is useful if an animal uses its body to dig, this reduces the size of the burrow needed for the same size of animal. Animals with appendages can use them to dig, scoop shaped ones will do a better job similar to shovels, but many shapes can work if the animal will use the burrow for a long time. An option used by some animals is to eat what they are burrowing through, which might be the animals main food source if it eats detritus or smaller creatures living underground.

Animals fight for a number of reasons: to catch prey, fend off attackers or possible threats, or fighting for resources such as territory, mating opportunities, or food. Whether an animal fights and how hard depends roughly on the resources at stake and how good a chance the animal has to win the fight: fighting to keep small number of offspring alive will generally get much more intense then a fight to kill one particular prey animal when another will likely show up soon enough.

The Sun Tzu quote about winning without fighting applies to animals, fights take energy, even a winning fight can result in injuries that may cause more damage then the fight is worth, and losing a fight is obviously less preferred then not having the fight in the first place. Because of this, intimidation and avoiding fights are quite common, if an animal can scare off a rival or attacker, or avoid being noticed by a threat, the animal avoids the costs o fighting. Intimidation uses various ways to make an animal seem threatening, such as loud noises or posture to seem larger, or looking like a different, more dangerous kind of animal, or various body displays to show a willingness to fight. Interactions over time have led to poisonous animals having particular bright color schemes which warn attackers off, which some nonpoisonous animals have evolved to have as well to gain the intimidation benefit. Camouflage, fast movement, and finding or creating places to hide are used to avoid threats. Camouflage could be similar color effects like countershading (underside of animal is light, upper side is dark, to blend in water with a light sky seen from below or dark deep water seen from above) to color changes (Cephalopods are particularly known for this) and exact copying of some environmental thing (Some insects look almost exactly like leaves, sticks, etc.). Sound muffling and smell muffling can be used as well.

Predators catching prey will need to kill an enemy, this means weapons to kill whatever they are catching, and an ability to get close enough to use them. The second part usually means ambushes or speed or a combination, ambushes rely on ability to stay hidden plus a sudden burst of power for an attack, chasing relies on short sprints to longer runs of speed to outrun prey. Potential prey animals, if they don’t have have similar weapons themselves to catch animals, are typically adapted more to avoid predators then fight back, and will tend to have some or all of good senses to detect threats, speed, ability to hide or camouflage, intimidation, or armor or thick skin to make predator attacks ineffective.

Territory or other resource fights can be lower stakes, depending on the exact resources being competed over. If within a social group, group members provide some benefits to each other (otherwise they probably wouldn’t be living together) so competition within social groups might also be less intense. Though “might” is the important word, some of these competitions do still get intense and deadly, especially if the resources are valuable. If a type of animal doesn’t use weapons for other purposes, it may have special display structures for these types of fights (such as many antlers), or may just use its existing body in a more aggressive way. Fights within social groups, or at least within a particular species, can often seem ritualized, with a specific fighting method and sequence of steps used.

If animals do fight, the weapons and defenses will evolve based on what threats the animal faces, balanced with how much energy and nutrients it takes to grow and maintain the organs, or any other disadvantages they have. Weapons and defenses will tend to adapt to each other, effectiveness will be balanced, as usual, with the energy, nutrients, and other resources needed to grow and use the organs in question. How weapons adapt resembles how human militaries adapt over time to each other, and a lot of animal weapons and protection resembles human equipment, particularly pre-gunpowder weapons. \\
Some weapons:

• Teeth, beaks, jaws, and other mouthparts: Animals have to eat, which often means breaking down food into smaller pieces. Since eating is a regular thing animals do, the body parts that process food experience a lot of wear and other physical forces, and will often be built out of tougher material and shaped to handle these physical effects. Such toughened, properly shaped organs will be easily adapted into weapons. Since the mouth is the body part that takes in food, these parts will most likely evolve to by in the mouth. Movement to open and close a mouth might give various benefits when first evolved, but get strong enough muscles, and various types of jaws and biting become part of an animal’s weapons (and other food processing.)
  
  If you read about prehistoric life, you’ll see lots of “Somethingdon” and “Nameodon” style names. Don and Odon mean tooth, and vertebrate teeth (if they have them) are quite distinct between different animals, plus tell a lot about that animal, how it lives, size, etc. The same is more or less true for other specialized mouthparts if an animal has some. There are a few patterns in shapes, though. For attacking soft tissue, flat or pointy shapes work well, edges allow slicing and points allow piercing to quickly cut something important. Rounded blunt parts are better for breaking shells or other hard parts, putting a lot of force on an area without breaking thin sections of the mouthpart. For grinding (used for plants, usually), flat shapes work well, For scraping, a flat surface perpendicular to how the tooth moves works well. Mouthparts used to attack large prey or fights will probably have to handle powerful forces, thick bodies or strong attachment will be common as a result. Less specialized mouthparts might evolve to a mix of features (small blunt tips with edges can be useful for both crushing hard parts and piercing or slicing soft parts, as an example). Even mouthparts adapted for things other then cutting can be dangerous weapons if enough force is put through them, they are often parts of displays or body language to scare off opponents as a result.
  • Claws or equivalent will have a similar shape to teeth, if used as weapons.
• Holding in place, crushing. Raw physical force might be used as a weapon (as with constrictor snakes) or hold an animal in place for something else to happen (many animals will hold prey in place with appendages while eating it). To do this an animal obviously needs a lot of physical strength, and some way of positioning themselves so that the opponent can’t escape. Piercing hooks, claws, or teeth might help hold the target in place by piercing it.
• Poison: “Venom” is poison injected into a target as an attack, more general poisons might be found elsewhere in an animal to fend off attackers. Poisons have a number of ways of harming a victim, they might directly destroy cells (particular cells or broad destruction both occur), or might interfere with particular signaling (such as venoms that cause paralysis) or metabolic reactions, they can be highly specific in what they attack. Many injected venoms are proteins, while defensive poisons are not, as digestion will destroy many proteins. Offensive poisons are typically injected into the target after piercing it with an appropriate organ (tooth, stinger, etc.) These might be small and hard to see, but still can pierce enough to enter skin.
  
  People often are scared of venomous animals for understandable reasons, they can be small but extremely dangerous for their size, easier to accidentally be attacked by then rarer and more visible physically dangerous ones. Venoms do have the advantage of killing without a need for lots of drawn out fighting or physical power. However, they also take a lot of energy to produce, venom takes time to replenish, and highly specialized venoms often are only useful against small groups of animals, plus targets might evolve resistance.
  * Horns (antlers, and similar) and Ramming. Horns are most famously found on big land herbivores, though a number of other kinds of animals also have these. Horns are often display structures as much as weapons, used to scare off others or for attraction as much as actual fighting. If they are used for fighting, a pointed, thick shape works well to keep them strong and more effectively injure the target. Ramming without using horns might instead use hard body parts plus cushioning to take the hit.
  
  If an animal doesn’t rely entirely on running away or hiding, or their own weapons, some sort of armor is the main defense. Thick could be a complete, hard shell, hardened skin coatings or plates or similar, thick skin, or anything in between. Poison or other nasty chemicals, spikes, or other methods might evolve to hurt attackers as part of a defense. The big tradeoff to hard body parts as protection is of course having to carry extra weight around, plus the need for nutrients and energy to be used to grow these defenses. Many animals as a result only have hard defenses where attacks are particularly dangerous and/or most likely to strike, such as vertebrates having a skull to protect the very important (brain, plus sense organs) and likely to get struck (since animals usually move forward) head, a less covered rib cage covering other important internal organs, and no bones enclosing the legs, which are relatively less important and need to move. Joints (like arthropod shells have, or flexible edges of scales or osteoderms) allow flexibility, but present a weak spot to attack.
  

Senses are anything used to gather information from the outside world. You may have heard of human senses like vision, hearing, proprioception, and others, other animals have these as well, plus some weird ones. Here’s roughly how different senses work:

• Sight: Information from visible light, infrared, near ultraviolet. Visible light and nearby electromagnetic radiation on the spectrum have a number of useful qualities for animal senses. It can see small details, radiation like microwaves or radio lacks fine enough resolution to do this. The sun emits more visible light than other parts of the spectrum, and Earth’s atmosphere is transparent (mostly) to it, so there is a lot available to see with. It interacts differently with many materials, so reflected or emitted provides lots of information about where it came from. It also doesn’t destroy or damage most biological molecules as higher energy electromagnetic waves (like ultraviolet or gamma rays) do, so it can be safely interact with a sensitive sense organ.
  
  Light sensors range in complexity from a simple light/dark sensor which can tell shadows and night/day cycles, to full on eyes that can see several colors and fine detail. All eyes have to have a receptor, usually this is a molecule that absorbs some range of frequencies. When light is absorbed, the molecule enters something called an excited state, this excited state then reacts with something in a neuron to send a neural signal, or chemical signals in simpler life forms. Eyes that see a picture need some way of resolving images. Some of these eyes (including vertebrate and cephalopod eyes, the “lens in front of a ball” eyes) use a single lens and sheet of light receptors, the retina, in back to do this. The lens focuses the image, the array of receptor cells processes light from different directions. Some other animals (including snails) use a pinhole eye, where a very small opening produces the image. Compound eyes use lots of tubes next to each other to see different parts of surroundings, building an image from these.
  
  Light comes in a range of frequencies, and different materials reflect or emit a different combination of these. Color vision is how animals detect these differences. If the animal has one type of receptor that sees brightness, it is equivalent to black and white vision. In animals with multiple color receptors, the types of receptors respond differently to the various frequencies of light, the animal then combines this information to make color. Humans have three receptors, one reacts most strongly to blue and gets weaker away from this color, one the same with green, and one the same with yellow. Red light activates this last receptor but not the first two, so emitting different combinations of red, blue, and green light is a good way to simulate all colors for humans, including RGB displays and subtractive color printing using Cyan, Magenta, and Yellow (Which absorb some colors and leave the right combination of red, gren, and blue) . Other animals may have other numbers of receptors, “dichromatic” “trichromatic”, “tetrachromatic” etc. describe the number of types of receptors each animal has.
  
  Heat vision is actually a form of sight, all bodies radiate electromagnetic waves, the exact amount depends on temperature. At earth surface temperatures, the most radiated are infrared, so infrared based vision roughly sees the temperature of the object emitting it. Some animals see near ultraviolet, the least dangerous frequencies closest to visible. Both of these use the same sorts of receptors, with the same considerations, as visible light, though heat vision needs some protection from the animal’s own emissions from body heat.
  
  Light is dispersed and/or absorbed a little bit by air, more by water, and dirt obviously blocks light almost completely, this allows long distance sensing in some environments. It is obviously affected by the day/night cycle, it is likely the first light sense in life was useful for telling day or night. Animals in well lit areas obviously get the most use out of sight. Animals living in poorly lit areas such as deeper in the ocean, or ones active at night, dusk, or dawn, might either lose sight, or develop vision that can pick up what light is there, either by catching more light or having eyes optimized more for sensitivity to any light then for color vision or other detail. The deepest ocean and underground far from tunnel or cave entrances lose all light and animals can be blind at little cost, though many animals in these areas travel to other environments where vision is still useful.
  * Sound and pressure changes or vibrations: Sound is made of pressure waves, which quickly compress and expand materials while travelling through them. Sound is produced in quite a lot of different ways, and has a huge range of frequencies, so sensing the strength and combination of sound frequencies brings a lot of information about the environment. Sound can also travel relatively far, including around objects, even more so in water than air. Not strictly sound, but closely related, and non continuous pressure waves or other vibrations. These, if an animal experienced them, can be sensed in similar ways and also tell a lot of information about surroundings.
  
  Sensing sound relies on nerves that respond to motion or vibrations, these nerves send signals when their orientation changes, which occurs when movement caused by pressure, flow, or other changed position happens for one section and not another. Some of these nerves might respond to any kind of motion, which can detect pressure or the existence of sound. Individual frequencies can be detected by neurons attached to structures that only vibrate when a particular frequency of sound hits. A general physical effect called resonance makes this possible, every structure has certain frequencies of change that make it vibrate much more than others, with the right design, the range that creates this response can be kept narrow, allowing accurate detection of some frequency. The principle has been used in radios (for picking up only desired radio frequencies), avoided in large structures like buildings (where resonances can destroy the structure, methods are used to reduce the response, in animals, ears have evolved with a lot of structures such as hairs that each can narrowly hear particular frequencies. Have lots of these, and an animal can have a good hearing range and accurately detect particular types of sounds.
  
  Sound travels a good distance through almost all environments, though is more easily dispersed then light. It travels best through water, moving quickly and losing little energy with distance. Air is less effective at transmission, as well as dirt, but these still transmit sounds relatively well. Sound also transfers more easily between materials with similar properties, so if sensors are inside a body, land animals need a way to efficiently transfer the energy from air to body, while water to body and dirt to body happen much more easily.
  * Chemical Senses: Includes smell and taste. Receptors have a molecule shaped in a way that interacts with very specific types of other molecules. When such an interaction occurs, the sensor molecule adjusts, and sends a signal. Scientists used to think shape was the determiner, a correctly shaped molecule fits into the receptor and attaches strongly enough to send a signal. Now, other mechanisms are also involved with some detections. Animals have a wide range of sensors, allowing a wide variety of molecules to be sensed.
  
  Chemicals don’t travel as fast as sound or light, and spread out more where they travel, so they won’t produce as precise a picture of surroundings as light or sound. However, they last a long time, and if detection is good enough, very small amounts can be found. This makes chemical senses good for detecting that something has happened in an area.
  * Body Position, Movement, Gravity: Proprioception is the fancy word for sensing where all the parts of a body are in relation to each other. Fast moving or larger animals in particular need these senses or something similar, they are very important for proper balance and direction control.
  
  Several senses together detect these things. Sensors inside a body detect pressure and muscle contraction, together these can determine how body parts are positioned in relation to each other. Gravity, acceleration, and rotation can be detected with fluid filled capsules, or heavy material. Both of these will accelerate slower then the rest of the body, and will be pulled different then the body by gravity, this relative motion and pressure can be detected by movement detecting neurons.
  * Pressure, touch, temperature, and other contact senses: A variety of senses found on skin or equivalent, some of these might be found internally for various body sensations. It is not known how touch senses work, distortion of the neurons causing a signal to be sent may be involved. Temperature sense relies on a collection of different molecules that change form as temperature changes. The equilibrium between different forms of any particular molecule will change smoothly as temperature changes rather then at a single point, added to having lots of molecules, and a wide range of temperatures can be sensed.
  
  You might hear that “The temperature you feel senses heat flow, not temperature’’: This isn’t strictly true, but there is a reason for the saying: Since skin is a boundary between the body and the environment, it takes a temperature between them. The exact temperature depends on heat transfer rate, a faster loss of heat from your body requires a lower skin temperature (heat conduction is faster between your body’s interior and the surface of the skin if the temperature difference is greater), so skin temperature makes a good proxy for heat loss.
  * Pain: Pain detects tissue damage. Animal pain is sensed when certain nerve cells are damaged, sending a strong signal.
  * Internal body senses: Hunger, thirst, heat, and other senses use equivalents to external senses to get information with the body. A cavity or organ forced to expand is detectable by pressure sensors, such as fullness after a meal or needing to pee. Internal chemical senses can detect if there are too much or too little of some chemicals, stimulating control processes in an animal’s body or need to eat, breathe, pee, or similar.
  * Electrical, Magnetic. A few animals can detect electric or magnetic fields. Electrical reception only happens in water, since other materials do not conduct electricity well enough for an electric field to be detectable at any distance. Magnetic sensing is used for longer distance travel and is usable in any material.
  
  Electric fields are sensed by certain molecules, these fields cause a structure change that makes the molecule function differently, this change causes events that set off a nerve signal. The way(s) animals detect magnetic fields isn’t know: Possible methods include a magnetic magetial similar to a compas magnet, that rotates and/or presses on surrounding tissue to be detected by pressure sensors, or by electrical sensors that detect currents generated if the animal and magnetic field are moving differently, or by the effects on spin of subatomic particles (see descriptions of chemical bonding or quantum mechanics fir a more complete explanation.) Electric fields are useful for detecting nearby living things: nerve impulses, muscle contractions, and some other body functions create electric field changes, and animal bodies made of water with salt dissolved can conduct electricity, which interacts with existing electric fields. Magnetic detection isn’t as useful in the immediate environment, but can be used to navigate like a compass, some rocks are also more magnetic and this might be useful for smaller scale navigation.
  * Direction, sensitivity location of sensors: Most animals you think of have more then one sense organ: 2 or more eyes, 2 ears, lots of skin sensations around the body, maybe lots of electrical sensors on some fish. And of course, within each sense organ there are lots of whatever kind of cell does the sensing, plus things like compound eyes. Any organ takes energy and nutrients to grow, and more than one of a sense organ might just provide duplicate information not worth much.
  
  Aside from safety margins, extra sense organs serve a few roles. Different sense organs, positioned at different places on the body, will pick up different information. Bodies block light and sound somewhat, so eyes and ears pointing different directions give a wider field of view. Multiple small eyes, and multiple light sensors in on eye, pointing in slightly different directions allows an image to form. Contact senses need to be right next to whatever they are sensing, so need to be all throughout any parts of a body where that sense will be useful. Multiple receptor types detecting different things, as described for each sense, are needed for color vision, smelling or tasting different chemicals, different sound frequencies, etc. Overlapping or close together sensors sensing the same thing allow direction and depth perception. Your own depth perception partly relies on images from two eyes being slightly different, the differences allowing distance to what is being seen to be determined (Parallax and perspective are worth searching for more information on this.) Different strengths of sound, smells, or electric fields n nearby sensors work similarly, allowing a direction to be determined, more sensors provide finer information this way.
  
  Sense organs are obviously more useful if they can collect more information, particularly more information of interest: Eyes might be higher up to see farther and over objects, chemical sensors in a mouth provide important information about food, similar logic applies to others. An animal’s body interferes with some senses, so they may need to be, or at least be more effective, on the surface and not be covered by protection. Locations that don’t get attacked as much may be helpful. Being close to a brain or equivalent means less nerve cells are needed to transfer information, and possibly a somewhat faster reaction time (depending how long nerve connections are needed to organs that carry out the reacting.) Making sense organs movable (either the organs or body part they are attached to) can help with all of these, organs can be moved to an area of interest to collect information, moved somewhere else if protection is needed, moved to a different interesting area, and so on. It is like the typical animal head with a brain, mouth, and lots of sense organs exists for some of the previous reasons: Some animals (the bilaterals) tend to move a particular direction, catching food means moving towards it so a mouth naturally pointed this direction, the animal would find it useful to sense what was in front of it and which way to go, so sense organs were most useful towards this front end, and a cluster of sense organs meant a brain nearby made the most sense.
  
  Brains of course are where decisions get made. Which leads to:
  

At one end, we have the Clever Octopus, elephants that never forget, playful dolphin, and Clever Crows. On the other end, sponges, bryozoans, and other attached ocean filter feeders don’t think much or at all. In the middle, everyone reading this page. Intelligence is difficult to define exactly: memory, problem solving skills, tool use, learning, adaptability to new situations all play a role, and different animals considered intelligent are good at different things. Scientists can test intelligence by seeing how animals perform various tasks, such as the mouse in a maze, or escape or tool use tests.

Any multicellular life needs a way for different parts to coordinate. Even in the simplest types, cells that move together or form coordinated shapes will do better than ones that act independently even while clustered. If the life form has senses, this information has to be processed in some wary to be useful. To accomplish this, various communication methods are used. Chemical communication is the most basic, using various signaling molecules for this role, which diffuse or are pumped to appropriate receptors, cells with correct receptors receive a signal and act accordingly. Hormones are an example, in humans and many other animals.

Chemical signals could in theory produce more complex decision making and intelligence, and seem to do so in some organism like fungi and slime molds. However, animals have evolved to use more precise and faster electrical signals. These take the form of nerves most of the time. Neurons, the cell that transmits signals, have a long main tube with several branches on both ends. Signals are transmitted by ion exchange along the tube, which transmits a charge, when the end is reached the neuron releases chemicals to another connected neuron or to another type of cell that does something, in neurons these chemicals set off another electrical signal. Neurons can deliver a signal to exactly where it is needed, instead of diffusing like chemical signals, and electrical signals travel at higher speed then diffusion or most fluid flow, but the signal disappears quickly. The chemicals that communicate between neurons, called neurotransmitters, can act like chemical signals/hormones, lasting longer and diffusing over a wider area. Learning and memory are thought to come from neural connections being strengthened or weakened to form particular pathways, but the nervous system is complex and other processes may play a role. Whatever these processes, they influence how and where future nerve signals travel, which leads to adjusted information processing and decision making.

Neurons in animals with simpler systems tend to form webs, where neurons are spread somewhat evenly throughout the body. However, in animals with more neurons, they tend to get concentrated in clusters; a small number of clusters scattered in the body, a large central brain, or both. Most neurons deal with basic body functions, either sensing or control of muscle movement, with some additional body function regulation thrown in. For this reason, brain size tends to increase with body size as more neurons are needed to coordinate and control all the parts of the body. Components of the brain associated with particular roles tend to be larger if that role is more important or involved, such as areas associated with a sense an animal uses a lot of that is particularly strong, or with balance if an animals has a more unstable body shape or movement style. Mixed in with senses, movement control, and basic functions will be higher level decision making, connecting the sensory information being taken in with the resulting actions to take. As brain size increases above an "expected" brain size in a particular type of animal, there are typically more neurons to do this sort of decision making, allowing more complex decisions. More neurons also means more possible pathways to strengthen and weaken, allowing learning and memory, instead of fixed behaviors from birth. The result is generally more intelligence in larger brained animals, however, this is not a hard rule, and the exact arrangement of neurons, plus how they individually work, greatly effects how intelligent an animal gets.

Neural tissue eats up a lot of energy, in your own body about a quarter of the energy you use goes to your brain, so intelligence has to have a good payoff, an animal's environment and lifestyle benefiting in some way from adaptability, memory, and the like. Oceanic nonmoving filter feeders don’t move or make much of any decisions at all, so intelligence tends to be low. Social Animals are often more intelligent than solitary ones, as keeping track of members of their group and navigating relationships can get complicated. Animals in complex environments use intelligence to navigate them, remembering routes and figuring out patterns for where things are. Predators are thought to generally be more intelligent than herbivores, since their food sources can move and have to be tracked or searched for, plus figuring out safe and powerful ways to attack. Animals with more generalized diets are likely more intelligent, this allows them to adapt to new food sources and adjust among a greater variety.

The existence of intelligence also adds requirements for animal care. While your typical pet or zoo animal won't be fixing head injuries or designing rockets, almost all will need some sort of mental stimulation: interesting environments to explore, objects to play with, other animals to interact with, and such. This even includes some perhaps unexpected ones, like many fish, look up info on your particular animal of interest for exact details.

Almost all animals interact with members of their own species. How intensely varies. At the low end, some animals may simply happen to be clustered near each other, or occasionally meet for mating purposes. Some come together in temporary groups, such as temporary hunting packs that share a catch, schools or baitballs of fish that help avoid or scare off predators, or large mating groups that come together at one time or place. The social animals that most people think of are more permanent groups, such as herds (troops, pods, flocks, whatever name applies to your animal of choice) or large family groups. At the most extreme are insect hives, and similar groups in other types of animals. with a complex society, several castes, and lots of members cooperating on various things. Not often classified as social but representing a grouping are a few animals made of lots of individuals whose bodies act something like different organs in a larger organism, this type of group is called a colony.

Grouping up, like anything else animals do, has tradeoffs. Disadvantages include that more animals in a single space means more competition for resources. Animals must also manage interactions between each other. Advantages of grouping up are that members can work together in ways that benefit everyone in the group, in situations similar to economies of scale or public goods if you’ve learned some economics. Examples include working together to get resources they could not individually, like building things or hunting large prey if carnivores, or that one animal sensing something can communicate this to others so they all benefit. Animals can specialize, some taking different roles like different hunting positions, or some animals watching for danger while others do something else, all the way to different lifetime roles in a hive or colony.

The combination of competition for resources and cooperation can make social interaction rather complicated, so more social animals are often more intelligent than less social equivalents. Tightly organized groups like hives might act like a larger organism, over a long time interactions have evolved so that everyone coordinates without one individual being in charge. In looser groups, several methods exist to smooth social interactions. Common is hierarchy, where higher ranking members get more resources than lower ranking ones, animals will compete for positions in the hierarchy sometimes but otherwise accept a position. This can reduce fighting compared to all animals competing all the time for resources, though managing place in a hierarchy creates its own stresses.

For animals to interact, they must communicate. Communication can use any external sense an animal has, how much information can be sent depends on how finely the receiving animal can sense, and how much variety can be produced or detected. Scent glands/pheromone glands produce chemicals, these can last awhile to communicate across time. Different chemical can have specific effects, but producing a great variety is difficult. Body position, sometimes color, and light displays are used for visual communication. Body position, for animals that can move, has a good variety and uses existing structures, so postures, dances, and other such displays are extremely common. Further structures often evolve for such display purposes, such as otherwise odd looking crests and horns. Light producing organs use light producing chemical reactions to glow, often using specific organs, color changes adjust the amount of shape of pigment containing structures. These require specialized structures, so aren’t as common, the difficulty of making these structures often means less variety. Lots of movements make sounds, more controlled sound can be produced by vibrating membranes, or vibrating air or fluid pockets, the shape and materials in these structures affect which sound frequencies are produced, multiple frequencies can be produced by changing the structure. Sound travels a long distance and is useful for long distance, fast communication. Touch is used with rubbing or nuzzling, though animals often have to have a lot of trust to let others get up close, and it isn’t useful for communicating with lots of other individuals at once.

Cross species social interaction is somewhat common. Most well known to readers are human and domesticated animals, but different nonhuman animals will sometimes join together. Examples are some multispecies herds, or occasionally different types of animals teaming up to hunt. Animals must be able to follow each other's communication, which will make this process less common, but combining different capabilities, or different requirements, is beneficial. Multispecies herds, for example, might have some species as better lookouts and some as better fighters, and different food requirements reduce competition.

Reproduction is one of the defining features of life, animals are no exception. Like other multicellular life, animals reproduce by creating a smaller, separate body that grows into a full grown organism. A few can reproduce asexually, one animal producing offspring on its own, almost all reproduce sexually, combining genes from two parents in each offspring. Thus an entire collection of Sex Tropes. Sexual reproduction allows more mixing of genetic material, creating more variety in offspring, though the two sets of genetic material need some way to meet, which involves risk and energy.

Sexually reproducing animals start from a single cell, created by merging a cell from each parent. The merged cell divides to form a mass of cells, this cluster grows large enough to form a small body, portions of which specialize into different organs. Chemical signals control how this specialization goes, based on the location of cells within the structure, ultimately the whole process is controlled genetically in some nonobvious ways, there are often not specific genes for, say, a tentacle or lung, but combinations that produce a particular pattern. Mutations in these genes are a common way to evolve unusual new body types. At some point, the body is large enough and has the correct organs to live as an independent organism.

In the simplest form of reproduction, reproductive cells are released by parents, randomly meet, carry enough starting nutrients to form a small, independent offspring, and that offspring than goes on to live with no further assistance from the parents. This type of reproduction has the advantage of the parents needing little energy per reproductive cell, they can produce a lot, some of which will survive. Over time, reproductive cells have specialized, some are larger and carry nutrients to start growth, these are eggs. Others are smaller and cal usually move faster, these are sperm. In animals that specialize in producing one or the other, egg producers are females, sperm producers are males, you are probably aware of other differences that have come from this division in your favorite animals of choice. Producing both is an option evolved in some animals, it requires these animals to maintain organs to produce both but adds flexibility.

Some animals invest a bit more energy into each offspring, these can’t throw out as many reproductive cells, but the ones they do release have a better chance of surviving. Eggs can carry more nutrients, often the egg cell is in a larger structures such as the chicken eggs we eat, that carries lots of starter nutrients for a good sized offspring. Parents can guard eggs, maybe burying them, maybe watching them or carrying the eggs, or dropping the egg structure and growing offspring inside their own body. When the offspring hatches or is born, parents may further protect and take care of it for some time, thus many Parenting Tropes. In species with separate males and females, differences between the two are heavily influenced by how much effort each puts into their offspring: Similar amounts of energy result in more similar behaving sexes and behaviors (pair bonding, quick mating for both, etc.), larger differences in effort lead to the opposite (harem equivalents, eating the partner, different sized genders, etc.)

Simply spitting out reproductive cells on the chance some will meet works for some animals, but most have to meet up close in some way to bring sperm and eggs together, a.k.a. mating (...Sex Tropes). This also allows animals to choose who to reproduce with, though in some sparsely populated areas animals will go with whoever they meet first as otherwise they might not reproduce at all, At minimum, the animals can stay near each other which makes sure sperm and eggs are close as well and can meet, animals can do internal fertilization where one type of cell (usually the sperm) goes into the other’s body, using a range of different methods in different animals. In between options exist, where sperm is given to the receiver, who puts it on their own eggs or into their own body. These methods are important for land animals, without water reproductive cells can’t move freely to meet.

Sex and dating are of course important to most of you, so how animals choose mates gets a good amount of attention in media, often in rather questionable ways. Stereotypically (at least from what the original author of this paragraph has seen) it is often about competition and dominance, fighting to find mates with “good genes”, meaning genes with those same things. If you stumble into these, remember that animals have an enormous variety, including an enormous variety of mating and reproduction methods, which all works reasonably well in particular circumstances ^note  In a somewhat commonly known example, a lizard has three types of males using three mating methods, one male with lots of mates, pair bonding, and males that mate quickly and don’t stick around. This gets repeated somewhat often because of a rock paper scissors relationship when different types of these males compete with each other , and that “good genes” simply mean genes which are more likely to survive and reproduce, involving lots of tradeoffs and a lot of Boring, but Practical advantages that won’t necessarily show up in stereotypical social competition. And a lot of pop science writing applied to human society or human relationships is a crap source for information.

Since there is so much variety, summing up all possible interactions isn’t going to happen in a single paragraph (just about all Sex Tropes or Love Tropes that can exist in animals have been seen at some point. Including things like rape or homosexuality or killing other's offspring.), so here are a few patterns for animals, though as always remember there are plenty of exceptions and oddities. The stereotype is that, if an animal can choose mates, they’ll pick based on what partners are available, going for ones with some sort of survival advantage (though what this might be depends on the animal). However, lots of animals will choose partners for things with no immediate benefit for survival (many bird songs, colorful crests, dances or other displays, etc.), there are a few functions these might serve. One is communication, to tell potential partners that a mating partner is in the area and available. Another might be as a demonstration of other good qualities, displays/sounds/smells/etc. can show that an animal is otherwise in good enough shape to pull it off, presumably the ability to tell which animals are otherwise healthier/in better shape/etc. compensates for the cost of an otherwise useless behavior or structure (though so called “honest signalling” often gets exaggerated or applied questionably in above pop science writing, so be very careful when seeing it argued. This also applies to other fields like economics where choices are examined). Also possible is random chance, if animals evolve something mating related, and an attraction to this thing, the mating structure or behavior can become what reproduces in a runaway, hard to change process even if it isn’t as efficient.

Behaviors around mating, such as how many partners to have, how much time to stay attached to one, whether to enjoy a Mantis Mating Meal, etc. are closely tied into an animal’s social structure and what parenting the animal does, these usually evolve together. Mating Season Mayhem, having a time and place where big groups come together to find mating partners, has the advantage of making a bigger variety of partners easier to find (similar to having a market for buyers and sellers to come together in human economics), and particular times and places might provide better conditions for raising offspring, but does require animals to be able to reach this location. More partners in a short time increases the genetic variety of offspring, it also increases the number of offspring if the animal puts little energy into growing and raising them afterwards (If an animal puts more energy into raising offspring, the limit becomes how many it can feed and raise rather than amount of mating, though variety can still be useful.). Reproduction opportunities are of course valuable, something a lot of animals fight over, different animals have developed a number of ways to compete, or stopping others from accessing a mate once found (Mate Guarding is a term often used), or otherwise restricting competitor's ability to reproduce (like the killing of offspring, or how hive insects use various methods to ensure only particular queens exist in a hive). In social animals, the competition between these goals can produce some interesting interactions, read more about the particular animal in question if you see an interesting one.

Two animals have reproduced, a new creature is hatched, born, or otherwise come into existence. Now it has to grow to adult size. Growth rates vary from animal to animal, as a general trend, animals with faster metabolism will likely grow faster. Food availability is also important, the more food, the more an animal can eat, the more it can grow. Still, there is wide variation. Animals with parental care will be fed or otherwise taken care of for part of this time, animals in social groups are often cared for by members of the group.

Some animals are born like miniature adults, some go through changes. The most extreme of these changes are a process called metamorphosis, where the animal goes from a larval form to a completely different body shape. Caterpillar to butterfly is the most well known example, other insects, some bony fish, starfish, amd a number of more obscure animals do something similar, including a number of attached, sessile animals that have swimming larvae before changing to settle down. In between are creatures like frogs with tadpoles or young wingless dragonflies, which noticeably change but still kee[ big elements of their bodies from young to adult forms. Being hatched or born adult like obviously requires less change over time, saving some energy and requiring less genetics or signaling for development. However, changing over time allows different forms to specialize into different roles, or do things when young not necessary for adults. Insect larvae often specialize to eat and don’t use energy on reproductive organs, sessile nimal’s larvae can swim to find new locations, before attaching somewhere as an adult. Among many animals, metamorphosing or not, adults and young eat different foods and possibly live in different places, reducing competition with themselves.

At the other end of the lifecycle, animals face a lot of deadly threats, plus aging in many. Lots of deaths are of course caused by diseases, attacks, accidents, extreme weather or lack of food or other bad luck, and other such things, animals in human controlled environments can almost always outlive animals in the wild. However, some internal controls do seem to limit how long animals live even if nothing external kills them, though mechanisms of aging aren’t well understood. Some animals die after reproducing, cephalopods and some insects are the most famous, the tradeoff here is likely that putting energy into reproduction instead of surviving gives more offspring, which compensates for the original parents dying.

Disease and injuries are constant threats, they can kill directly, or weaken an animal to where it dies some other way. Animals have a number of ways to survive and heal from them. Protective systems like skin or acidic or alkaline stomach stop many microbes and parasites from entering in the first place, usually either with barriers that physically stop an infector from getting in, or chemicals that poison or destroy it. Large enough parasites can be physically removed, as with horse or cow tails that can swat flies, or birds and whales rubbing themselves in dirt to clean off skin parasites. Some animals specialize in cleaning parasites from other species, usually behavior evolves so the animal being cleaned allows this to happen.

If microbes get into a body, animals have further defenses. Some chemicals inside a body can harm microbes. Specialized cells can eat them, other cells can attack infected existing cells, which are part of a virus reproductive cycle. Vertebrate have additional components to this system, that can adapt to specific invaders and more effectively target them, cleaning up the disease agent much faster if exposed again. This is the basis for vaccines, which expose the body to something harmless that activates the immune system as a disease would, so you catch the actual disease your body fends it off far faster. These disease fighting systems also attack cancer cells, some smaller parasites, and other random things that may enter the body. To tell an outside body from a part of the animal, animal cells have signaling molecules on their surfaces that invading disease microbes usually lack, immune cells can also respond to common molecules found on disease causing microbes to detect them.

To heal from injuries, an animal needs to remove damaged tissue if it can’t be repaired, and regrow whatever was missing. Doing this takes nutrients and energy, and some body parts on some animals can’t be fully healed if damaged. To remove unrecoverable tissue, some will be rubbed off, sometimes regrowing tissue will push it away, immune cells might digest some if the tissue comes in small enough pieces. Regrowing tissues means lots of cell division, with chemical signals to tell what type of cells should grow where, and chemicals such as bone or shell are secreted as needed. The process is about the same as normal growth, since regrowing what is damaged is what the animal is doing.

If an animal isn’t killed by an attack, accident, or infection, or other bad events, a few patterns have been found for how long they can live. Larger species tend to live longer, though smaller animals within a species tend to live longer also. Animals that face fewer threats/have some protection, like flying or shelled ones, tend to live longer, likely the benefits of reduced aging matter less when something else has a good chance of killing it anyway. Slower metabolism animals can live longer, though this isn’t as consistent a trend, it seems the lowered speed of chemical reactions means those associated with aging also happen slower. Some cnidarians are unique in switching between two forms, each of which effectively rebirths the animals, these theoretically can live forever this way. Some others don’t seem to age at all, though they still suffer other causes of death. The longest lived animals as far as we know are some several thousand year old sponges and corals, out of larger animals some deep sea sharks may be the longest lived.

Several tabs up described how an environment affects the animal, but how does an animal affect its environment? What happens if you place one in an environment or take one away? (Careful about doing this in Real Life, ecology can get quite complicated and weird things happen.)

Obviously, everything an animal does affects its surroundings in some way, large or small, but there are a few big categories of ways animals influence things around them. Most of these also apply to other lifeforms you might add, but this is useful notes animals, so animals will be the ones talked about.

One way is to directly physically change its surroundings. Burrowing, building, knocking things down, moving objects around, that sort of thing. In extreme examples, animals can create whole new environments, such as coral (and occasionally other animals) building reefs or beavers creating lakes by damming rivers. Another less extreme but important examples are burrowers mixing and aerating soil/seabed sediment, which can change nutrients available to plants or algae, and allows different living things (including microbes, fungi, and such) to use the soil or seabed.

The second big effect on surroundings is through eating. Animals have to eat things, and can be eaten by other things (not just predators, also parasites, somewhat scavengers as well.), making what they eat less common and what feeds on them more common. Animals that can learn will also change their behavior in response (hunting differently. Changing activities to avoid predators or remove a new parasite, that sort of thing.) Sounds simply enough, but the knock on effects when food and feeders themselves eat things and are eaten by things, and those organisms are eaten by and eat other things…..eating behavior is a powerful way that animals can influence their surroundings. If an organism like trees or coral that is the basis for the environment is part of its food connections, an animal might be vitally important=ant for maintaining an environment (often happens with predators of things that eat important plants or similar), or might cause a collapse if too common or rare.

Larger scale and longer term, sound counterintuitive effects occur as eaters and eaten adapt to each other. The existence of creatures that eat a certain type of lifeform tend to increase that type’s diversity, both by reducing competition for resources which allows room for other living things to use them, and by forcing adaptation, as an eater can’t optimize as well for a variety of types of food as for a single type. Longer term, life forms eating other life forms are a big driver of evolution, as features useful for defense or getting food now have value where they would be wastes of energy and nutrients otherwise. It is thought, for example, that the evolution of good senses, hard body parts, faster movement, among other features were driven by the appearance of predators of animals back in prehistory. These features have proven useful for other things once evolved, and make up a big component of animal diversity today.

A third big animal influence in this useful notes is by moving things around. Animal’s ability move makes them much more important in this role then most other organisms (or moving microbes which can’t move that quickly). Within an environment, animals might help spread things out from a single source, such as moving pollen and seeds around which helps some types of plants reproduce. Over a larger scale, migrating animals might move diseases from place to place, or move nutrients if they tend ot eat on one place while pooping, peeing, and/or dying in another (leaving a decomposing body in the new place). A spectacular example of the last is Whale Fall, where corpses of whales which tend to live near the surface might fall to the ocean floor, feeding large numbers of many kinds of animals from scavenging the carcass, on an otherwise nutrient poor ocean floor.

A final effect is on interactions with living things that aren’t eating. This could include harmful interactions (parasites technically means this, this useful ntes has mostly used to narrower sense of animals that eat from another without killing it, like bloodsuckers.) or helpful interactions. Obviously, removing or adding an animal on one side of an interaction effects those on other other. These could include cross animal social cooperation, competition between different types of animals for things like territory, and a range of behaviors that are hard to summarize here.

How important an animal is depends on how much overlap its activities have with other animals (same with other lifeforms), and how large an effect its activities have. No two animals in an area will do exactly the same thing for long, in these situations one will outcompete another, but there can be varying amounts of overlap (such as two animals that eat similar food, but are active at different times, or have different preferences of both types are available, vs. animals that might occasionally eat the same thing but mostly have different food sources). Extremely important animals can be called keystone species, removing them destroys or greatly degrades the environment they were in, some examples include major predators, including ones who’s prey eat the major plants or other organisms making up an environment, or environment builders described above, or on the invasive species side, an animal that can eat something that previously had no threats. By contrast, an animal that eats food also covered by lots of other animals, that is also not all that common, and does no building or burrowing or similar won’t have much influence.

Horses, Cows, Dogs, and many more…farm animals, woprk animals, pets, people use animals in a lot of ways. Other intelligent species in fiction likely would as well, if they are similar to people. Animals can do a lot of things better than people, but being able to regularly work with them takes some skill, and certain types of animals are better for their roles than others. Breeding further specializes animals for particular roles, because people do a lot of things (protection, often feeding, navigation, medical care among them) for animals that wild animals would have to do themselves or go without, human bred animals can get a lot weirder and have traits wild animals would have a hard time surviving with, to better fill intended roles.

Food animals, whether for meat or for products like milk, eggs, or weirder ones like blood (yes, really), are useful because of differing nutritional requirements between animals and people. Humans are flexible eaters (though other intelligent species may be different, do what you like in your own stories), but some foods are hard to get value out of, like grasses, leaves, or various waste products. Animals that use these food sources efficiently can convert the energy and nutrients in them into something people can use (Ancients who first used these animals wouldn’t have known the chemistry, but would have noticed the overall effect.) Animals can also rebalance nutrients in a way more fit for humans, mainly converting carbohydrate heavy plamt diets into something more protein rich. For a food source to be useful, it should mostly eat less valuable food (So grass or leaf eaters make good livestock, meat eaters poor livestock), grow quickly (which means more of what it eats goes into food products vs. energy for living, plus faster response when increasing or decreasing herd sizes), and provide a larger fraction of edible material vs. bones or cartilage or similar. Fished or hunted animals can be a bit more flexible, but in practice animals with the above characteristics will be more common, and also easier to switch to farming if needed. In general, products like milk or eggs are usually a more efficient use of food, they can be produced continuously, allow the animal to be used for other things, and end up overall using less feed, but meat has a different nutrition mix and does taste pretty good…

Other animal products, like wool or leather, or manure (for fertilizer, possibly some chemicals), can be treated similarly. Animals often supply multiple products, all of which are worth taking into account to know the value of an animal. Good use of seemingly useless or less valuable parts are a good skill to have.

Work animals (draft animals obviouss, but also riding, specialized hunting or herding or other job animals, and arguably pets) are useful for doing things people find difficult. Other animals often have a different mix of senses that can be useful (better smell in many animals is a common one to take advantage of), are physically stranger (draft animals), faster running (riding horses, gods for some roles), or come with particular skills (hunting cats and dogs that catch pests, lab animals that can be fast bred and easily experimented with). And of course, companionship (pets). The same food concerns apply to work animals as food animals, though less extreme, work animals often have enough value from what they do that a more expensive diet is fine, and a longer life is often beneficial to spend more time doing useful work, slower growth is more tolerable as a result.

How tightly humans control animals has a range. Some animals are hunted or fished with no people interaction outside of this. Some animals are controlled but still the same as wild versions, this is how most fish farming works, little interaction with people occurs for these. For roles that need more interaction, some wild animals act in a way that is friendly to people. Original domestic cats are thought to be an example, descended from wildcats that got along with people and were tolerated for killing vermin until they developed into the pets we know today. Animals that can learn might be tamed, where a captured animals is taught in various ways how to behave around people, taming will be common in area with a good supply of the desired animal in the wild. Full domestication takes lots of breeding, certain traits that allow living with people are enhanced, and the animal becomes different to its wild relative and adapted to living with people. “Feral” animals occur when human controlled ones escape, various animals have differing abilities to survive on their own in different environments.

Which animals can be fully domesticated can be somewhat random. Horses, for example, obviously can, closely related zebras have not seen successful domestication. A mix of how aggressive the animal is and how social they act (meaning people can use the animal’s social tendencies to get desired results) play into it, but other factors are not known. Similar communication, including body language, helps.

Animals bring their own issues in addition to the food, care, and attention needed to use them. They cause pollution, such as bodily wastes. Important today is methane, a major greenhouse gas produced by a lot of livestock. Animals can obviously wander off by themselves, leading to various legal issues/disputes for a society to address. Many diseases come from animals, living around them can make disease transfers more likely. Feral/stray animals can become problematic invasive species, cats and pigs in particular are well known for this.
If you are making up a domesticated or tamed animal for some intelligent being to use, you'll probably do perfectly fine by borrowing from existing human used animals, but here are some pointers anyway which might help. Obviously, an animal should be able to fill its roles well: a draft animal should be physically strong and preferably able to work for a long time, a hunting animal should have some senses, physical skills, or other abilities the intelligent species lacks, animals grown for products should produce lots of that product, and it should be something unique that can't be easily produced elsewhere. You'll figure out something plausible by looking at existing animals, and/or by understanding the animal body functions in general (Endurance for draft animals probably means a consistently high metabolism, for example, and breathing and circulation which can supply oxygen for a long time). Animals should have the right personality and/or ability to be trained, kept, and/or controlled. Animal breeding and diets aren't often as thought about, but inventing animals which breed quickly if needed and can eat foods the intelligent species can easily get are plausible ways to go. Other then that, imagine away.

Sponges on Tree of Life project

Non-moving filter feeders, these are probably the closest to the earliest animals. Sponges have a similar form, a water channel or series of channels on the inside, lined by a layer of tissue, that coats a layer of structural material of some sort, followed by an outer lining. The inner layer is lined by cells with cilia, the cilia pull water through the inner channel, where particles and microbes are absorbed as food. The cells in these layers can transform into each other. unlike most other animals where cells become fixed into a role once specialized. More complex sponges have lots of small channels, giving more surface area for the cilia to pump water and absorb nutrients. A few use webbing to capture prey, though exactly how this works is not known. Many others have symbiotic algae as a food source. Sponges use a number of structural materials for support, some use an elastic protein called spongin similar to collagen, some use minerals, many have a mix of the two. Many sponge larvae are swimming, using cilia, others drift, they lack specialized organs just like the adult forms. Most sponges capture sperm inside themselves and grow larva there, rather then releasing eggs to be fertilized in the open as many other similar animals do, they can also reproduce by budding or regrowing from fragments.

If you are wondering whether sponges the animal relate to sponges the cleaning tool, yes, they do. Spongin is elastic, and the numerous water channels and holes in some sponges are excellent for holding water, combine them and you get the soft, water soaking thing found in kitchens and cleaning equipment. Sponges were used so much for this that they were overfished in some areas, synthetic materials got rid of this problem and are the main material for cleaning sponges today.
Sponge groups:

• Calcareous Sponges: These use Calcite as a skeleton, they have a wide variety of shapes.
• Glass Sponges/Hexactinellids: They use silica as a skeleton, and lack spongin. Tend to be found in deeper areas or polar regions. Have an unusual cell structure, where internal components of several cells combine within a single membrane.
• Demo sponges: The vast bulk of sponge species. Silica and/or spongin for support, many different shapes, found in most water environments.

Comb Jelly family Tree

You’ve probably seen these if you’ve watched deep sea documentaries: hollow, transparent swimming animals with curved rows of waving hairs on the side: they often fill the role of ‘generic swimming animal” shown for a quick shot but not commented on. The main body itself is quite small, with a simple mouth and digestive system, the large cluster of cilia on the sides moves the animal and helps funnel food in, mostly small plankton. The tissue of these animals is jellyfish like, held together by a similar material.

The transparent cilia carriers on comb jellies can have a number of shapes: lobes, ovals, twisted, or squash shapes are some examples. Comb Jellies have a variety of reproduction methods, but all produce adult like young. They have been found throughout oceans, but not in freshwater or land environments.

Cnidarian family tree

Coral, Sea Anemones, Jellyfish, and some others, plus a very odd group called Myxozoa described below. Cnidarians (apart from Myxozoa) have an unusual reproduction system, cycling between a fixed in place form and a free floating or swimming form; a few species can in theory live forever by changing between these forms. Cnidarians are radially symmetric, looking the same if the animal is rotated around an axis, and have a digestive system where food goes in and waste goes out through the same opening, many have some sort of tentacles or arms utilized in capturing prey. Cnidarians are also characterized by the possession of cnidocytes, cells that are shaped like tiny harpoons that serve to inject venom into their prey.

Unlike other animal groups in this useful notes, the common names of cnidarians (jellyfish, coral, hydras, etc.) are found in multiple evolutionary branches, so the below categories don't correspond exactly to official scientific groups.

Many Cnidarians have an unusual reproductive cycle, which switches between an attached form (called a polyp, think the standard Sea Anemone or coral shape) and a floating or swimming form (called a Medusa, think the standard jellyfish image)), where neither of these is an obvious "adult" form. Many can also reproduce asexually, or the more typical eggs + sperm forms larva method.

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Jellyfish

Tropes:
Electric Jellyfish

Used often as background animals swimming in an ocean, or documentaries as a generic ‘simple animal”. Jellyfish are not any particular branch of cnidarian, instead are found in several groups. The tentacles sting things, and draw food towards the jellyfish mouth where it is eaten. Their pulsing swimming motion takes advantage of water turbulence, helping the cup structure relax. While the swimming Medusa form is far better known, Jellyfish do use the full Medusa-Polyp cycle described above, a polyp forms from sperm and eggs, grown on the sea bed, and produces several circular layers that split to form an individual Medusa apiece.

Jellyfish stings: The sting actually injects a structure into the victim, this structure releases poison. To get rid of the poison, the best way is washing with seawater..yes, a sea creature’s sting is solved with seawater. This washes off the jellyfish structures without popping them as freshwater might.

These days, with lots of ocean creatures dying off for various reasons (pollution, temperature, acidification, overfishing) jellyfish have been filling in the gaps. Not too economically useful, and can cause further problems if jellyfish eat too much larvae of other creatures. Though many sea turtles eat the things, they too are negatively impacted by the ocean's changing climate and pollution, so may not be enough to curb the jellyfish.
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Hydras

Smaller then jellyfish and without as long of tentacles, these are the closest thing to a "basic cnidarian" that exists. Some could theoretically like forever be switching between Medusa and polyp form.

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Siphonophores

Tropes:
The Worm That Walks

Also not a specific branch of cnidarian. Siphonophores form big colonies of specialized animals, in a vaguely similar way as cells form multicellular life, and live much like jellyfish, eating and often stinging what they can catch.

Among their number are some of the few really huge non-vertebrate animals. One of them is longer than a blue whale, though not as heavy and technically made of lots of animals.

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Sea Anemones

Both of these types of animals attach to the sea bed. Anemones are predators, stinging their prey with tentacles before before eating it, some also contain symbiotic algae. Anemones are more like other animals then cnidarians in simply having a larva then adult form, rather then the two stages of Medusa and polyp as described above.

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Sea Pens

Filter feeders. Some look something like feather pens, others look like generic sponges. They are found in a lot of water environments, more often in deeper water. Like Anemones, they have larvae and an adult form. Coral

Makers of reefs, with lots of beautiful, colored shapes where lots of other life makes its home. These shapes are actually rocky structures built by coral, the actual animals are small bodies not too different from sea anemones, with tentacles surrounding a mouth. Coral feed on a mix of symbiotic algae and catching whatever floats by.

The reefs are much more known than the animals themselves, creating fantastically productive ocean systems that produce a lot with often limited nutrients, and act as breeding grounds for many animals elsewhere in the ocean. Not surprisingly, these are threatened by pollution, temperature changes, and acidification, a common effect is “bleaching” where coral lose their algae, go white, and die, causing problems for everything else living nearby.

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Myxozoa

Vastly different in shape to other animals, their place within life was not known until genetic analysis showed them to have evolved from Cnidarians. These are extremely small parasites, with very simplified organ structures, some even have simplified cell structure compared to other animals. They do keep the stinging cell cells that other cnidarians have, though repurposed for other functions. Reproduction is by spores

Placozoan tree Placozoan relationships to other animals are not worked out, this project picked one possibility, but a different position on the animal tree is easily possibile.

The closest thing to a Blob Monster, though much smaller then usually seen in movies. They were discovered only around 1880, and are very small (a few mm across), as a result not much is known about them. Placozoans are pancake shaped, with a few types of cells organized into layers. They use cilia to move, and eat by engulfing food on one side of the pancake shape, then excreting enzymes to digest their food, which appears to be microbes and other debris. The structure is consistent enough that only genetic testing tells species apart. Poisons have been found for protection from predators. They mostly reproduce by budding, but can perform sexual reproduction occasionally if conditions are stressed.

As mentioned, the exact relationship of placozoans to other animals groups is not well figured out. Their extreme simplicity suggests an early diverging, primitive type, their cell layers resemble a simplified version of comb jellies or cnidarians. Genetic testing sometimes suggests a close relationship to cnidarians or bilaterals, the exact relationships are still not nailed down even in these analyses. Obviously they have lost complexity if this is the case.

Xenoaceolomorph subgroups

This is a small group of flatworm like animals. They were originally classified as flatworms, which were assumed to be early bilaterals. However, genetic evidence points to flatworms and this group actually having separate ancestry, It is thought that this group diverged from other bilterals early on, but some evidence points to them being related to echinoderms instead. Al of them live in salty or brackish water.

They lack a lot of internal organs and are generally flattened top to bottom. They are missing some internal organs found in other bilaterals, such as a body wall or brains, some lack a standard digestive system. Eating is split between eaters or small things and parasites, mostly living near the bottom of whatever water they are in. Reproduction is sexual, all members are hermaphrodites.

Family tree

Roundworms and lots of other little tiny things, including lots of parasites. You don’t notice them day to day unless you or a pet gets infected, but they are one of the most common animals in the biosphere, found in all environments, including on you. Some estimates suggest they have about a million species, but scientists have only discovered a few tens of thousands. Probably the most unusual environment is in the deepest mine in the world, where some nematodes eat bacteria living on the rock. It is thought they descend from worms introduced when people first dug the mine. Nematode diets are pretty much anything small they can get their hands on: bacteria, small particles like skin flakes, decomposing material, bits of flesh and body nutrients as parasites, etc.

As the name roundworm suggests, these are pretty simple animals, a worm that is round, with a standard gut and head, with sometimes an organ for sucking liquids. Nematode mouths are somewhere between a Flower Mouth and a regular jaw, with several components that open and close together.

Technically, nematodes are part of a bigger group, the nematoida, which includes several similar worms.

This group of animals is extremely recently discovered, specimens were only fully captured and analyzed in the 1970s and 1980s. They are extremely small, and live by attaching themselves to the sea bottom, hiding in crevices, and feeding on debris. The animals are named for an outer shell called a lorica, which has one big opening for the head. These organism have most of the standard internal organs, including brains, but do not have blood circulation. They have typical male and female reproduction, with larvae that mostly are kind of like adults, but swimming to find a location to settle.

Evolutionary tree

More worms, living in the ocean, eating random bits of stuff. Mainly included because of their name, meaning “Penis worm”. (Same base as “Priapism”.) This both references their general shape, and a particular organ.

The Kinorhyncha, close relatives, live in mud instead, and eat random bits of stuff there. These are more obviously segmented, but still wormlike.

Both these animals have an outer hardened cuticle, which has to be molted.

Velvet Worm tree

Despite the name, these look something like caterpillars, and unusually are fully land based, though must have water based ancestors as their relationship to arthropods seems to go back to when these were water living. Instead of legs, they are supported by cones of their body extending to the ground on which they wriggle. These lower portions are hardened, while the rest of the skin is softer.

They are mostly carnivores, ambushing prey, and some show herdlike behavior, defending regions and developing a dominance hierarchy.

Tardigrade Tree

Also called water bears. They look something like an armadillo bug if such a thing existed, with several feet attached to shelled segments bigger near the middle and smaller at the head and end. And they are very, very microscopic. Mostly they eat microbes and plants, a few eat fellow tardigrades.

Their main claim to fame is surviving all sorts of nasty stuff: high and low temperatures, drying, outer space, quantum entanglement, among others. A simple genome helps with this, the animals also are able to enter a dormant state that can tolerate these conditions and restart under better conditions. They also have a number of proteins that preserve cellular structure.

Tropes:
Big Creepy-Crawlies

In many ways, the stars of the animal show, living in almost all environments, by far the most species both discovered and estimated (even leaving out insects, they still have the most known species of any animal group), filling almost all possible ways of eating and living. Prehistorically, members of this group were the first animals onto land and into the air.

Arthropods are covered in a hard exoskeleton formed from a material called chitin, sometgimes with minerals mixed in, and form in segments, you can see these clearly in a lobster shell or on centipedes or millipedes. In most creatures the segments have evolved to merge together into a few large body sections, as in three part insects or two part spiders. This exoskeleton commonly has appendages, forming legs, arms, claws, antennas, or wings based on the creature. This skeleton provides protection and support, but for whatever reason never evolved to grow with the animal, requiring moulting. The danger during moulting and slowed growth limit arthropod's modern size. Compound eyes are common, with lots of different lenses adding up to a single picture, but a number of eye structures exist in the group.

With such a large, well known group, it's worth writing up arthropod subgroups as their own sections.

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Chelicerates

Chelicerate Tree
Tropes:
Arachnid Tropes
Scorpions

Arachnids and horseshoe crabs, named after their mouthparts. Arachnids include spiders, scorpions, and ticks, among others. Prehistorically, these may have been the first animals onto land, legs and organs easily adapted to book lungs made the transition relatively easy. Today, horseshoe crabs still make this transition, going on on a beach to mate and lay eggs, while arachnids mostly live on land permanently.

Chelicarates have 2 body segments, scientists call these a cephalothorax and Prosthoma. The chelicarae that they are named after can take a number of different forms. Many of these animals use book lungs, a breathing system where flat, body fluid containing structures are stacked together and surrounded by air.

Some groups of Chelicarates:

• Horseshoe Crabs: Live in water, eating particles on the bottom. They have a very distinctive appearance, with a large round shell in front, a smaller rear section, and tail at the back which can be used to flip back over if needed. Often said to resemble the earliest arthropods, haven't changed much in a long history.
• Ticks and Mites: There are two groups of these which may or may not be close relatives: acariforms and parasitiforms. Ticks are famously parasites that can spread a number of diseases, Mites are more diverse and have a variety of eating habits, including herbivores, detritovores, and some predators. These groups are small with a dome shaped shell covering most of their body. There may be a lot more mite species then currently known, as they are small and not easy to find for study.
• Scorpions: The useful notes link above has more information. Carnivorous, somewhat long bodies, large claws and stingers, the last two in various combination are how they catch prey. Most live in deserts, most prefer nighttime activities. Young are taken care of somewhat, until they are strong enough to hunt on their own.
• False scorpions: Closely related to scorpions and resembling them apart from lacking a stinger tail. Some can spin webs.
• Spiders: The 8 legged, webspinning things we all know and love. Or maybe find creepy. Spiders are all carnivorous, eating small animals. Not all spin webs, but all use silk in some way. Unusually, legs ar3e moved using fluid pressure instead of direct muscle action. Some spiders breathe with insect style tubes that diffuse air into the body, instead of book lungs of most chelicerates.
• Harvestmen: Daddy long legs are in this group. Look like spiders, but don’t produce silk and have a more flexible diet, mixing in plants and detritus with meat.
• Sea Spiders: Live in water as the name suggests. These are unusual in having no large abdomen like structure, instead having a thin central body that often looks like a tube or extension of the legs.
• Whip Scorpions, Whip spiders, and others: Lots of smaller chelicarate groups look like scorpions or spiders or something in between, but with some variation (diet, limb structure, etc.)

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Myriapods

Myriadpod Tree
Tropes:
Creepy Centipedes

Centipedes and millipedes, plus a couple of less well known groups. Arthropods evolved from ancestors with lots of segments, this group preserves the large amount of such segments most obviously, though myriapods actually grow by adding segments over time instead of starting with all of them. Myriapods use tubes in their bodies to breathe, similar to instects, tube entrances are near their legs. A few lack eyes.

Myriapod Groups:

• Millipedes: Plant eaters mostly. 2 legs per segment (Each segment forms from the merger of 2 segments which had the expected pair of legs).
• Centipedes: Mostly predators, some have poisonous bites.
• Pauropods: Far fewer leg pairs then most centipedes (9 is most common), these mostly eat fungi, occasionally roots or other debris, found underground on in leaf litter.
• Symphylans: Also shorter then centipedes or millipedes (around 16 segments). Mostly eat detritus and plant debris. Lack eyes.

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Crustaceans

Crustacean Tree
Tropes:
Crustacean Index
Crabs, Lobsters, and shrimp resembling the ones you eat are the stereotypical members, but crustaceans are a much more varied group then this (Decapods, the group with said members, are between 1/4 and 1/5 of crustacean species.) The rest include lots of shrimp and clam looking organisms, living a wide variety of lifestyles in the ocean. Crustaceans are quite likely the most common form of animal on earth by total mass, as two groups (Copepods and Krill) make up most zooplankton in the ocean, larger crustaceans are also relatively important middle steps in ocean food webs. A few members live on land, the wood louse might be the most famous of these (well, and insects, but this section is on the more common meaning of the group.), some can also breathe on land and water and switch between the two for various purposes.

Crustacean bodies typically have small number of segments, the exact amount varies from group to group. Almost all have legs, which might be shaped like fins or help swim, regular legs for walking, or a mix for multiple ways of moving. Swimming might also use a flattened tail. While cooked shrimp are curled, most bodies are straight with segments, looking like short millipedes, a few use single smooth shells instead which look more clamlike.

Crustacean groups:

• Tantulocaridians: Small parasites that mostly attack other crustaceans. Unusual life cycle which has the larva attach to a host, extracting body fluids, then become either a parthogenic attached adult or free swimming gendered adult.
• Oligostraca: Another group of very small animals. Includes some parasites, plus some sediment dwelling animals. One group of parasites are tongue worms: these lack a hard shell, but grow by moulting, this and DNA evidence points to them being crustacean descendents.
• Thecostraca: Barnacles and relatives. Free swimming as larvae, then attach tightly to something as an adult, grow a hard shell, and live off filter feeding. Some of the “relatives” live off of body fluids. Removing barnacles is very important for larger sea creatures and for human built ships, the creatures add weight and drag, greatly slowing down whatever is carrying them if they attach to something that moves through water.
• Malacostraca: Includes the decapod group described above, plus krill, and many other kinds of shrimp besides. Also includes some land dwelling members, such as the wood louse, many members are also able to breathe air and walk on land, using hardened gills or book lungs to do so, though most only do this close to the sea shore without long term living on land. Wide variety of forms, so if you want ideas for a standard crustacean like creature, this is the group you'll be looking at.
• Branchiopods: Some more kinds of shrimp. Unique in having gills on the mouthparts, in addition to internally or elsewhere on the body.
• Copepods: Very small shrimp. Very common part of plankton, the next step up past floating algae in ocean food chains.
• Remipedes: The closest relatives to insects. Long, lots of legs, can travel on land short distances.
• Hexopods, including insects: As mentioned, these evolved from crustaceans. If you know anything about animal classification, you'll know there are a lot of these, so they get a whole section to themselves just below:

Tropes:
Insect Index

By species, the biggest group of animals, more than all other animals put together by about 2-3 times at around a million. Estimated species undiscovered could be something like 5 million. Yes, bugs and bees and beetles are pretty important. They are descended from crustaceans, unfortunately their fossil record is thin so how this happened is not well known. Around 350 million years ago, ish, flight evolved in some insects, and they have been massively diversifying since that time. The extreme number of species seems a mix of small size allowing lots of specialization, fast breeding cycles allowing lots of changes, and flight, as birds and bats similarly have lots of species compared to other similar groups. Some estimates come from finding insects within species of tree, and discovering new insects at a decent clip, with new species occuring every time a new plant species is checked. Such extreme association with some other life form shows up a lot, with insects that pollinate a single species, or parasitize one particular type of animal, or burrow in a particular plant, repeat this over and over, and you get a lot of animals.

Insects follow a similar body plan. They have three main sections, a head with eyes, antennae, the biggest chunk of neurons (insects have several such clusters, the biggest is the closest to a brain in other animals), a mouth, all the usual parts, a thorax where the legs and wings attach where several internal organs hang out, and an abdomen which is usually the longest section, with more internal organs. As the name hexapod suggests, insects have three legs, running motion is usually by lifting one tripod (a front, a back on one side, a middle on opposite side) and planting it, followed by the other. Insect wings probably evolved from lost crustacean legs, which would have become lumps that could develop into full wings, though some other guesses exist. To move, the thorax near the wings is flexed and rebounds, wings can flap very, very quickly. “Physics can’t explain how insects fly” is repeated somewhat commonly: though the fluid flow is complex, the physics is understood pretty well and not out of the realm of other fluid mechanics questions. In most species, wings can be folded onto the body, taking up less space, allowing camouflage, and such. Insects breathe by letting air through tubes in their body, oxygen is absorbed and diffused into their cells. Circulation has a heart pushing body fluid through a tube, which then becomes intracellular fluid moving through the whole body until absorbed again.

If there’s a possible lifestyle that involves living on land and not being too large, some insect probably lives it. Plant eating, fruit eating, scavenging, blood sucking….forests, tundra, desert…you get the idea. They are highly important species in most ecosystems, eating, shaping the environment, and acting as a source of food for other creatures, the fact that “insectivore” is a common term should make their value as food obvious. Some lifestyles are highly unique to insects, by far the most pollinators are in this group, as are so called parasatoids, which inject larvae into a host that is gradually eaten to death as the larva grows.

Though caterpillar into butterfly is a common trope, not all insects have this sort of metamorphosis. Many of the earlier groups hatch as forms similar to the final forms, though lacking things like wings, as the animal grows and molts its body shape changes into the final form. The larvae of these types are called nymphs or naiads usually, while the full metamorphosing ones are just called larvae unless they have a special name (like maggots or caterpillars). Many insects actually spend most of their life in these larval forms, eating and growing until they change to the flying form for a quick burst of reproduction.

Most groups of insects have “opter” or “optera” or such in the name, based on the greek word for wing. (Compare helicopter or pterosaur) And now, the different types:

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Early Hexapods and non flying insects

Proturans Bristletail Tree Springtails Silverfish and Related tree
Springtails, silverfish, and a number of other random bug looking things. They have a similar body shape apart from lacking wings, though some have mouthparts inside the body. Most of these live in forests among random stuff on the ground, eating various things there, silverfish sometimes hang out in the roughly similar environment of walls and furniture. They can be a sign of the environmental health of such areas, and walking out of old furniture can be a surprise, mostly they aren’t common in media or well known.

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Paleoptera, or dragonflies and mayflies

Pterygota Tree Mayflies
Tropes:
We Are as Mayflies
Dreadful Dragonfly

In prehistoric life shows, carboniferous giant dragonflies are a common sight. With no larger animals (a.k.a. Vertebrate fliers) in the air, they could get much bigger than current flying insects. In modern times, dragonflies still catch things in the air using those well known long wings. Mayflies are known for not living that long, once their larval forms turn into adult forms, they mate and die very, very quickly.

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Zoroptera, or angel insects

Relationship Tree
Soft bodies, with a particular antenna shape. One of many groups of insects to live in leaf litter or under logs. Not too many species.

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Dermaptera, the Earwigs

Earwig Tree
Flattened insets, letting them fit in small spaces. In the wild, live under logs or leaves or such, eating random bits of plants. You may have seen them crawling out of furniture, where bits of food and the medium tight fit of cushions and furniture parts mimics their preferred environment.

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Plecoptera, the Stoneflies

Stonefly Tree
A less concentrated version of We Are as Mayflies applies to this group, most of their life is spent as larvae in water, eating random bits of plants or very small animals in lakes and streams, and only a few weeks in the adult flying form.

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Orthoptera, or Grasshoppers, locusts, crickets, and related

Orthoptera Tree
Tropes:
The Swarm

If you want an alternative to Strong Ants, Grasshoppers jumping really high is for you. Like the ant trope, it is an example of the square cube law, muscle strength increases by cross section, or the square of the size, the mass to jump increases as the cube, smaller animals can jump further compared to their size than similarly equipped large animals. Locusts are of course known for swarming and eating everything in an area, as in one of the plagues of Egypt, or a general expression. Crickets, named Jiminy of course, are the noisemakers of the group, chirping away using their legs.

All of these animals have a similar shape, long bodies, long back legs. They mainly eat grasses, sometimes other plants, as the habits of locusts suggest. Depending where you live, they may be offered as food, The Bible has locusts as an allowable thing to eat. The cricket chirp is mainly a mating call, sounds like this are used by other members but not as famously.

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Mantises

Mantid Tree
Tropes:
Mantis Mating Meal
Slaying Mantis

The praying mantis is probably the name you think of, Oversized Mantises are also common fantasy creatures, or at least some sort of giant insect borrowing the name, most likely based on a martial arts association.

Most mantises stand angled upward towards the head, with long arm looking front legs, these are used to catch prey. Though they are known for a trope named after another arthropod, this behavior isn’t a guaranteed thing, instead something that happens part of the time in some species, common enough to be noticed but not most of the time.

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Roaches and Termites, or Blattodea

Roach Tree
Tropes:
Termite Trouble
Creepy Cockroach
Hive Caste System

You’ve probably heard of the memetically hard to kill cockroach, or the wood devouring termite. Roaches tend to be omnivores, not too specialized, flat, and nocturnal, which makes the house pests good at getting into buildings, not being seen, and eating all sorts of random food or other items that you probably don’t want eaten. A lot of termites do in fact eat wood, though a lot also eat random debris. Bacteria help them digest this wood, and methane release from these bacteria is a big source of this gas in the atmosphere.

Most of these insects are quite social, roaches have lots of communication and group interaction, while termites are a famous example of an insect hive system. Termites have both a reproducing male and female, instead of a queen with smaller drones as found in other hive insects. Some termite nests can get quite large, building up into several foot or several meter tall mounds, influencing the surrounding area.

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Stick Insects, or phasmatodea

Phasmid Tree
Insects shaped like sticks. Or maybe leaves. Plant eaters, with a famously camouflaged body resembling various plant parts. Some can get quite long, though their thin body shape means they aren’t as heavy as some more compact insects.

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Webspinners, or Embioptera

Webspinner Tree
Spiders aren’t the only things that make silk, nor silkworms. These insects live in clusters under the webs found in their name, and mostly eat random plants or other debris. The Mantis Mating Meal could be named after them, as males once in adult form go to a different cluster, mate, and die, sometimes being eaten afterwards.

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Ice bugs and gladiators, or notoptera

Tree for both groups
This group has a small number of species, found in very narrow regions of the world. Ice bugs, as you might guess, live in very cold areas, and are so adapted they are killed if the temperature gets a little bit above freezing. They mostly eat plants and debris. The gladiators are recently discovered, though new insect species discoveries are a very normal thing, this was a rare time a completely new line of insects had been found. The gladiators are carnivores, something like mantises, found in various spots in southern Africa.

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Lice

Lice and relatives tree
Tropes:
Lice Episode

Not just the bloodsucking hair living bugs you want to get rid of, some also eat books, or stored food. And of course, a lot live out in the wild, eating bits of plants or other material as many other insects do. Some of them are among the few live birthing insects, and a few produce silk. Lice that infest animals get very specialized for the animal they attach to, species exist that live on most kinds of birds and mammals.

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Thrips, or Thysanoptera

Thrip Tree
What lice are to mammals and birds, thrips are to plants. Some eat debris or small prey, some pierce into plants and suck up fluids, some eat pollen or other plant material on the surface, among these are many crop pests. Many are social, the groups living in growths they cause on plants. They are good at resisting insecticides and avoiding predators that might control other pests, making them a bigger challenge to deal with.

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True Bugs, or Hemiptera

Bug Tree
Tropes:
Cicadian Rhythm

Not those fake, pretend bugs, these are the real thing. (Scientists like to name groups “true somethings”, but you often see it in greek as the eu- prefix, as in “eusocial”) In this group you may have heard of cicadas, bedbugs, and aphids.

As you might guess from the list, these bugs have quite a variety of lifestyles. Bedbugs and a few others suck blood. Aphids feed on plant sap, others food on plants in general. Some are predators, such as the assassin bug, some of these pierce their prey and such out body fluids, or inject enzymes to digest the target. You may notice how many of these involve piercing something and sucking something out, a key feature of this group is a piercing beak that both injects and extracts liquid, with a separate channel for both roles.

The cicada noise comes from a vibrating structure that is popped in and out.

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Ants, Bees, Wasps. and sawflies. or Hymenoptera

Tree for Hymenoptera
Tropes:
Bees
Bee People
Insect Gender-Bender
Insect Queen
Hive Caste System
Ant War
Strong Ants
Wicked Wasps

Probably the insect group that shows up the most in media. Though termites and social wasps (think hornets and yellowjackets) exist, most hive insect/hive society tropes are based on bees and ants, there’s just something darn impressive about insects forming such organized societies, even more impressive when you remember no one is actually in charge despite the Hive Queen trope, the whole colony is self directing, relying mainly in pheromones and some simple tricks for organization. Plus swarms do make an impression, which these insects are prone to do.

Sawflies, often forgotten, are the original members of the group. Most live the typical life of generally plant eating insects, but at some point the ability to inject larvae into other organisms involved. This created a new type of organism, the parasatoid, which injects larvae into a living host. The larvae than eats the host from the inside until fully grown, when they emerge and the host dies. (Parasatoid is the name because it isn’t exact parasitic, the host is guaranteed to die, but unlike predators the host isn’t killed right away. This won’t happen to you, they go for roughly similar sized targets as themselves, insect larvae that eat gigantic you are just regular parasites.) A nasty way to go, but since most of the targets are other insects, and most such organisms specialize in a single type of organism, these parasatoids can be useful as highly directed pest control. Wasps evolved from insects with this way of living, in many including bees the organ that injects the eggs changed into a stinger, thus creating wasp and bee stings and all their fun. Wasps have diversified into other roles, among them are ants where flight was lost in most members.

Members of this group fill most possible lifestyles, some being herbivores, some predators, some eating random debris, the mentioned parasatoids, regular parasites, etc. Bees are well known for pollination, grabbing nectar from a plant, picking up pollen, and taking it to the next flower. Using honeybees for this gives pollination, noney, and was together, making a rather useful creature to raise. (Though there is some concern about honeybees replacing more specialized pollinators, often better for a particular plant.), some wasps do this as well. Ants are often among the most numerous organisms in any particular environment, both by numbers and total weight, stereotypically swarming either to fight, or eat whatever type of food that particular species happens to like; some ant’s small size and ability to eat human based food makes them house pests.

Ant colonies are stereotypically underground, but they can make their homes by boring into other materials also. Bees are known for the honeycombs, in a kept beehive these are found on plates, but in the wild will squeeze into any reasonable spaces, bee infestations in walls of houses or trees or the ground are known. Social wasp hives are often paper or a similar hard material, some hardened by wasp saliva. Like beehives, they have individual cells where various things happen. Unlike termites, hive queens are a thing in these animals, one (sometimes a few) large reproducing female lays the eggs, small male drones act as mates and that’s about it. In ants, queens and drones are the only members that may have wings. Other specializations depend on the species, workers and fighters are common, sometimes specialized workers exist that, say, take care of larvae, and sometimes different types of fighters can be hatched.

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Strepsiptera

Strepsiptera Tree
A group of parasitic insects with an unusual way of living. Females live inside another insect, creating a structure to block host defenses, and feeding off body fluids. Males are free flying, and livie very short lives that consist of finding a female and mating. Larvae are raised partly inside the female, before leaving and going after a new host. Hosts are, understandably, often hurt badly by this infestation.
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Beetles, or Coleoptera

Beetle Tree
Tropes:
Japanese Beetle Brothers
Tough Beetles
Scarab Power
Thunder Beetle

“What have you learned about God?’ “He had an inordinate fondness for beetles” is the quote, for beetles are the insect group with the most species. Fireflies, scarabs, and ladybugs might be the best known, some other fun ones are the goliath beetle, heaviest insect, and the bombardier beetle, which can spray hot material at attackers. With such a wide variety of species, beetles as a whole are not surprisingly found in just about all land environments, with species specialized to eat just about anything possible.

The main characteristic of beetles is the wing covering: most insects have 4 wings, in beetles two of these have developed into the shell we’re all familiar with, covering the other pair when folded, and providing extra protection to the animal when not flying, also protection for wings in tight spaces.

A lot of crop pests are beetles, such as Boll weevils for cotton, or a number of tree boring beetles that infest cities or forests occasionally.

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Neuropterida

Neuroptera Tree
Tropes:
Antlion Monster

Lacewings, dobsonflies, snakeflies, and others, but the one mentioned in this trope is the antlion, an ant eating (where the name comes from) insect known for the ambushes it carries out. Many of this group have large wings that don’t fit particularly well on their bodies. Contains a number of predators.

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Butterflies and Moths, or Lepidoptera

Lepidoptera Tree
Tropes:
Butterfly of Death and Rebirth
Butterfly of Transformation
Moth Menace
Macabre Moth Motif
Pretty Butterflies

The plain caterpillar becomes a beautiful butterfly. Or a not so beautiful moth, but that’s a less common story. Lepidoptera means “scaled wing”, these scales are what let the animals have the wing patterns they are known for, and their body is also covered with these. Most are herbivores of some sort, many are pollinators, and they live in most places in the world, but many moths eat random bits of debris, including the ones that hang out in houses and eat bits of clothing or other things you really don’t want them eating. The Hungry Hungry Caterpillar gets the idea right, like many larvae lepidopteran ones will eat lots of plants, before going pupa and adult forms, this eating can cause major problems with crops.

The wing pattern can be used for camouflage, bright colors warning of poison, looking like larger animals, or nothing at all.

Silkworms are a part of this group, they are a type of moth larva.

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Caddisflies, or Trichoptera

Caddisfly tree
These live most of their lives as larvae in freshwater, adults live for a much shorter time. The larvae are important animals where they live, eating a variety of foods and being food themselves for lots of fish and other animals.

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Flies, or Diptera

Fly Tree
Tropes:
Flies Equals Evil
Messy Maggots
Super Fly Reflexes

Another “true” group (see true bugs), officially known as the true flies. These have a bad reputation among people, thanks to mosquitos biting and spreading diseases, annoying houseflies and fruit flies and such, maggots, and several species whose larvae live in people and animal’s skin, causing problems in the larger animal, this is called myiasis or being fly-blown.

Flies have just one pair of wings unlike other insect’s double set, and with this pair plus good senses are very agile in the air, thus swatting a fly as a show of reflexes. Fly’s dodging comes heavily from detecting air currents, which solid objects such as a hand generate more of than flexible or perforated objects, thus flyswatter designs with lots of holes. Pieces of cloth can also do the job if flicked or whipped in the right way.

Fly larvae eat lots of types of foods, but detritus and dead things are most common, thus many types of maggots. A few of these maggots can be used medically, sterilized (very important!) members of some species preferentially go for dead tissue while leaving living tissue mostly alone, and also have bacteria killing chemicals: these can remove dead tissue and reduce infections in some ulcers or other difficult to heal wounds. A lot of adult flies have sucking mouthparts, which can feed off of liquid as in fruit flies, or, as with mosquitos, suck blood. Thanks to fast reproduction and being well studied, some fruit flies are used as lab animals.

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Scorpionflies, or Mecoptera

Scorpionfly Tree
Named for the end of theri abdomen which looks like, surprisingly enough, a scorpion tail. These look like the other group of flies, but instead live in moist areas and eat dead animals, or other random bits of what is available.

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Fleas, or siphonaptera

Flea Tree
Tropes:
Flea Episode

Notice the a-ptera, so no wings on these. Fleas are flightless, but are known for jumping seemingly absurdly high compared to their body size, another example of Strong Ants style Square-Cube Law in action. All of them live on mammals or birds, sucking blood. Larvae also live on skin, eating dead skin cells and other debris that accumulates. Like many blood suckers, they are known to transmit diseases (bubonic plague/black death being the most well known.)

Gnathiferia Tree

Microscopic, mainly water living animals, with unusual hard mouthparts. Some are attached, some float, and some swim, eating up bits of debris and small animals. The mouthparts open into a brisly looking front looking something like a vase, the rest of the body can have a variety of shapes, though always symmetrical, such as conelike with a tail, or cup like.

Flatworm and relative Tree
Worms that are flat. This is not just being silly, flatworms have lost a lot of the internal organs that most bilateral animals have. Among these are a respiratory system, flatworm’s flatness lets them abrosb oxygen and lose carbon dioxide and some wastes through diffusion through the skin. They were thought to be primitive relatives for some time, but DNA studies have placed them as spiralians that almost certainly lost internal organs at some point. Some can be cut in pieces and regrow, among other unusual properties.

The most well known of these are probably tapeworms, among many parasites, lots of flatworms are free living as well, living in water or moist environments. The free living types have a variety of lifestyles, some parasites do the bloodsucking thing, some like tapeworms and flukes live inside hosts, feeding off surrounding fluids.

Annelid Tree
Tropes:
Instant Leech: Just Fall in Water!
Pitiful Worms
Worm in an Apple
(Neither technically have to be annelids, but you probably imagine something earthworm like))

Earthworms and leeches, among others. Includes a number of deep sea worms, attached to the seafloor or nutrient sources such as sunken whale corpses or hydrothermal vents (including the red and white tubeworm pictures), polychaetes are the main group of these. Annelids grow in segments, each segment repeats some organs with a similar structure, with blood vessels, nerves, a digestive system running through, and similar connecting organs running through; they were once united with arthropods for this reason, but molecular analyses don't support this. Most have little hairlike structures called setae off the sides to help them move. The larger than normal ring you see on earthworms is a reproductive section, used to store eggs.

Both ocean and earthworms often burrow into the ground, allowing circulation into seabeds and soils and improving their quality, in addition to the usual feeding roles. Decomposers/debris eaters are the most common, some are predators or herbivores. Leeches are known for bloodsucking, though many are predators also.

While some of this group can regrow from cut up pieces, earthworms aren’t among them. Most whose reproduction is known take care of eggs, providing nutrients and/or protecting them in some way.

Mollusc Tree
Tropes:
Mollusk Tropes

Most widely known with mineralized shells (Arthropod shells are made of chitin, a protein, with some minerals added in some groups), though some groups have lost these. Mollusc body shapes have a structure called a foot, which you can see in their names (-pod = foot), a general muscular structure that has evolved into a number of roles, a scraping tongue with hard minerals in it called a radula, and a mantle that grows a shell in animals with one.

The bigger mollusc groups are described below:

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Squid, Octopuses, Cuttlefish, and Nautiluses, or Cephalopods

Tropes:
Cephalopod Index

Tentacles and jets are the main features of this group, they are also known for high intelligence compared to other animals, especially octopuses. Tentacles, the “foot” on cephalopods, can be used for lots of purposes, their main use is holding prey while the animal eats it, squid arms are longer versions used mainly to reach out and grab prey. Hooks or suckers found on tentacles improve their ability to grab things. Jetting is a unique way cephalopods use to move, they expand their mantle (the “main body” of the organism, if you are looking at a whole one) to draw in water, then squirt it through a funnel. This does take a lot of energy to move fast, so most use fins or general swimming for slower movement, orwalking/crawling with tentacles as many octopuses do, while saving the jet for drifting movement or for fast bursts to escape or catch things. The squid/Octopus/Cuttlefish branch is known for intelligence, nautilus’s many have this as well but haven’t been checked as much. Also unusual to cephalopods are vertebrate like eyes, with a lens, single eyeball, retina, and such, though the retinal nerves are arranged differently.

Shelllessness is a recent thing in prehistoric terms, most cephalopods until a bit after the cretaceous extinction had shells, a group called belemnites is the ancestor to the current soft bodied forms. A bit of hard structure remains in a “gladius”, a bit of cartilage found in squid, and a small cuttlebone made of mineral in cuttlefish. The reason for shell loss is not known, sonar using whales seeing shells far more easily than soft bodies, or an increase in agility against predators that could get through shells, have been proposed. Nautilus ancestors obvious avoided these problems. The skin of the shellless cephalopods at some point evolved fast color changes and texture mimicry, you can find videos of amazing octopus camouflage in particular on youtube.

Nautiluses and octopuses mainly live near the seabed, octopuses commonly eat shelled organisms but can attack other types occasionally, Nautiluses scavenge and sometimes grab live animals when they can. Squid live up in the water column, grabbing larger animals to eat, and themselves are common food source for larger animals. Cuttlefish also tend to stay near the seafloor, going for shrimp and crabs mostly but eating what they can catch. Food is eaten with a cartilage beak (yes, really, it is shaped like a bird beak) and radula scraping.

Nautiluses tend to reproduce slowly and live long, while the other cephalopods die on reproducing and grow very quickly.

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Clams and Mussels, or Bivalves

Tropes:
Clamshells as Mouths

You probably think of these as food, but of course there are lots of types that aren’t commonly eaten. In these creatures, the mantle secretes a two part shell, with a muscle and ligament attached to open or close the shell as needed, and the other usual organs. Most are filter feeders, and can be used to clarify water thanks to said feeding. Reproduction involves releasing sperm into the water, some species release eggs as well which quickly fertilize and hatch, in others females draw in sperm to fertilize their eggs.

While these organisms are known as stay in place filter feeders, most are able to swim using their shells, or burrow into the seabed using shell and foot movement and using a siphon to draw in water for feeding.

Snails and slugs, or Gastropods

Tropes:
Speedy Snail

One should not underestimate the humble snail. Cephalopods get attention for cleverness, bivalves and squid by food lovers, but snails have their own charms. (including food lovers, yes.) They are among the more species rich of animal groups, and live in just about every environment on earth from deep sea to desert, including several moves from water to land. Land snails are mostly plant eaters, maybe not surprising for s stereotypically slow animal, while water snails eat most types of food.

Snails are known for coiled shells, but some like limpets have flatter versions, or coils with a very small coil and large entrance, if you’ve ever collected seashells you will know the different shapes. Their main weird bits of anatomy are asymmetrical organs, plus a process called torsion that during development twists the rear section around, making some tail organs near their heads instead. The foot is used for movement, on land snails it gives off mucus to help the snail move. Stalks on some snails carry eyes and the ability to smell. The mantle creates the snail’s shell. Losing a shell has happened many times, creating different kinds of slugs.

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Chitons

These have a shell made from several plates, allowing some flexibility in the shell. They move somewhat like snails, crawling along whatever surface they live on, though they only live in the ocean with some adapted for occasional time in the air. Most chitons eat by scraping material off rocks, such as algae, a process which can erode the rock underneath.
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Monoplacophorans

Rare, sea floor organisms, these have a cone shell with the body underneath, and are assumed to resemble the earliest molluscs the best. They had been assumed extinct until organisms were found in the 20th century.

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Tusk shells, or Scaphopods

These are named for their long, bent cone shaped shells (shaped vaguely like tusks, surprisingly enough.). They have somewhat simpler organs compared to other molluscs, and live on the seafloor, eating single celled organisms that fall from above.
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Aplacophorans

These are a deep sea dwelling group. They resemble worms or slugs, having lost shells at some point. Some are predators of worms or cnidarians, others eat detritus.

Brachiopod Tree
Resembling clams, but with a different ancestry and different shell anatomy, with the two halves acting more like a mouth than side structures as in clams. The creatures attach to the sea floor using a long structure, and the shell either hangs in water, or extends out of a burrow for feeding. Larvae tend to resemble small adults, but swimming in the water.

Brachiopods are rarer today than in the past, they were the main clam type animal at points in prehistory, but clams have generally replaced them for various reasons as time went on.

Phoronid Tree
A small group of worms, they live on seafloor, stand upright, and use a web looking structure on top to catch food.

Bryozoan Tree
A couple groups are usually included, with some different internal anatomy, but scientists aren’t sure if they are completely related. They are written as one group for simplicity.

Bryozoans are another group of attached, filter feeding ocean animals, with a variety of forms from stalklike to kelplike to others, the name means “moss animals” and some do look like this. Reproduction releases sperm that are either captured by bryozoans acting as females (in most species individuals can switch between the two) or fertilize free released egg. These hatch into mobile larvae that filter feed and/or rely on yolk until they find a place to settle down.

Ribbon Worm Tree
A group of usually very thin worms, mostly living in sediment, most act as predators, using a proboscis to capture prey. They are among worms that can be broken into pieces which regrow into complete animals. They don’t have a heart or blood vessels, instead using a tube network to move body fluids around, acting to both circulate nutrients and control movement of the animal.

Echinoderm Tree
Starfish are easily the most well known members of this group. Members are restricted to salt water and live on the sea bottom as adults, but are found at all depths of the ocean. The most unique feature of echinoderms is probably their water tube system, which combines movement and what would be blood circulation in other animals. Water is moved back and forth in the tubes, the inflation and contraction causes different parts of the animal to move, this also circulates nutrients and waste. Along these parts are lots of small tube feet, by inflating the feet in the right way they can be moved back and forth to walk along the seafloor though this movement is very slow. Mineralized plates just below the outer skin layers make up a skeleton, these plates might be jointed or fixed in place depending on species. Echinoderms have swimming, side to side symmetric larvae, when they change to an adult form, they change to a five pointed or sided symmetrical body instead (This is easily visible in five pointed starfish, internal organs follow the same pattern in less obviously five sided groups.)
Echinoderms are one of those animal groups that can regenerate from losing major body parts, a few species can grow from a chopped off piece to a whole organism if the piece is large enough.
The branches of echinoderms are listed below:

• Starfish: Most well known of the echinoderms. Shaped like a five poined tar with different sized limbs. These mostly eat shellfish, which they move slowly along the seafloor to eat.
• Brittle Stars: Looks like spindly starfish. These eat detritus instead of live prey. Unusually, their senses are distributed in the arms, rather then having specific sense organs,
• Sea Urchins: These have an outer shell coated in spikes. They eat a mix of algae and slower moving animals that they can catch on the seafloor.
• Sea Cucumbers: Range from long and worm like to sluglike to squat. Also no sense organs. Mostly they eat detritus, some also filter feed.
• Crinoids: Filter feeders, they can move slowly, but mostly act like attached filter feeders in other groups. They are also called “Sea Lillies”, as they resemble plants. Some have stalks, some have “leaves” attached to a central body on the ground.

Acorn Worm Tree
Acorn worms and pterobranches. Acorn worms are wormlike, but have an unusual anatomy where some organs are in a large lobe in front, as a result they look something like earthworms with a collar, smaller front, and larger back, but the mouth is actually on the collar. These draw water through their mouths and filter feed it, releasing it from gills. The mostly live burrowed in the seabed.

Pterobranches have a similar body shape, but have lots of tentavles/feathery things sticking out the top to help catch debris. They are closely related to extinct grapholites, described in Prehistoric Life.

Lancelet Tree
A small group of wormlike animals, mostly seabed burrowing filter feeders. They resemble tunicate larvae and early vertebrates, with a nerve cord down the back, plus a cartilage rod. Though they have small fins that help swim, they mostly burrow into seabed with their mouths sticking out, filter feeding.

Tunicate Tree
If you go back before other apes, before elephants, before birds, frogs, sharks, and other fish, your closest relatives are…some coral looking tubular filter feeding things. Yes, really. The resemblance is in lancelet like larvae, with a nerve cord and cartilage rod down the back, but as they become adults they lose these features.

Most tunicates (sea squirts) are attached filter feeders, forming a roughly tubelike shape where water can flow through and be filtered. Some (salps) are free floating, often transparent, and act as predators. A few called larvaceans don’t change much from the larval form, instead adapting the swimming tail to help catch debris.

Vertebrates are a relatively successful group as animals go, members live in just about every environment, including being one of two groups with flyers, and the group has more species than most other types of animals. However, vertebrates get disproportionate attention in media, probably for a couple reasons: 1. Most media is made by and for one of their number and 2. Vertebrates get much larger than other animals. For prehistoric life, add that bones and teeth fossilize well. Combine these, and most animals you interact directly with/notice will be vertebrates, with media reflecting this.

The name of the group comes from the backbone, made of either cartilage or bone, in living vertebrates this extends into a full skeleton. Bone is also found in structures called osteoderms, embedded in skin, or as part of scales, both of these help armoring animals that have it. The skeleton is the main reason vertebrates can get so big, it provides support that doesn’t exist in soft bodied animals, and can grow continuously unlike arthropod exoskeletons which must be molted. Land vertebrates and descendants have a very consistent body structure, almost always four limbs of some kind, occasionally lost in some species, plus the usual rib cage, skull, hips, etc. Fish are a bit more variable, with differing numbers and shapes of fins and a variety of body forms. Worm like shapes have evolved in a number of branches, most famously in snakes and eels.

Outside of the skeleton, vertebrates have an adaptive immune system (This allows memory of previous diseases, allowing a faster immune response. It is what allows vaccines to work), camera eyes, and a full circulatory system using hemoglobin.
Vertebrate evolution is a great example of Early-Installment Weirdness, the first creatures were very small lancelet like animals, if they’d stayed like this an alternate intelligent species might call them “fin worms” or "chord worms" or similar. Instead, over tens of millions of years, they got bigger, full skeletons, fins including the classic fishtail, jaws, better senses and bone evolved, plus possibly some soft features that don’t fossilize; at around 400 million years ago some of the hugely diverse fish became the biggest things in the ocean and some others were able to walk onto land. Most of PrehistoricLife describes vertebrates, you can check out more details there. (Or The Other Wiki, or books, etc.)

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Hagfish and Lampreys, or Cyclostomes
Cyclostome Tree
Tropes:
Lamprey Mouth

Most vertebrates have jaws, hagfish and lampreys are the exception. Cyclostome roughly means “circle mouth”, a good description of the lamprey mouth. Both are relatively low metabolism animals, hagfish tend to scavenge, though diets are somewhat flexible and they occasionally grab other meat, lampreys have some bloodsucking parasite species, some species that filter feed as larvae and die shortly after reproducing as adults. Both are eel like, and hagfish even lack backbone that defines vertebrates, though they do have skulls which show membership in the group.

Hagfish are known for making lots and lots of slime, partly as a defense against predators, and their skin makes good leather. Lampreys are sometimes eaten, though have few other uses.

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Sharks and Rays, or Cartilaginous Fish
Cartilaginous Tree
Tropes:
Threatening Shark

These have a cartilage skeleton, not as hard as bone but lighter, and some muscles also attach to skin. On that skin are toothlike scales, helping reduce friction and providing some protection. For buoyancy, oil in their bodies is used. Most cartilaginous fish fertilize internally instead of releasing eggs and sperm into water, some hold eggs in their body to hatch and a few give live birth. This is a problem with pollution, overfishing, accidental killing, and other such issues, as these populations take time to recover. Cartilaginous fish are found in most ocean environments, but not rivers apart from occasional trips by some members, and deep sea species are rare. Among these are a couple of low metabolism, long lives species, the Greenland shark and sleeper shark. Current the largest fish in the world are filter feeding sharks, the whale and basking shark, the secon of which has likely spawned a number of supposed sea monster sightings.

The branches of Cartilaginous fish are listed below:

• Chimeras This group mostly has the standard fish shape, but a thinner tail with far smaller fins. Mostly they eat things off the sea floor, and use front fins to help with propulsion.
• Sharks: The big predatory sharks are most famous, but sharks have a similar range of lifestyles as other kinds of fish, including filter feeding, scavenging, large and small. Most of variations on the standard fish shape.
• Rays, Sawfish, Guitarfish: These animals have larger then normal fins, rays and skates use them to swim through water. Guitarfish and sawfish are intermediate forms, between a ray shape and standard fish shape, using a mix of fins and tail to move themselves. Rays are mostly filter feeders, sawfish are predators, guitarfish eat food they find in sediments.

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Ray Finned Fish
Ray Finned Tree
Tropes:
These Tropes Are Fishy

About half of vertebrate species are in this group, if it wasn’t mentioned in another group of fish in this useful notes, it goes here. Ray finned fish have a few traits: a bony skeleton with relatively thin fin bones, and a swim bladder, a bag of gas the fish can inflate or deflate to control buoyancy. Most are covered in relatively small scales. Some early branching fish have chunkier scales, a few (including sturgeons) have partial cartilage skeletons, a few branches are smooth skinned.

Food lovers and makers may know about oily vs. non oily fish: the oil in the fish helps the fish float and stores energy, useful for fish that must swim long distances. Less intense swimmers don’t need this oil and become whitefish. (There are some weird ones like fatty catfish, of course.) Caviar is the eggs of certain species, most bony fish reproduce by putting lots of eggs and sperm into the water to fertilize, though in a large group there are always exceptions.

Ray finned fish are an extremely diverse group in shape, size, what they eat, etc., and live in every type of water environment. The branches of the ray finned family tree are also diverse on their own, if you want to find lesser known fish, any phylogenetic tree of the group should lead to a fun Wiki Walk. Below, a few archetypes are described:

• Air Breathers: Catfish, Bichirs, mudskippers, and a few others. These have evolved various ways of breathing air, often they live in shallow water, and/or water with little oxygen where this ability is useful. Some can spend decent amounts of time out of water.
• Eels and other worm shaped fish: Most are in a group called elopomorphs. They might have thin fins along their body, or nothing. As described in the main section of vertebrates, this shape allows more flexible movement and fast acceleration, but they can’t swim quickly, eels end up as ambush predators or eaters of nonmoving things as a result.
• Fast Swimming predators: Tuna, swordfish, marlins, and many others. These move fast and have a very streamlined shape as a result. Many have faster metabolisms and higher body temperatures then most fish, to support this increased activity.

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Lobe Finned Fish
Coelacanths Lungfish
There are only a few species of these, lungfish and ceolacanths. Coelacanths are a famous animal thought extinct, but rediscovered in the 20th century, extinct relatives had been found as fossils from only long before. Lungfish live in some places in the southern hemisphere.

The main characteristic of lobe finned fish is thick fins, as the name suggests. They also have lungs, as the name lungfish suggest, though in Coelecanths this has evolved into an oil and air filled organ that helps flotation. Lungs most likely evolved first and later some became swim bladders in ray finned fish. There are a few other random differences with ray finned fish as well. Lungfish tend to live in rivers and can go dormant when they dry out, while coelecanths drift and swim through water and grab fish.
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Amphibians
Amphibian Tree
Tropes:
Reptile and Amphibian Tropes

It isn’t known if modern amphibians have a common ancestor different than land vertebrates different theories of ancestry go either way, but they make a useful group for this useful notes.

Frogs, salamanders/newts, and small snakelike creatures called caecilians are the modern amphibians. They all must lay eggs in water, and tend to be found in moist environments as a result. Many go through metamorphosis, starting as a swimming tadpole before changing to a land dwelling form. Amphibians often have thin skin, this skin is used for breathing in addition to lungs, but is also sensitive to outside chemicals. Amphibians are sensitive to pollution for this reason, and also cannot live in salty water.
The major amphibian groups are:

• Frogs: The familiar wide mouthed, jumping, wide mouthed, tailless things. The most species rich amphibian group by far. Most are carnivores eating whatever they can catch, there are kinds that live in just about every land environment on earth, even deserts.
• Salamanders: They look like lizards with slimier, smoother, skin. Includes kinds that eat a variety of foods. Tend to live near rivers, a few are fully water living with gills kept from being a tadpole. Many are found in cooler areas, living lifestyles that lizards would live in warmer ones.
• Caecilians: Burrowing worm or snake looking animals. They tend to eat whatever small creatures they find underground. Most give live birth, some of these use an unusual feeding system of growing extra skin layers that the young scrape off to feed on.

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The Great Reptile Mix-Up

Land vertebrates that don’t need to lay eggs in water, or at least had an ancestor with this quality, are called amniotes. Some have returned to water, but these are either live birthing in some way, possibly keeping eggs inside the body, or return to land for reproduction. Includes mammals, birds, and rept…actually, we run into some issues here.

No group has been more messed with by phylogenetic classification than “reptiles”. When the original linnean system was created, land vertebrates naturally divided into furry warm blooded animals with milk (mammals), feathered warm blooded animals with wings (birds), thin skinned cold blooded animals laying eggs in water (amphibians), and cold blooded animals with drying resistant eggs often with scales, shells, or osteoderms (reptiles). This system still makes sense for vets and zookeepers and other people working with modern animals. However, it is not phylogenetic, birds and crocodiles have a common ancestry that lizards don’t, turtles may be associated with this group also.

Prehistoric animals mess this up even further. Because of various bone features, and later because cold blooded but laying eggs out of water was a stage modern amniotes clearly went through, but the animals didn’t have mammal or bird features, lots of prehistoric creatures were called “reptiles”, such as dimetrodon, therapsids (“mammal like reptiles”), dinosaurs, pterosaurs, large sea creatures, and many others. The whateversaurus tropes exists for this reason, -saurus mean lizard, and is a common suffix used for these animals. However, the phylogenetic problem still exists, lots of creatures will be more closely related to birds than lizards, probably also turtles, and “mammal like reptiles’ are closer to mammals than anything else, and equally related to birds and modern reptile groups. Classifying them by traits doesn’t work well either based on what we know, cold blooded lizards, live birthing possibly warm blooded ichthyosaurs, high metabolism fur like thing covered pterosaurs, feathered dinosaurs, turtles, and somewhat more active crocodile ancestors would not get classified into one group while leaving out birds and mammals, if someone were doing it from scratch today.

So, though Prehistoric Life uses the term reptile in this older sense, the official classification term is now used for diapsids, and this useful notes will describe group within it separately.

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Mammals and other synapsids
Mammal Tree
Tropes:
Mammal Tropes

Mammal Evolutionary Tree
Amniotes split into synapsids and sauropsids early on, the groups are based on molecular evidence, and fossils where they have different numbers of holes in a skull for lightening and provided room for muscles. Synapsids have an up and down history, the first land vertebrates to get really big, than replace by the other ‘psids for a couple hundred million years over a couple mass extinctions, than with a third mass extinction became the big animals again. And at least one of them is reading this right now.
Mammals are probably the most tropperiffic group of animals, even if you don’t cheat and leave out all the society, culture, technology, romance, etc. tropes. They make out our livestock, pets, several pests, and the big wild animals we stay away from, or smaller wild animals we watch do things. Mammals are defined by milk, fur (at least in ancestors), and some skull structures such as a different ear and jaw shape. Most have live births, in marsupials like kangaroos to a small infant that crawls into a pouch and grows further, in placentals into a more or less partly grown baby that can survive with some care outside. Milk gives us dairy products and breastfeeding, it must supply lots of nutrients and energy, and be a good consistency for babies to drink. Milk production and live birth means mammals provide good parental care by animal standards, definitely no ‘release reproductive cells into water and hope for the best” as lots of other groups do, or “die after reproducing”. Fur is useful as insulation, it also protects from sun and provides color for displays. In your own mouth are four kinds of teeth (Incisors that cut, to molars that grind, canines and premolars in between)), this is an unusual feature of mammals, most other vertebrates have just one kind. The teeth are adapted in different animals for different kinds of food, but the use of four different kinds of teeth for specialized roles is similar. However, this makes teeth harder to replace, most mammals only have a limited number of sets over a lifetime, so losing or breaking teeth can get quite badly.
By species, rodents and bats are the biggest groups, including a number of pests, or just annoyances as with bats in a house. Mammals include eaters of most types of foods, you probably know roughly which groups eat what. Insect eating is more common in smaller animals, larger ones tends towards larger prey or plants or a mix. Grass eaters are common livestock, a very useful quality as grass is one of the few foods people don’t handle well, allowing the use of an extra resource. The largest animals around today are a group of baleen whales, as described in the “size” section of this useful notes.

Mammals are really well known in society, you are probably familiar with most kinds of larger or medium sized ones. If you want to use a lesser known one, the various groups of smaller insect eaters found in most branches are a good bet. Here's a quick description of the main groups:
* Monotremes: Platypus and echidnas. Lay eggs, lower body temperature and metabolic rate then most other mammals, milk is secreted onto the surface rather then through a glad directly to the young.

• Marsupials: Young are birthed very early (similar to fetuses in placental mammals), go to a pouch and live on milk until they grow to a larger size. Includes kangaroos, possums, wombats among others, mostly found in Australia. Marsupial equivalents to many kinds of placental mammals exist or went extinct very recently, apart from aquatic ones, almost certainly because air breathing infant in a pouch plus swimming underwater do not mix.
• Xenarthra: Sloths and armadillos. Slower metabolism than most other mammals, name is from unusual joints in their spine.
• Afrotheria: Elephants, Sea cows (including manatees), aardvarks, and some less well known ones. As the name suggests, originally came from Africa. Includes many members that resemble other mammals from divergent groups in other places. Have molars that are continuously replaced, rather then finite number in a lifetime most mammals have.
• Euarchontoglires: Primates, rodents, rabbits. Also less well known flying squirrels and treeshrews. This group doesn't have any particular features in common, it was discovered through genetic means rather than physical anatomy. If looking for less well known mammals, rodents or some early diverging primates (like lemurs or tarsiers) are a good place to look.
• Laurasiatheria: Most of the famous big mammals come from here, including both hooved mammal groups and the carnivorans (which include most mammal land predators), these three groups are all closely related. Also includes bats, a big group of insect eaters called eulipotyphlans, and pangolins. Whales come from hooved mammas, and seals from carnivorans, so two of the major aquatic groups are also here. Like above, this group was discovered through genetics and doesn't have any obvious body features in common.

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Lizards, Snakes, Tuatara, or Lepidosaurs

Lepidosaur Tree
Tropes:
Hollywood Chameleons
Lovable Lizard
Malicious Monitor Lizard
Snake and Serpent Tropes

Probably the animal group that most resembles the earliest amniotes, though still evolved over time. Lepido- means scaled, and scales are a key feature of these animals, made from keratin on the surface, sometimes bony support. Most lizards walk in a sprawling motion rather then legs under the body. This is likely related to their slower metabolism ten mammals and birds, meaning they spend a lot of time resting. The resulting side to side motion restricts breathing. Legless lizards have evolved a few times, snakes are the most well known and species rich, all of these groups have organs adapted to work in a longer, thinner, body.

A number of animals in this group have venom, most famously poisonous snakes, and entire subgroup of the group seems to have venomous ancestry. Constriction is an unusual method of killing, found in several snakes, the method takes a lot of energy and seems to shut down blood flow or and heartbeats of the victim as its killing method.

While a "generic lizard" lays eggs after mating, has a slow metabolism, etc., the group has a lot of variety, and unusual treats and exceptions can be found somewhere. Many lizards can reproduce with just a mother, some give live birth, some have higher then usual metabolism, etc. Lizards in addition have a great variety of eating habits and live in most environments (including a few sea snakes and some heavily aquatic lizards), though they tend to be less common in cold areas.

A quick summary of lizard groups is below:

• Sphenodonts (Tuatara): Tuataras are the only remaining animal in this group, found in New Zealand. Might be 2 species or 1 species with 2 separate, different looking groups. Look like lizards on the outside, but different skeletal structures point to a long ago divergence. Most known for a “third eye” on top of the head that can sense light and dark but not images, though many other vertebrates have this organ in a less visible way.
• Geckos: Mostly carnivores. This group includes some animals with sticky feet, that can hang of vertical surfaces or upside down. Unique among lizards in heavily using calls to communicate.
• Skinks and relatives: Variable group, many members have smaller legs then usual and burrow effectively.
• Lacertoids Includes a variety of types of lizards, plus legless amphisbaenians. :
• Anguimorphs: Among these are several groups of legless lizards and monitor lizards. Has a few members with venom (komodo dragons seem to, plus some gila monster relatives.) Monitor lizards have somewhat higher metabolism and more activity then other lizards.

Iguanas and relatives: Chameleons are the most well known individual member. Includes lots of herbivores in the iguana group.

• Snakes: The most species rich and widespread of legless lizards. Predators, often of rodents, a high fraction have venom of some sort. For lesser known snakes, look to smaller species.

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Turtles
Turtle Tree
Tropes
Index in a Half Shell
Turtle ancestry is tricky, they for a long time seemingly showed up fully formed in the fossil record, and don’t have several features scientists normally use to classify land vertebrates, best guess was they were primitive/early diverging branch on the amniote . Today, molecular studies mostly point to the bird + crocodile group as close relatives, but a few point to lizards being closer.

Whatever their ancestry, turtles are obviously defined by their shell, created of bone or a thick leathery material. This shell obviously provides protection, but also limits movement, limits space for organs, including space to breathe, and requires nutrients to grow. In a more intimate issue, turtle penises must be really long to reach between the mating couple. Most of these problems are less extreme in water, fins can provide propulsion for example, and turtle lineages have evolved from land to water and back many times. Turtles also lack teeth, having beaks instead.

Most turtles eat whatever is available, with land turtles eating more plants and sea turtles eating more meat, as might be expected from how easy walking is vs. swimming. Among the more specialized turtles are leatherbacks, eating jellyfish. Metabolism is generally slow, apart from some sea turtles that maintain a high temperature partly through physical activity. The combination of slow metabolism and protection is probably what allows turtles to live such a long time, close to 200 year lifespans have been measured in a few.

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Crocodilians
Crocodilian Family Tree
Tropes:
Gator Load of This Index
The crocodile group, pseudosuchians, has a lot of interesting prehistoric members which are worth reading about, but the only remaining members are the alligators and crocodiles (and gharials and caimans) we all have heard of today. All of these mainly live in rivers, eating fish, land animals they can ambush, or a mix of the three. Crocodiles and alligators are differentiated by random bits of anatomy, caimans also have some of these and are usually smaller, and gharials have a very thin snout that trades ability to catch land prey for fast closing to catch fish.

While fish are a big part of lots of crocodile’s diets, catching land animals is what they are famous for, and they have a lot of anatomy to do it. Crocodiles are famous for a very strong bite, and for their ridiculously weak jaw opening muscles, this bite is used to hold animals in water to drown them, followed by the crocodile eating, sometimes rolling to separate bits of meat to swallow. They are adapted to swim low in water, eyes and nostrils on top of the head. Greyish slight greenish (exaggerated by pictures) color adds camouflage, resembling logs or natural river color. The plate like things in their skin are called osteoderms, which are bony and include structures for good heat transfer and other functions in addition to protection.

Crocodiles have a lot of anatomy suggesting a more active ancestry: 4 chambered hearts, found in birds and mammals, plus the ability to stand and walk upright and some can even gallop in a way, though they can't do this for long without tiring. See prehistoric life for more details.

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Birds and other Avemetatarsalia
Bird Tree
Tropes:
Bird Tropes
Dinosaur Tropes
Terror-dactyl
Dinosaurs

Another highly troperiffic group, including the Pterosaur, a bunch of large prehistoric land animals you played with as a kid, and modern birds are no slouch either, such as the majestic Eagle, the Circling Vultures waiting for something to die, Chicken which all meat tastes like, Clever Crows or Creepy Crows as fits your preference, and many others.

Birds today are known for being warm blooded, having beaks, and feathers most obviously. Other adaptations include air filled bones, very efficient lungs, and large breast muscles used for pulling the wings. All in some way contribute to flying, beaks and air filled bones lighten the animal, powerful chest muscles work the wings, high metabolism and efficient lungs give the bird energy to fly, a high power to weight for animals, feathers form the airfoil of the wings and keep birds warm to help maintain a high temperature. The lungs have a very different structure than what you are familiar with, instead of lungs that draw air in and out and absorb it, birds have a rigid organ than absorbs oxygen, and a couple systems of air sacs that expand and contract, all attached by tubes. These sac’s movement is timed so that fresh air is always flowing through the oxygen absorber, while stale air bypasses this organ and is exhaled directly. This helps birds handle much lower oxygen levels than almost all animals can handle, they can fly at altitudes where humans need extra air. Fligthless birds maintain these adaptations. The evolution of many of these features is a subject of interest for paleontologists, a lot show up in bird’s flying relatives Pterosaurs as well.

Outside of flight adaptations, birds have a sound generating organ called a syrinx, allowing the variety of calls they are known for. Their brains seem to make more efficient use of space then equivalent mammal brains, and lots of birds are studied for the resulting intelligence. They show a variety of mating strategies, and all have some amount of parental care.

Subgroups of birds are often quite diverse with a variety of lifestyles, Birds as a whole eat a variety of different foods and live in a variety of environments. The closest to a "generic bird" is probably a roughly pigeon or parrot sized animal that mostly eats insects, worms, and other small animals and/or nutritious parts of plants, but a few archetypes are listed below.

• Flightless Birds: Large flightless birds such as Ostriches and Cassowaries are most well known, most of these are in a group called Paleognaths, but a number of smaller flightless birds exist as well. (Some domesticated birds also have a hard time flying, but their wild relatives are perfectly able to do this) Flightlessness is more common on islands or other isolated areas, where lack of access to other land mammals allows such birds to evolve. These birds typically lose their keelbones (where wing muscles attach), and have smaller wings, but keep other typical bird features.
• Passerines: This group includes about half of all bird species, if you are looking for less known birds, this is the group to look at. They have a unique toe arrangement that makes perching easier (“perching birds” is the common name for the group), otherwise they resemble the “most common bird” described above.
• Waterbirds: Waterfowl is the major group, plus loons, penguins, grebes, and other scattered species. Birds that regularly swim have webbed feet for easy paddling, species that dive regularly are more heavily built to allow sinking, and have more difficulty flying as a result.
• Birds of prey and similar: Most are in a group called Hieraves. These tend to swoop down on prey from above. Beaks and claws are the main weapons, using either pressure or cutting to kill prey. Known for very good senses, in whatever environment they are adapted to.