Tetrastomes

Multicellular life on Nemoros doesn't easily fit into the three kingdoms of Earth life. Many mobile organisms - those belonging to the kingdom Mobilida specifically - still engage in chemosynthesis and often even photosynthesis, possessing the "chemoplasts" of more plant-like organisms. The chemoplasts are likely to have evolved as a result of a symbiotic relationship with microsomes, organisms belonging to a domain of single celled chemosynthesisers. These chemoplasts can also engage in hydrogen respiration, so are essential for supporting the active lifestyle of mobilids. These animal-like organisms actually have far more chemoplasts than "plants" do, although they are often lower in melanophyll (the main photosynthetic pigment on Nemoros), instead being more specialised towards their role in respiration. 

A typical tetrastome

Most large mobilids belong to the group Tetrastomata. Tetrastomes are characterised, at least basally, by tetrahedral symmetry, four long limbs, the presence of mouths at the end of each limb, and feather-like filaments used in digestion.  

Spherical or near spherical symmetry is quite common in the seas of Nemoros; the heavy plant life prevents fast movement, so for most floating organisms there is little evolutionary pressure for a differentiated top or bottom, or front and back. For most bilateral organisms, gravity provides the up-down distinction, whereas the direction of movement provides the front-back distinction; pressures that aren't nearly as strong in the oceans of Nemoros. 

Circulatory and respiratory system

Like most  Nemorosan "animals", tetrastomes engage in hydrogen respiration. They obtain hydrocarbons from other organisms, eaten with their four mouths, from which energy is obtained through reactions with atmospheric hydrogen taken into the body. Unsaturated hydrocarbons with double or triple bonds are where most energy is obtained through this process, especially acetylene which many plants use for energy storage. Nemorosan respiration produces methane as a byproduct, which replenishes the methane absorbed by plants during photosynthesis and chemoautotrophy.

Tetrastomes have methane filled tubes throughout their bodies used for the transport of nutrients and gasses. There are numerous pumps in the middle of their body used to drive the movement of this circulatory system. Their circulatory fluids contain no hydrogen carrying substance, so they're only able to carry whatever hydrogen will dissolve directly into the methane. 

Tetrastome internal anatomy. Each limb shows a different organ system;
in reality, all organs are present in all limbs

The vast majority of tetrastomes lack lungs, instead exchanging respiratory gasses through the skin. This is inefficient, but given the dense atmosphere and the relative unenergetic nature of most tetrastomes this poses few issues.

Digestive system

Unlike the vertebrates of Earth, tetrastomes don't have a complete digestive tract. Instead of beginning at one end of the body end exiting through the other, once food is digested it comes back up the digestive tract and out through the same opening it entered. Because of this, tetrastomes have to wait until a full cycle of digestion is complete before eating again. 

After food enters one of the mouths, it goes through the "feather stomach" and into a second stomach further back used to chemically break down food. Once the food has been sufficiently dissolved and chemically altered, it comes back into the feather stomach where the tendrils absorb all nutrients from the liquid into the bloodstream. This process can take some time, and once it is completed the food is regurgitated. 

The feathers of the feather stomach are quite agile. They're able to leave the body through the mouth to manipulate food, and can expand in size under hydrostatic pressure. Some nutrients are absorbed outside of the body this way, especially if they need little digestion.

Muscular system

Tetrastomes have two different types of muscles working together by opposing each other. Each contracting muscle is also accompanied by an extending muscle; a muscle that provides movement by pushing instead of pulling. 

The extending muscles extend with hydraulic pressure, supplied by the circulatory system. The contracting muscles, however, don't truly contract; they're actually a type of shape-memory tissue that curls in a spiral spring-like pattern. When opposing contracting muscles and the accompanying extending muscles stretch out the coils, they're able to return to their original shape by being exposed to certain chemical conditions initiated by the nervous system. A similar type of tissue is present in the hearts of tetrastomes, although they don't form a spiral, instead bending and creasing to change their internal volume and cause a change in fluid pressure. 

Reproduction

In their larval stage, Tetrastomes are tiny, spherical creatures. They are often released into the sea or air as spores, only growing larger later on far from their parent. The vast majority of tetrastomes reproduce sexually, finding a mate while still in this larval stage. Once a suitable mate is located they permanently attach to them, eventually forming a large chimeric adult with a roughly homogeneous mixture of cells from both individuals. 

Chimeric tetrastomes only mix their genes when flowering; the rest of the time the two genotypes remain distinct. This "flowering" is a periodic event where sexually mature chimeras grow reproductive organs over their bodies, which they completely lack most of the time. In these "flowers" larval non-chimeric tetrastomes are produced, possessing a mixture of genes from both "parents" (who are really a single chimeric organism). When released, they spread via wind dispersal if the organism is land based, or drift through the methane if the organism is aquatic. Eventually, they will find larvae released from other individuals to bond with, and the cycle continues. 

In many more advanced organisms, chimeric adults play a role in finding suitable mates for their larvae to bond with. Higher tetrastomes can be far more intelligent than their larvae, so this greatly helps in ensuring their offspring fuse with only the best mates. In most species that do this, the chimera finds a healthy mate that meets its specifications, and engages in cross pollination with it, trading larvae. Not long after the process is complete, the now chimeric larvae are released into the air. However, some species do care for their young and instead plant them somewhere they can guard. 

Most advanced tetrastomes are also capable of asexual reproduction, growing chimeric flowers that produce chimeric larvae containing cells identical to each "parent". Whether an individual will prefer to reproduce sexually or asexually depends on how well it is thriving, and can change in response to various factors like food availability and frequency of injuries. This way, evolution occurs faster; if an individual has particularly good genes, they will spread rapidly in asexual copies, whereas in sexual reproduction there's a chance of the specific genes governing these advantages being lost in their offspring. On the other hand, if an individual is particularly ill suited to the environment, sexual reproduction offers a greater chance that at least some of their offspring will be fitter. Also, it ensures that when a population needs to change quickly to adapt to new conditions, there is greater variation within the population to select from, whereas once they have become optimally adapted change is slower and the gene pool is relatively stable. 

Senses

Most tetrastomes are blind, due to the lack of light that reaches into the oceans the first tetrastomes evolved in. Instead, they navigate using sonar, using the same organs utilised by many lineages for jet propulsion, a trait that has been retained by almost all groups even after moving onto land. Even with enough light on the surface to see, their sense of echolocation was already so well developed there was little evolutionary pressure for the development of vision. The fact that the moon's atmosphere is so dense contributed to this, as echolocation is more efficient in denser atmospheres than thinner ones.

At the end of each limb are three horn-like protuberances, jointed at the base, which vibrate in response to sound waves. These are their primary hearing organs. They are attached to extremely sensitive sensory organs that can detect even the slightest change in position. 

Tetrastome diversity

Although the most primitive tetrastomes have tetrahedral symmetry, many lineages, especially those that live on the sea floor or on the ground, have moved away from this ancestral form. The most noteworthy are the trilaterians, who use three of their limbs for walking, and mainly the forth for eating. While digestive organs do still exist in the legs, they are much less developed than those of the "head". This way, trilaterians have moved away from their ancestral tetrahedral symmetry, and instead posses triradial symmetry. 

Unlike some other worlds with low-oxygen atmospheres, like Amthalassa for example, surface tetrastomes have no need for an exoskeleton or even scales to protect them from UV radiation. While the moon does lack an ozone layer, the tholin layer serves a similar purpose, blocking most ultraviolet light from reaching the surface and protecting life from its harmful affects. The fact that they typically breathe through the skin also provides an evolutionary hindrance to the development of a protective covering. Most also lack endoskeletons, since under the low gravity there's little need for structural support for all but the largest organisms. 

The earliest tetrastomes possessed swim bladders, allowing them to remain buoyant in the methane oceans they inhabited. While it was originally filled with a heavier mixture of gasses, many tetrastomes, especially more "advanced" ones like trilaterians, have swim bladders filled with hydrogen. With the moon's thick atmosphere, this allows them to swim through the sky as if they were swimming through the sea; in fact, there are many organisms lacking these hydrogen swim bladders in the skies, staying airborne through their own muscular effort with relative ease. Often, they will look more like fish than birds.