Other Phyla

In addition to hydratozoans, there are numerous other animal phyla that inhabit Xenosulia. Most animals have a similar optic nervous system as hydratozoans, in addition to similar pneumatic muscle tissue, which they likely inherited from a common ancestor (in fact, in most cases genetic evidence confirms this). However, hydraulic muscles are rare in non-hydratozoans, and those on land tend to be much smaller, lacking good skeletal support.

 
This is far from a comprehensive list; there are countless animal phyla on the planet, far too many to cover here. This includes the various worm-like phyla, in addition to a great number of aquatic animals.  

Mycozoans

These organisms resemble the fungi of Earth, and are often referred to as such. Despite belonging to Xenosulia’s animal kingdom, they lack any muscles or a nervous system; evidence shows they branched off before these were developed, rather than having lost these features. 
Mycozoans are detrivores, feeding off decaying organic matter, and are especially common on the planet’s night side. On the night side, they are able to feed off of dead air-algae that has been blown over from the day side and fallen to the ground, giving them an ample supply of food. 
Like hydratozoans, mycozoans have separate diploid and haploid generations; this is believed to have been an ancestral trait of animal life and has been retained by almost all groups. The diploid generation is the primary generation, with the haploid generation only consisting of microscopic – but still multicellular – spores that are spread by wind dispersal. 

Xenospongozoa

These large sponge-like organisms are similar to the sponges of Earth, sedentary filter-feeders, but unlike the sponges of Earth there are many terrestrial linages. Wind passing through their pores can leave various particles behind, including the nutrient rich air algae. They can grow to large sizes, and are a common sight in the open savannas of Xenosulia, where they can often be almost as common as trees. Xenospongozoans are abundant on both the day and night sides of the planet. 

Like mycozoans, they reproduce by wind dispersal, releasing tiny haploid spores into the air. 

Spherozoans

Spherozoans are tiny animals who basally have spherical symmetry, but some groups have become radially symmetrical. The most “primitive” spherozoans have a body plan consisting of a central sphere with numerous equally-spaced legs sticking out in all dimensions for locomotion and the gathering of food. Their legs are typically arranged in a polyhedral formation. 

This body plan has been retained by many slow-moving aquatic groups, where distinction between directions isn’t really relevant. However, most terrestrial lineages have developed a distinct top and bottom, with the lower legs adapted for locomotion and the upper legs specialised for food gathering. This has happened in multiple different orders independently, most notably in hexahedrites and octohedrites. Sight has also developed in these groups, with eyes at the ends of the feeding limbs. 
Food is eaten through mouths in the arms, which enters the main body through an oesophagus running the length of the appendage. In most radial terrestrial lineages, only the upper limbs have retained this function, although some groups graze with their feet. 

They are covered by a flexible outer cuticle, protecting them from desiccation. There is an air filled layer beneath this cuticle, allowing oxygen to be absorbed directly into the more permeable skin underneath; holes in the cuticle – usually three per segment – allow for passage of air in and out of this layer. 

Air absorbed by the inner skin enters the circulatory system; this is an open circulatory system, in which the body cavity is filled with hemolymph rather than contained entirely in blood vessels. Suspended in the hemolymph is the oxygen carrier hemoflavin, a protein unknown on Earth that, like hemerythrin, is immune to carbon monoxide poisoning. However, it is susceptible to hydrogen poisoning. Hemoflavin is yellow in colour in both its oxygenated and deoxygenated state, and although its oxygen carrier is iron the protein’s colour comes from its structure and not the metal. 

Without a hard skeleton, the body of a spherozoan is supported entirely from the hydrostatic pressure of the hemolymph. There is a layer of muscle under the inner skin which can increase internal pressure by contracting against the hemolymph, making the animal much more rigid. 

There are countless different species of spherozoan, greatly exceeding the total number of hydratozoan species, and they occupy a niche similar to the arthropods of Earth. However, unlike many insects, no spherozoans are capable of flight. 

Unlike hydratozoans, although spherozoans have both a diploid and haploid generation, it is only the haploid generation that are motile. The haploid generation is also far more longer-lived than the relatively short diploid generation. The diploid generation are fungus-like in appearance, spawning numerous spherozoans from tiny holes before dying, and in some spherozoan taxa the haploids may produce numerous diploid “fungus” in their lifetime. 

Cardozoans

These organisms consist of a rigid external shell that bends in the middle, the two halves of the body joined by a “hinge”. The subphylum, Cardoptera, is an example of one of the handful of times in which flight has evolved. All cardozoans outside of this subphylum are aquatic, and it seems likely that cardopterans developed flight straight out of the water, with an intermediary surfacing and gliding stage. The flight of cardopterans is similar to the swimming of aquatic cardozoans, involving the flapping of both halves of the body. 

Cardozoans are known as “hingeflies”, due to the appearance of their entire body consisting of just two wings joined together at a hinge. However, this is just their outer shell, and their anatomy is actually more complicated than this. Inside the shell is an – often tentacled – worm-like organism, parts of which emerge for the purposes of feeding and mating. The eyes themselves are often attached to eye-holes in the shell itself, right at the hinge, although in the class Maculocardita there are also eyes distributed across the wings. They may have additional sensory organs that can be retracted, such as tentacles covered in taste receptors or a stalk eye, but all the senses needed for flight are available to the hingefly while its safely retracted inside its shell. The shell itself consists of a woody material, made of a substance similar to cellulose or chitin. 

Terrestrial cardozoans are usually small in size, due to limitations of their respiratory system, and show an even greater range of diversity than spherozoans. They fill a similar niche as flying insects do on Earth. 

Like spherozoans, only the haploid generation of cardozoans are motile, and the diploid generation is typically shorter lived. Diploid cardozoans appear like a woody branching fungus, often red in colour, on which growing cardozoans are often seen hanging from. Usually cardozoans are folded up at this stage of development, and may resemble seeds. They can typically fly immediately upon detaching from this branching structure.