Creating aliens is easy.
Creating believable aliens is hard.
Star Trek has so far gotten away with humans with funny foreheads*, because everyone knows that actors somehow have to fit into the costumes and the budget isn't good enough to get Alien-quality special effects. However, that has never been a restriction with written fiction. The only excuses for not giving readers some really freakin' awesome extraterrestrial creatures are laziness and lack of knowledge, the first of which can be fixed by a good smack upside the head, and the second of which is about to be fixed right here.
*Not entirely true anymore; as Pint has pointed out, Enterprise has a couple of CGI aliens, and Voyager had Species 8472. But these are still rare.
It's usually best to go through the various topics in order, but if you have a specific idea that you're basing your aliens around, you might want to begin with whichever topic that falls under.
Table of Contents
The World (i.e., The Home Planet)
Paths to Sapience
Society & Psychology(in progress)
Sample Creations(in progress)
The World. It is where your aliens first crawled out of the primordial slime. It is the place they evolved to live in, and its particulars can have a profound effect not only on anatomy, but science, technology, religion, and myth.
Do your aliens live on a high or low gravity world? If they live in high gravity, they'll be short and sturdy. If they live in low gravity or underwater, they can be tall and thin, but anything that lives underwater will have a snowball's chance in the Sahara of developing a technological society.
What is the axial tilt of the world? This is not just an academic question -- it affects the global temperature range, and the variation in temperatures in any given area. With no tilt, temperature will remain nearly constant, and only vary by with the small changes in distance from the sun as the planet proceeds on its elliptical orbit. Because of the axial tilt that earth has (well, and a lot of other things like the greenhouse effect, but let's keep it simple right now), we have a tropical zone with wet and dry seasons, a temperate zone with 4 hot, cold, and in between seasons, and cold polar caps. A world with little or no axial tilt would have essentially no seasons, and little or no variation in the length of days. A world with a greater axial tilt would have more severe extremes of temperature and lighting. In the most extreme case, at the peak of summer the geographical pole is pointing almost directly at the sun, so half of the planet is illuminated constantly, and the other half is constantly in the dark.
Additionally, inclinations greater than 32 degrees or thereabouts will result in significant seasons across the entire globe. Thus, unless you have very little water or extreme geographic isolation from a large body of water (provided by high mountains surrounding a certain region and blocking the airflow, or a long distance across the continent to the nearest ocean), seasonal rainfall will result in a near lack of permanent deserts.
How much of the world is covered in water (or some other biological solvent; perhaps you prefer liquid ammonia with dissolved rubidium chloride to our boring water & NaCl oceans)? How are land/water distributed? Water stabilizes temperature- a planet with a single huge supercontinent will likely have massive temperature swings between night and day in the middle, with temperate zones existing only near the coast.
Which part of the world did your aliens evolve in? Where they live isn't necessarily important- a sapient species will probably spread over nearly the entire globe as humanity has. Now might be a good time to sketch out a few maps, or at least imagine one.
What temperature range can your aliens survive in?
PATHS TO SAPIENCE
All creatures are not created equal. In the vast majority of cases, sapience just isn't worth the effort. It takes a special kind of organism to evolve intelligence, so, unless you aim to create a planet inhabited entirely by mindless, yet cool, animals waiting to be discovered by other aliens, you should probably have some idea of what is required before putting in the effort to design a really freakin' awesome galactic conqueror, only to discover that it couldn't open a pickle jar, let alone subjugate humanity.
Mobility is a major source of the type of problems intelligence can help to solve, such as the need to find food. As cool as they may seem, sapient plants really aren't all that likely. Toughies like figuring out which neighbor is more or less likely to bash your head in if you try to steal his mate, however, suddenly make basic survival like finding food look minor. An earthworm has enough intelligence for that.
Interesting levels of intelligence appear almost exclusively in social animals, at least in our single real world sample. The only exceptions I know of are octopodes and large squid.
For animals whose food is scarce, (e.g., tigers) social groupings are likely to be more trouble than they're worth. Any help that friends might provide is balanced by their wish to eat your hard-earned grub. Social groupings are most advantageous to animals whose food is plentiful. In the case of herbivores, this means that there are more sets of major sensory organs to watch out for predators. In the case of carnivores, this means that Very Large Prey can be taken down more easily. However, as large carnivores require lots of wee beasties to feed them, intelligent alien carnivores would have to come from a very special sort of ecologically rich world, and would likely have a much lower population growth curve than humans.
We have established that food, both acquiring it and avoiding becoming it, are major causes of the creation of social structures. But just as scarcity can destroy any chance of social order, so can overabundance. When food is ridiculously easy to find, having a friend to point out the massive piles of food littering your habitat isn't going to provide much of an advantage. Lookouts for predators are still useful, but it doesn't take a lot of grey matter to run for it when you notice that everyone else in your herd has started doing so. A balance must be found.
Enter fruit. When it ripens, fruit is ridiculously abundant in a limited area and for a limited time. Therefore, friends aren't competition. However, because the time and place are limited, creatures with enough brainpower to remember where to find it have an evolutionary advantage- and so do the creatures who learn to follow them around. E.g., in a social group.
If you eat fruit, it's good to be smart, it's good to be social, and neither one precludes the other. But what if your home planet doesn't have fruit? Heck, it may not even have plants, strictly speaking. Not a problem. Anything that is locally plentiful yet hard to find will work just as well. Dolphin-feeding fish, for example. Or enormous Woolly Mammoth carcasses being slowly picked apart by packs of scavengers.
But what about the octopodes? Hm. That is a conundrum. My theory, concerning reef octopodes, at least, is that their food is obscenely plentiful, but requires intelligence to successfully consume. Ever try opening a clam shell with your bare hands? Not an easy thing to do.
But what about actual physical restraints on intelligence? Assuming your aliens' brains use cells equivalent to neurons (if that isn't true, I really can't say anything about it), a brain with sufficient excess neurons to do something interesting must be relatively large. Absolute size isn't as important, to a point. Obviously, something the size of a beetle won't be able to contain much processing power. However, larger animals require larger brains to control their bodies- it's the stuff left over that counts towards intelligence. Humans don't have the largest brains on the planet by far- what we do have is the largest brains compared to our body size.
Since a sapient alien must be of respectable size (a medium size dog looks to be about right as the lower limit, but any reasonably exact cutoff point is really just pure speculation), an insect-style exoskeleton isn't going to work out very well, except in low gravity. They become too heavy and inflexible as their size increases- part of the reason Mothra only appears in movies. Of course, nature isn't restricted to extant Earthling forms. Fenestration (that is, creating gaps in the structure such that it more closely resembles vertebrate ribbing than a solid suit of armor) could help a lot. Or perhaps your alien could construct silicon dioxide fibers like a sponge with which to weave a light fiberglass exoskeleton. It seems to me that breaking an arm would be a bit more extreme if it causes a shower of sharp toxic microcrystalline shards, but if you can come up with something plausible, more power to ya. Until then, I'm sticking with the idea that most sapient aliens are either going to be mainly endoskeletal or mainly soft-bodied, with perhaps bits of exoskeleton tacked on.
So, we've identified the major environmental factors leading to intelligence. But these factors occur all over the Earth! Why is there only one known sophont (us) inhabiting the planet? Some people would say it's because we've become so dominant as to take away any other creature's chance. But why, then, didn't an intelligent animal evolve before us? Because Humans have Hands. Obviously, interestingly sophisticated technology, or even basic tool use, can't come about without some way of manipulating ones environment. But perhaps not quite so obviously, the ability to use tools probably doesn't come about because of increasing brainpower*; rather, increasing brainpower comes about as a result of the ability to manipulate ones world. Anthropologists once thought that human's brains grew larger first, and then that new intelligence led to a drive to release the forelimbs from walking, the development of dexterous hands and bipedalism, etc. But why would anyone's brain grow larger in the first place? Brains are expensive pieces of equipment. The use up valuable energy that could be better spent, say, chasing the Hot Australopithicine Babe two caves over, or running away from those nasty Saber-toothed cats. Fossil evidence and more complete studies of our Great Ape friends have provided a more logical explanation. Proto-humans had opposable thumbs while their brains were still the size of a chimpanzee's, and heck, living chimpanzees have more opposable thumbs than we do. The enormous evolutionary advantage afforded by prehensile appendages pressured humans to free their forelimbs from ambulatory uses, and the massive increase in sensory stimulation led to bigger brains that could think of things for those fingers to do. Cross-referencing with the Technology section, any alien with any level of technology, no matter how primitive it is or how strange the alien may be (and stranger is better), will have some way of manipulating its environment built in. Telekinesis does not count (where would the brainpower come from, anyway?). Perhaps it extrudes pseudopodia. Perhaps it employs a prehensile phallus (how's that for sexual dimorphism? Imagine only males being capable of tool use...). I've got one species on the drawing board that employs prehensile eyestalks. But whatever their form, sophonts will have hands. Concerning the octopodes again, why haven't they developed technology with all of those wonderfully versatile arms? Refer to the WORLD section. It's hard to refine metals underwater, thus giving strictly aquatic creatures a snowball's chance in Death Valley.
*Larry Niven provides an interesting counterpoint with the Grendels of Beowulf's Children. Parasites infect the creatures' brains, causing them to grow extra lobes similarly to how Ribeiroia trematodes contribute to limb deformation in Pacific Treefrogs.
Besides the scenarios described above, there could be any number of other factors pushing your aliens towards sentience. Most will probably be helps and additions to the factors already in place, but a few might provide complete replacements or exceptions to the usual "rules". What follows is purely speculation, and I have no way of testing its validity at this time, but it provides a nice example.
Revisit, for a moment, the fact that humans have the largest brain in relation to body size, and then consider what is required for a new human to come into the world. Were a baby's brain to grow full-size in utero, its head would be unable to squeeze out. To avoid this problem, human babies are born entirely helpless, without even a fully developed visual system and only the grossest motor control, and a significant fraction of brain development occurs after birth. Ergo, parents are required to take care of the child for a very long time. Besides encouraging a longer lifespan for the adults of the species, this puts quite a bit of strain on the parents (ask anyone with a child of less than 18 years), and it seems to me that this encourages the development of larger social groups to support the parents while they take care of the baby. Thus, the details of how humans reproduce provide an evolutionary pressure towards complex social structures, which as is explained above promote intelligence. One might go so far as to say that they require sophont-level intelligence after a certain critical point of complexity.
After contemplating General Biology for a bit, you would do well to consider what details of your aliens' lives might alter or add to the drive towards sapience.
What do your aliens look like? Start out with something general, the details will fill in by themselves. You probably already have a few bits worked out from the PATHS TO SAPIENCE section. Number of legs, major external sensory organs, etc. are a good way to start. And remember, life has lots of ways to do things. The humanoid body plan isn't the only one, and your aliens don't have to use it (nay, they should not use it!), nor do they have to be bug-eyed and slimy.
What range of shapes, colors, and sizes do they come in? Are there differences between genders? (E.g., human males tend to be taller and more muscular than females. On the extreme side of sexual dimorphism, some deep sea fish have big fat all-mouth females, and itty-bitty males so insignificant that they permanently merge with the female when they mate.) Are these variations within a single species, or are the aliens polytaxic? (E.g., Neanderthals, Homo sapiens, and Australopithecines all living at the same time.)
Why do your aliens look like this? Refer to the WORLD section. If they live underwater, they'll tend to be streamlined. If they live in a desert, they might have thick skin or plates to reduce water loss. If they live on mountains, they'll need to be able to balance on precarious precipices. As long as it's not an evolutionary disadvantage, sex can produce some interesting frills- giant crests or spectacularly colored tails, for example. Or perhaps your aliens have some novel appendages not seen at all in Earth-life; giant fronds of skin held up by spines as external lungs and temperature regulators, for example.
What and how do your aliens eat? I've used all manner of methods. Pick from the list, or come up with something mind-bogglingly original: Internal mouthparts, external mouthparts, swallowing things whole, injecting food with gastric juices and sucking up the sludge as it dissolves, everting its stomach and engulfing things. What the creatures eat will influence their dental hardware, if they have any, and their digestive system. Herbivores will tend to have longer, more complex digestive systems than carnivores, and flat teeth for cutting plant matter (assuming plant matter that is similarly complex and fibrous as Earthling plant matter). Earthling animals typically either swallow gravel or have rough grinding teeth as well. Your aliens might use something else, but they'll need some way to break down their food. Carnivores have sharp thin teeth for tearing meat and usually smaller grinders, though some might have large rough grinders and massive jaw muscles for crushing bone, with snakes being a special case. Many snakes have lots of small backwards-pointing teeth that act like ratchets to keep their whole-bodied snacks from crawling back out.
Are your aliens ectothermic, endothermic, or somewhere in between? Endotherms require much more food than ectotherms, but they have a lot of advantages. Almost everything that stands upright, especially bipeds, will be endothermic, as they will be more exposed than sprawling animals. Flying creatures are especially vulnerable. Large animals will be able to retain body heat fairly well, but they'll also have trouble warming up if they do lose too much heat. Comes the first rainstorm, and your giant ectotherm will be chilled straight through, lying lethargically in the sun for many hours in an attempt to warm back up afterwards. Time during which a smaller, faster endotherm can come along and eat it at leisure. Smaller animals (i.e., animals with lots of surface area) will not be able to retain much heat, so their temperature will fluctuate wildly with the slightest gusts of wind. That's why lizards remain sprawled near the ground, where they aren't as exposed. Ectotherms are certainly possible, especially if they live in a very stable environment (deep caves, perhaps), and can present some very interesting sociological weirdnesses, but endotherms are much easier.
What is their lifespan? A simple variable that can have far-reaching social consequences.
What and how do they breathe? Almost certainly, they'll employ oxygen at some point. Why? Because oxygen is the best electron receptor there is. Ever wonder why there are no carbon or sulphur breathers? There are. But none of them are much more complex than bacteria. The reduction of oxygen releases many times more energy than its nearest competitor. It's perfectly possible for your aliens to live in an anaerobic atmosphere, and even to have backup anaerobic metabolisms (see the Herdbeasts of Labyrinth), but they'll need some way to store oxygen for an aerobic metabolism if they are ever going to get enough energy to move.
On the other hand, one could reverse the usual oxidation-reduction cycle, making the 'plants' oxidizers and the 'animals' hydrogen-breathing reducers, as Hal Clement did with the Mesklinites. Hydrogenation of acetylene and unsaturated hydrocarbons releases a lot of energy.
Some structure will be needed for gas exchange, unless all of the aliens' inputs and wastes are in liquid or solid form (one could eat sulphates or nitrates as oxidizers rather than breathing diatomic oxygen, for example). Gills, internal or external, are useful for small creatures, as are spiracles + tracheae, while lungs are generally better for larger or more active creatures. It's all about surface area: the more you have, the more nutrients your creature can take in and the more wastes it can expel. Small creatures, or creatures with unusually large surface areas compared to mass (see the Stromatandros) might make do with absorbing and expelling gasses directly through their skin.
What is your aliens' primary sense? Light works very well in a thin earthlike atmosphere. But under a far denser atmosphere, like Venus's, sound might provide better resolution. Sound can also travel around corners, and can be used in the dark, both of which are very useful in cave or jungle environments. If we allow active senses rather than just passive ones, sonar can also be used as a weapon. For some reason, active vision is rare in Earth animals - far rarer than active hearing - but there's no reason your aliens couldn't have light-emitting eyes for night vision, like certain deep-sea squid which have sacs of bioluminescent bacteria living in their eyes. If it doesn't have to worry about remote detection, or for some reason light and sound are simply unavailable, touch can be quite useful. Octopodes have sensitive enough tentacles that they can identify objects just as accurately by feeling them over as they can by looking at them. For an aquatic creature, electromagnetic sensors like those used by sharks can make good secondary senses, but the lack of resolution makes them unsuitable for a primary sense. Chemical senses (smell and taste) are slower and easily disturbed by breezes, but they too make good secondary senses. If you can construct an environment which has predictable or nonexistent air disturbances, a chemical sense could be handy as a primary sense, such as in an ant colony or mole tunnel- or, once again, in deep caves.
Where is the brain located? Unless you can manage much faster types of nerves than we have, it should be near the primary sense organs (or vice versa) to minimize the time it takes for nerve impulses to reach it, but it should also be protected. Notice that us humans have all of our major sensors clustered in a little ball on top of our bodies, all within a few inches of the brain. Sensors should also be wherever they will do the most good. High (or low if the alien's food and/or major predator is typically underneath it) or forward, or both, as they can see more from a height, and most animals have more need to know where they are going than where they have already been.
What do they use as their manipulators? Recall the end of the PATHS TO SAPIENCE section. Hands, tentacles, lips, eyestalks, phalli....
How many limbs total? You should've answered this question in the APPEARANCE section. E.g., earthling vertebrates all have four limbs. If your intelligent aliens have six limbs, so will all other creatures of the same gross classification, with perhaps a few exceptions due to loss of limbs through specialization.
Exoskeleton, endoskeleton, or no skeleton? You might've answered this in the PATHS TO SAPIENCE section. If your aliens are land dwelling and soft-bodied, they'll need exceptionally strong muscles to support their bodies.
How do they grow? With exoskeletons, growing can be a bit inconvenient - if they want to get bigger, they'll have to shed their shell, increase their size, and then get a new shell, unless they have some way of continuously re-sculpting their shell (which bugs and crustaceans don't). This leaves them vulnerable while they're in between shells. Metamorphosis can also be inconvenient, as they are immobilized and helpless for quite some time, but just because it's inconvenient doesn't mean it can't be used. A third possibility: they start out with lots of independent plates, like a human baby's skull, which fuse into a complete exoskeleton when they mature.
How many sexes? Only one is extremely unlikely, as evolution with asexual reproduction is mind-bendingly slow. About the only way I can think of to get a single-gendered species is that you start out with a two-gendered species that is also capable of parthenogenesis, and some disaster kills off all of one gender. This seems most plausible if gender isn't inherited, but environmentally determined, as in crocodiles (or any of a large number of other reptiles, for that matter). You don't have to be limited to two genders, but requiring more will make reproduction much more difficult. The best arrangement I can think of for more than two genders is where any two genders can mate, but each combination will only produce a certain subset. Imagine an ant colony where the queen could asexually produce workers, mate with drones to produce new queens, workers could mate with each other to make more workers, and workers could mate with drones to make more drones. Or something like that.
What is the physical method of reproduction? E.g., does it have a modified tentacle, dedicated genitals, etc. How do the necessary bodily fluids get from one to the other? Does the female store them for later use, or get pregnant right away? Eggs, live birth, or something else?
After birth, do they care for their offspring, leave them to the society, or leave them to chance?
How do the aliens feel about sex? For example, are they interested all the time, or do they go through estrus cycles?
Society & Psychology
Myrmemimoids / Formicidomimoids
Myrmemimoids (Ant Mimics) evolved in the Great Salt Desert*. The various species of Myrmemimoids range in size from a decimeter or so up to a meter and a half in length. Myrmes are active in varying amounts during almost the entire Sea/Land cycle. However, for the short time that the entire Great Salt Desert is completely submerged, Myrmes hibernate in underground tunnel cities.
These aliens follow a septapod body plan. Like most advanced septapods (on this particular world, at least), the foremost appendages have exapted into complex horizontally opening mouthparts. Within the mouth, rows of rough silica teeth are mounted on muscular plates on the roof and floor of the mouth, and grind together to process food. The external mouthparts are semi-prehensile. Two independently rotating three-lidded eyes are set into the side of the braincase just above and behind the mouthparts.
Appendages 3-4 are mounted directly to the braincase like the mouthparts. Appendages 3-6 bear two long clawed fingers each. Bony plates armor and support the aft section of the body behind the braincase. Appendage 7 has been reduced to a stumpy extension of the main body, but still bears two degenerate fingers with oversized “tail claws”.
Myrmemimoids are extremely social, and dig vast tunnel networks in the cemented sands of the Great Salt Desert. The major entrances and external “Sand Castle” structures are located as high as possible on hills, but minor entrances can be scattered nearly anywhere. Seemingly positioned at random, the entrances to Myrmemimoid cities are actually distributed to take advantage of winds to create a constant flow of fresh air through the city like meerkat burrows. Each entrance is capped with a large smooth dome or a tower similar to those built by termites. A central shaft runs several meters straight into the ground before u-bending back upward into an entry chamber, from which the tunnels of the city branch out. Each transition to a lower level makes use of a similar structure, but with the descending tunnel set at an angle rather than being perfectly vertical. Living chambers consist of flattened bubbles set just below the level of the residential tunnels, and alternating between left and right. Myrme cities are highly structured, with each level having a single well-defined purpose, which is almost universal between all Myrme species. Levels include food storage, sleeping quarters, nurseries, and farming and slave quarters in some species.
Some species are either carnivorous or herbivorous, but most Myrmemimoids are omnivores. Omnivores and especially herbivores are known to cultivate beds of fungus and algae in their cities. All species are highly territorial, and most stage wars between species either to capture a city, eliminate competition for food, or simply to take slaves. No Myrmemimoids, however, are known to stage wars between different colonies of the same species. Actual killing in these conflicts is rare, although significant mutilation is not; typically, forcing an opponent’s mouthparts to the ground is sufficient to gain submission. Social structure in Myrmemimoid colonies is determined by physical strength and burrowing ability. Shoving matches involving locking mouths and attempting to force the opponent backwards and to the ground, not unlike a less violent version of an interspecies battle, determine pecking order within a city, with the exception that the individual who tunneled the largest fraction of any particular district is given deference when in that area.
*The Great Salt Desert is a very unearthly environment to be found on one of my most recent, as yet un-named worlds. Nearly the entire basin is below sealevel, but it is separated from the ocean by a low landbridge. Every few years, the planet's two moons line up to provide enough tide to breach the landbridge, flash flooding the basin. Over the next year or two, the water evaporates and drains away, until all that's left are a few small hypersaline lakes scattered about a brilliant white desert of salt.
Stromatandros (blanket men) originated on a minor isolated continent (think Australia in terms of weirdness compared to the rest of the world). Their earliest ancestors were a form of Ediacaran, simple sheets of flesh that waved lethargically across the seafloor, powered by a combination of photosynthesis and filtering of plankton on their mucus-covered skins. These original animoids were more like ferns than animals, having multiple large folds and fronds of skin to increase the surface area available for photosynthesis and filtering. When true animals evolved and started eating each other, rather than going extinct like earthling ediacarans did, these primitive organisms developed several protective strategies. One was simply to grow very large, another was to secrete poisons, or just bad tasting chemicals, and the last was to become fast.
With the added size and mobility came a higher energy need, and the new organisms turned to passive predation of sessile seafloor animals, plants, and ediacarans by gliding over them and secreting corrosive digestive chemicals from their undersides, and then soaking up the nutrients as the trapped creatures dissolved. The creatures quickly adapted their large flat bodies into fins, managing to propel themselves much faster through open water by rippling the edges of their bodies in sine waves than they could by rippling along the seafloor. As the creatures grew larger, some of them turned to active predation, lying in wait under the sand for food to wander by before gliding over the food, pinning it down or engulfing it, smothering it in mucus, and dissolving it externally. Since the creatures still spent most of their time basking lazily in the sun, eating only when food conveniently wandered by or they required extra energy to reproduce, and otherwise moving only to escape predators, growing a complete internal digestive system was just a waste of energy.
In order to locate food when it was needed and to avoid predators, the primitive blanket organisms evolved a series of useful senses and a complex nervous system to deal with them. These included a set of four olfactory antennae, a nerve network which converted most of the underside into an enormous tympanic membrane, electrical sensors, multiple tactile sensor networks, orientation organs, and a series of primitive eyes, essentially just photo-receptive dots. Four of these photo-receptive dots developed into pinhole eyes, and then into simple lensed eyes. Eventually, two extra skin flaps developed on the underside, forming a channel in which to fold the creature’s food.
Next, some of these creatures began the move to land, growing thicker skin and a coat of thin hair to limit water loss. Those that remained in the water eventually developed complex eyes, but those that moved onto land stopped at stationary lenses. They also almost completely lost the electrical sensor system, which was pretty much useless for locating animoids through the thin air. The primitive land dwellers spent the majority of their time wallowing in shallow pools, fronds extended, soaking up sun and laying in wait for other animals to come by which they could pounce on and engulf. Pouncing on things obviously requires a lot of energy, and more so still when one has to violently flop one's entire body mass around to do so. The creatures diverged once more: one set became grazers, slowly gliding across grasslands and scouring them away like enormous living lawnmowers. The eyes of these creatures became extremely degenerated, but developed polarizing filters as a navigational aid. The others discovered that it was easier to fall on things than to pounce on them, and, finding numerous creeping things and foliage to eat on the way up the local tree-analogues, began climbing up trees like large puddles of strangely animated barf to get greater sun exposure, and to wait for things to go by underneath which they could fall on. In order to facilitate tree climbing, the creatures developed sucker-depressions and sets of independently operable muscles around their edges like hundreds of webbed-together pseudo-fingers. Over a very long period of time, nearly the entire main foot / body sheet split up, as did mollusks’ feet during the evolution of squid and octopodes, forming twelve tentacles tipped with sucker-fingers. This new-found dexterity resulted in a burst of rapid evolution in which the creatures became far more predacious, getting more energy from food and spending less time basking in the sun, and experienced a massive increase in relative brain size. The fronds were not lost, however, as they served double-duty as gas exchange organs, and the arboreal creatures retained the ability to photosynthesize when necessary/convenient. Thus, they became totally nocturnal, spending the days asleep on branches or wallowing in shallow pools, fronds extended and soaking up sun. One set of eyes set on opposite sides became more highly developed, resembling mammalian eyes save for the fact that they were immovable, while the other set became reduced in size, became recessed, shifted forward and down, and adapted to detect infrared radiation like a snake’s pits.
Herdbeasts of Labyrinth
Adult Herdbeasts range in size from a little larger than a domestic cat to a little over 1.5 meters tall. They have a thin external articulated exoskeleton of hydrated iron oxide, but their primary endoskeleton is composed of calcium carbonate. Rather than haemoglobin, Herdbeasts employ haemocyanin as an oxygen carrier, and sacs of perfluorodichlorooctane (C8F16Cl2) to store dissolved oxygen and nitrous oxide from the decomposition of nitrates for use in the anoxic zones of the cave systems in which they live. 4 semi-prehensile rough "tongues" surround the mouth, which has a horizontally opening beak/jaw. The main body is shaped similarly to a tail-less shrimp. The head, actually little more than a short stalk on which to mount the beak, joins underneath the body just behind the rounded front edge of the shell, rather than on the top, with the four limbs arranged radially around it. Most species of Herdbeasts are quadrupedal, but the interesting pre-sapient bipedal ones have shortened forelimbs that split into four three-jointed fingers, two of which are opposable, while the hindlimbs are long and thick with bird-like knee joints and two toes each facing forwards and backwards. The equivalents of lungs are feathery structures hanging from under the side edges of the shell. Unlike most cave animals, Herdbeasts are not completely blind, and four infrared-sensing pits are located just under the front edge of the shell.
Herdbeasts live in family groups similar to chimpanzee troops and care for their young, which are born from leathery eggs and fed by trophylaxis until they are a few months old. For herbivores and carnivores, this only serves to allow the infant time to gain the strength needed to eat on its own, but for chemovores it also serves the purpose of establishing a new colony of bacteria in the trophosome, a kind of second stomach, which convert raw chemicals such as hydrogen sulphide into food for the host. Only carnivorous species remain in highly organized social structures for long after each new set of infants has matured; chemovores and herbivores remain in loose clumps so as to be able to easily find mates, but each individual acts alone, and they exhibit no signs of social cooperation. Herdbeast technology is limited to basic stone tools.