The planet’s largest animals, especially on land, belong to the phylum Hydratozoa. While most higher groups have become bilateral, they basal body plan is radially symmetric, with the most obvious trait shared by such primitive hydratozoans being the compound eye ring surrounding the mouth. They usually also have multiple togues for catching food, and their mouth is positioned at the top of the body with an anus underneath. They have a water-vascular system somewhat similar to those of the echinoderms of Earth, which functions as both a respiratory and circulatory system as well as aiding in movement. They’re boneless, supported entirely by hydrostatic pressure, but more advanced groups have developed skeletal supports.
The first bilaterally symmetric hydratozoans looked something like a slug or sea cucumber, with their bilateral symmetry coming from the fact that one half of their body is adapted to sitting on the ground. Many such hydratozoans still exist today. A specific clade of these slug-like hydratozoans with an endoskeleton – mainly used for protection – eventually gave rise to more fish-like organisms capable of swimming, which later moved onto land.
The majority of terrestrial hydratozoans belong to the clade Tripoda. Although some of the more slug-like hydratozoans also moved onto land, most are very small and unable to compete for the same niches as tripods. Tripods descended from “fish” with only two paired fins, and as such there are no tripod species with four or more limbs. However, the tail has developed into a third limb, allowing them to have up to three legs – although there are many bipedal, legless, and even monopodal taxa. The first members of the clade are believed to have been tripedal, hence the name, although the line between a third leg and a heavily specialised tail can be very blurred.
Spinoptilite anatomy
Spinoptilita is a very successful group of tripod, with an active lifestyle, erect limb posture, and a type of warm-bloodedness that allows them to keep different parts of their body at different temperatures. Many of the features of spinoptilites are shared by other tripods, so this should give a good idea of general tripod biology.
Spinoptilites belong to a large clade called Sucodermata, characterised by thick skin protecting them from desiccation. This allows them to live further from bodies of water, although more basal groups still depend on water for reproduction. Most tripods outside of this group dry out very easily, and will shrivel up and die in contact with salt similarly to a slug. The vast majority of fully terrestrial hydratozoans belong to this clade.
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| Internal anatomy of a spinoptilite. The species pictured is the Occasian fin-backed tarus (Tilusa nusulu). |
Skin
The most obvious defining trait of spinoptilites is the presence of spiny integument on their bodies. These spines can vary in length, density, and distribution, and serve a number of functions. Defence was likely the original reason these spines evolved, but they serve a very limited defensive purpose in most groups. They also break up an animal’s silhouette, helping them escape the notice of predators or prey more effectively; this is especially true of species with longer spines. The least obvious but perhaps most common function they serve is in temperature regulation; each spine has a large blood vessel, and increasing the flow of blood into the spine can help cool the animal by allowing heat to pass out. Spines are usually thinner on one side to allow more heat out, which actually limits their defensive ability.
As sucoderms, the skin of spinoptilites is thick and very good at holding in moisture. It is also very flexible, and contains a layer of muscle underneath that allows it to change shape slightly or tense up. These muscles aren’t attached to the skeleton, and only seem to serve a role in changing skin texture or shape for better camouflage, as well as protecting the animal. It can also help an animal balance by changing the distribution of muscular fluids throughout the body, modifying the centre of gravity.
This layer of muscle means that developing tentacles isn’t that hard along fairly short evolutionary timescales, and they have evolved many times among tripods as a whole.
This layer of muscle means that developing tentacles isn’t that hard along fairly short evolutionary timescales, and they have evolved many times among tripods as a whole.
The skin of all tripods consists primarily of a protein that has been labelled by human researchers as cupitin. The properties of cupitin are similar to that of keratin, but it’s quite different in structure. The skin consists of outer layers of cells whose cell walls consist of the protein, underneath which is tissue consisting of wall-less cells.
Skeletal system
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| Skeleton of the fin-backed tarus |
Tripod bones consist largely of silica, rather than the calcium phosphate of vertebrate bones, and tend to be much stronger. Rather than possessing any bone marrow equivalent, that function is instead taken up by a specific organ, with the hollow interior of bones being used as a long-term energy store in the form of starch. Other than these differences, the bone tissue of tripods is very similar to that of vertebrates in terms of structure and function.
Their skeletal structure differs from a vertebrate’s in a number of ways. The first hydratozoan skeletons consisted of a internal “shell” with a number of interlocking plates, which was very effective at protecting the internal organs. However, in larger and more active animals such a skeleton can weigh them down, so many groups have developed a lighter skeleton with more gaps and holes.
In most sucoderms the skeletal plates of the torso have developed into thinner “ribs”, but have maintained triangular processes that interlock with the rib behind it to provide the skeleton with more strength than it would have otherwise. These processes have roughly triangular holes in them to reduce weight without sacrificing strength too much. The fossil record shows that rather than developing from projections growing from a more vertebrate-like rib, the processes are actually what remained of the broader skeletal plate as it lost bone. The ribs attach to a spinal column running underneath the body, rather than the back as in vertebrates.
The bones of what can be considered the skull have retained their ancestral shape, however, offering the brain a greater degree of protection. The front legs attach to the skull onto a specialised “shoulder blade” plate. The skull lacks any kind of jaw, with a mere opening for the oral proboscis.
The rear leg is quite different anatomically from the front pair. While the muscle is on the outside of the front legs, the rear leg has much broader hollow bones with space inside for digestive organs, muscle, and other tissues typically found in the torso. This is due to its development from the rear of the animal, with each rear-leg bone being homologous to the skeletal plates or ribs of the torso and skull.
The skeleton pictured is of a fin-backed tarus, which bears a number of distinctions from other spinoptilites. Two of the dermal spines have become ossified, and have attachment points on the skeleton, which can be seen in the image. They also have a horn at the tip of the skull, and the processes of the rear ribs have been greatly extended to protect and support the enlarged primary hydraulic pump.
Nervous system
The nervous system of hydratozoans, and most animals on Xenosulia in fact, is bioluminescent rather than electrochemical. This is far from uncommon on other planets; however, whereas most bioluminescent nervous systems encountered elsewhere involve the production of light at one end of the nerve, which is transferred across a kind of biological optical fibre, the nervous system of hydratozoans works differently. Instead, there is a kind of relay system where light is sequentially produced across the nerve.
Hydratozoan nerves are filled with photosensitive light releasing chemicals. When light is produced in one part of a nerve, it stimulates the release of the same chemical compounds further down the nerve; this bioluminescence only lasts for a brief period, which is followed by that part of the nerve becoming fatigued, unable to light up again until it’s recovered. This way, the part of the nerve that’s lit up is prevented from moving in reverse. Instead, it will continue to trigger the next portion of the nerve fibre to light up until the signal has run across the whole length.
More primitive animals have to rely on the diffusion of chemical signals throughout the body; these chemicals are similar to the bioluminescent chemicals in hydratozoan nerves, which they’re likely ancestral to.
The central nervous system consists of a brain at the front of the body and a nerve cord running through the middle. Since it’s located deeper within the body than the spinal cords of Earth’s vertebrates, it doesn’t need as much protection, with a flexible sheath covering it. Nerves branch from the primary nerve cord to the rest of the body, although the oral proboscis has its own distinct nerve cord. At the end of the oral proboscis is the oral ganglion, used for processing sensory information from the proboscis.
Muscular system
On a basic level, the muscles of hydratozoans work in a superficially similar way, contracting to generate force. However, at a closer look the way this is accomplished can be seen to be quite different. Rather than fibres of myosin and actin sliding together as in Earth animals, the muscle cells of hydratozoans and many other Xenosulian animal phyla are pneumatic. They work by increasing internal pressure, forcing the stretched-out cells to become more spherical and contract. On larger scales this looks similar to vertebrate muscles, with contracting muscle tissue becoming shorter along one axis but swelling in others.
The increase in pressure is caused by a release of gasses, mainly oxygen, from a protein contained within specialised organelles found in muscle cells, which forms bubbles. This realise is triggered by the same chemicals used in conveying nerve signals, and muscle cells are in fact homologous to nerve fibres and able to transmit light in the same way. The gas molecules bind back to the protein soon afterwards, reducing cell pressure again and allowing muscles to stretch.
While the above is true of the majority of motile animal life on Xenosulia, the muscular system of advanced hydratozoans has a number of features that sets it apart. Most notably is a system of hydraulic pumps adapted from the ancestral water-vascular system, which is linked to the cardiovascular system. The muscles themselves have both parallel and traverse fibres, allowing them to both contract and elongate. The pumps increase the fluid pressure in the muscles, which allows them to elongate even more forcefully. There is a hydraulic pump for each leg, as well as a primary hydraulic pump larger in size than the rest; this primary hydraulic pump is supplied with deoxygenated blood from the veins. This hydraulic muscular system gives tripods more strength than would otherwise be possible, but it can’t be used as dependably over long periods as normal contraction because of the need to build up pressure.
Circulatory and respiratory systems
The circulatory system of all “higher” hydratozoans is derived from the ancestral water-vascular system, as is the respiratory system. They have a closed circulatory system, with the blood contained in veins and arteries that branch and deliver blood to all the body’s tissues. The lung, as well as the hearts, are derived from the water-vascular system’s muscular pumps, just as the hydraulic pumps are. Unlike the water-vascular system, the circulatory system is closed off from the surrounding water (or air) in the outside environment, and contains fluid quite different in composition rather than just consisting of the surrounding seawater.
The single lung is derived from the gill pump of the more fish-like ancestors of tripods, and is located in the animal’s back. Numerous paired spiracles on the animals back are used for breathing, and while breathing in is generally passive (although some derived groups employ active inhalation), exhaling air is an active process. This is accomplished by the contraction of a wall of muscle that surrounds the lungs, forcing air out.
Each trachea has a sphincter at the base that allows for the control of airflow, and many groups have an additional set of sphincters at the spiracles, especially aquatic groups that might want to prevent the trachea from filling with water. In spinoptilites, there are also vocal folds present in the trachea, allowing for the production of sound. Many other groups have developed similar mechanisms of vocalisation independently.
There are numerous hearts, most often small, located throughout the body, but they fit into three main groups. One group, the arterial hearts, are located ventrally, along the animal’s chest and abdomen, and encased within the spinal column for protection. The arterial hearts are responsible for pumping oxygenated blood from the lungs. These are usually the smallest hearts, and are the reason why some vertebrae are larger than the others (since those are the vertebrae which contain a heart). The arterial hearts can number anywhere from seven to twenty, occasionally even more, and there are usually twice as many vertebrae as there are such hearts.
To pump blood, the arterial hearts successively contract in waves, from front to back. Each arterial heart is single chambered, so they depend on this sequential worming motion to work effectively. The arterial hearts of tripods and other “advanced” hydratozoans are derived from the major ventral artery of the more primitive slug-like hydratozoans, which pumped blood through similar waves of contraction. This blood vessel developed areas where muscles were more concentrated which over time became a long chain of hearts.
A second group of hearts, the lung hearts, pump deoxygenated blood through the blood vessels that branch through the lung, increasing the efficiency in which oxygen is taken up by the blood. The two lung hearts are located just behind the lung and act together as a single two-chambered heart. They are often, but not always, larger in size than the arterial hearts, likely because there are less of them – although in terms of total muscle mass they are collectively smaller.
Unique to spinoptilites and entomopterites are the cranial hearts, which are responsible for pumping oxygenated blood directly to the brain. These are located just below and behind the brain, and like the lung hearts they consist of a pair of single chambered hearts that act together to pump blood.
The blood itself contains pellet-shaped blood cells which use the protein hemerythrin to transport oxygen. It’s likely that hemerythrin outcompeted other oxygen binding proteins, such as the hemoglobin used by vertibrates, due to its immunity to carbon monoxide poisoning. Not only are levels of carbon monoxide higher on Xenosulia than on Earth, but the geological record indicates it was far higher at certain points in the past. Hemerythrin isn’t the only oxygen binding protein used, however; chlorocruorin is also suspended directly in the blood, with most tripods having a dual system of both proteins, although many groups have lost either one or the other. Blood cells are produced in a specialised organ, rather than from bone marrow, which also plays a role in the immune system.
Hemerythrin gives the blood of most tripods a purplish colour when oxygenated, although blood colour can vary depending on the varying levels of hemerythrin and chlorocruorin. Those with low or no hemerythrin typically have green blood, and the brightest purples are seen in blood lacking chlorocruorin.
Digestive system
The digestive system begins at the oral probiscis, present in almost all tripods, used for the gathering of food. In spinoptilites, much digestion takes place within the proboscis itself, but in many other groups these digestive organs are greatly reduced and the proboscis can be retracted into the head. The skin of the proboscis is generally more elastic than in other parts of the body, allowing it to stretch in length for more efficient food gathering. The presence of such a proboscis is likely why so few tripods have developed necks, in addition to the wide field of view the compound eye band offers.In side the mouth of the proboscis is a tooth covered radula, although this is lost in one of the two main spinoptilite branches. This tongue-like radula, present in almost all non-spinoptilites, is used to scrape up food matter through an abrasive “licking” motion. There is also a salivary gland, which is capable of producing very acidic secretions; this secretion is especially concentrated in the spinoptilite clade lacking a radula. In such spinoptilites, feeding is accomplished by first spitting acid onto their food, and then consuming the partially liquified biomatter. A dependence on this method of feeding is likely what led to the loss of the radula. Tripods basally lack any kind of jaw, spinoptilites included, and liquivory was developed to make up for this – however, jaws were developed independently in multiple tripod taxa.
Once food has left the mouth it enters the oesophagus, which leads to a muscular gizzard. This organ is lined with tough cupitinous teeth and grinds up food through a series of contractions, although there are certain groups that lack such teeth and need to swallow stones. The gizzard is usually located near the mouth of the proboscis, but in the animal pictured above, the fin-backed tarus, it has been pulled back into the skull.
At the base of the proboscis is an anterior stomach, which releases enzymes involved in the breakdown of food and plays a role in the absorption of water into the blood. After it has been processed by this stomach food enters the crop which stores food; it is also involved in fermentation in many herbivorous species.
The crops leads to the posterior stomach, where sulphuric acid is released for the breakdown of food. There are also enzymes involved, but many of the enzymes found in the anterior stomach can only survive in the presence of the weaker acid present there.
Beyond the stomach is the intestines, which absorb nutrients as well as playing a role in the further breaking-down of food, which leads to a tube known as the cnemic passage which makes its way through the rear leg. In most groups the cnemic passage is straight, but in many herbivorous groups like that fin backed tarus pictured above, this has developed into a secondary intestinal tract for the further breakdown and absorption of food. The cnemic passage leads to an anus at the back of the foot, where waste is excreted. This waste tends to be quite dry, with most excess water excreted out of the oral proboscis.
Nitrogenous waste is extracted from the blood by a kidney-like organ, where it is collected and stored in a specialised sac. When an individual is low on nitrogen, this storage organ releases some of its contents back into the blood. If this organ exceeds its capacity, these nitrogenous compounds enter a passage to the intestines where they’re excreted as solid yellowish crystals.
Senses
The majority of sense organs are distributed towards the front of the body, as well as the tip of the proboscis. These sense organs provide many of the same senses as those seen in the vertibrates of Earth, including sight, sound, olfaction, and taste. In addition to the specialised sense organs described below, pressure, pain, and temperature receptors and distributed throughout the skin. Pressure receptors are especially densely concentrated on the oral proboscis.Compound eye
At the front of the body is a large compound eye band; in spite of most spinoptilites having just one such eye, the fact it’s a compound eye in addition to its shape offers it a great degree of peripheral vision.
Like the compound eyes found in the arthropods of Earth, this eye consists of numerous small lenses that focus light on photoreceptor cells, with each unit unable to resolve images on their own. However, together they are able to form a mosaic of the animal’s surroundings. Compound eyes typically have a poorer resolution than the eyes of many vertebrates; however, spinoptilites and other tripods have numerous means of improving their vision. The most significant is their ability to quickly vibrate their eyes, which is accomplished by quick sub-dermal muscular movements. These vibrations are actually fast paced scans that allow the eye to pick up more visual data; visual data from each position in the scan is put together to make an even more detailed mosaic than would otherwise be possible.
Some of the other means by which vision is improved also depend on subtle movements of the eye. The compound eye is able to bend, so sub-dermal muscle can flatten the eye in certain areas so a specific object the animal is focusing on comes into better focus. The greater number of ommatidia directed towards the object allows it to be viewed with far greater clarity.
The compound eye has very good colour vision, especially the compound eyes of spinoptilites, with most spinoptilites being hexachromatic. However, these colours tend to be shifted to the red end of the spectrum since the planet orbits a red dwarf. It’s rare for an animal to be able to detect blue light, but they’re very sensitive to red light and most species can see well into the near-infrared. Although Xenosulia seems slightly dimmer than Earth to most humans, it actually receives the same amount of light because of its close distance, with a large part of that light being in the infrared and therefore invisible to humans. To an animal native to the planet, it looks much brighter, at least on the day side.
Simple eyes
In addition to the compound eye, there are numerous smaller single-lensed ocelli surrounding it. The ocelli of the head are divided into two groups; the lateral ocelli and the dorsal ocelli. These eyes evolved separately from the compound eyes, and are present in most tripods, although the ocelli of the first tripods lacked lenses. The ocelli of spinoptilites, however, do have lenses, which are primarily composed of hard and transparent cupitin. They lack the ability to resolve images, but have advantages over the compound eye such as a greater ability to sense changes in light and dark. They’re also much faster acting than the compound eye, since the neurons at the base of the compound eye photoreceptors have an inefficient mechanism for transmitting information, something that’s difficult for evolution to work around. Also, the compound eyes, while able to distinguish colour, have a comparatively poor ability to distinguish light intensity.
Sound
Sense of sound is complicated on Xenosulia by the strong winds. Sound travels more dependably through the ground, however, so more basal tripods with a sprawling posture have developed a pair of ears at the bottom of their body to pick up such sounds. There are periods of stiller weather, however, and areas more protected from the winds, so sensitivity to airborne sound isn’t useless. The ears under the body aren’t in an optimal position for picking up airborne sound; because of this, there is a second pair of ears, higher up, that serve this purpose.
Standing with a more erect posture, most spinoptilites have lost this lower ear pair, retaining only the upper ears. Raised off the ground, the lower ears serve little purpose, although in certain groups they have been repurposed for the detection of airborne sound. Because the need to detect vibrations in the ground still exists, spinoptilites have developed a secondary set of hearing organs in the feet, characteristic of the group.
Smell
While the oral proboscis, mentioned below, is able smell and taste at short distances, spinoptilites can pick up smells at much further distances using other sensory organs. Namely, olfactory receptors located inside the respiratory spiracles are used, this being an ideal location due to the rapid movement of air into and out of this part of the body. These sense organs are especially developed in certain predatory groups, where they can be used to aid in the location of prey.
Sensory organs of the proboscis
At the tip of the proboscis there are a number of sense organs too, assisting in the location of food. Numerous sensory hairs project from the tip, which not only help tactile detection but are also lined with taste receptors. The proboscis can smell, too, with olfactory slits near the mouth that are able to draw in air with the help of small internal bladders. These slits aren’t able to smell very well beyond short distances, but it is only used for finding food at a close range anyway. There are taste receptors on the inside of proboscis, too, right near the opening of the mouth.
The oral proboscis also has eyes near the end, similar in structure as the dorsal and lateral ocelli, but specialised for short range vision. Although the oral eyes of many spinoptilites lack lenses, in those where they are present these lenses are made of hardened cupitin. In spite of these lenses these oral eyes have a similar inability to resolve clear images as the ocelli do, although they’re able to see colour and in many species can make out rough shapes.
Reproductive system
Reproduction is accomplished via specialised organs in the oral proboscis. There are numerous glands near the mouth, capable of producing either male or female gametozoans; a Xenosulian term for the animal equivalent of gametophytes. These gametozoans are distinct organisms, representing the haploid generation. What is usually thought of as the actual animal is the diploid generation; in fact, all the anatomy described above applies only to them. The much larger diploid animals have cells with paired chromosomes, whereas the chromosomes of gametozoans are unpaired.
The gametozoans of most tripods are motile and radially symmetrical, with six legs covered in sensory hairs. Although tiny, they are usually macroscopic in size. However, female gamatozoans of many taxa have lost their legs and ability to move, instead remaining within the diploid parent. This is the case in spinoptilites, where mating usually consists of simply linking the oral proboscises together and allowing the male gametozoans to crawl in to the other animal and mate with it.
Soon after a male and female gametozoan mate, they die, the materials of their body being used in the initial growth of a new diploid organism with genetic material from both parents. In spinoptilites and most other sucoderms, the developing embryos migrate to a uterus in the skull.
The diploid generation of most tripods are hermaphroditic, and sequential hermaphroditism is common, especially in spinoptilites. The gametozoan producing organs in the mouth can easily change over time to be female producing or male producing, and there is nothing stopping an individual from having some produce male gametozoans and others produce females.
One feature that distinguishes spinoptilites from most other tripods is the phenomenon of eusociality with regards to the gametozoans. In addition to fertile males and females, spinoptilites also prouduce infertile males gametozoans; these infertile males play no role in reproduction, instead assisting the animal in various ways like worker ants in an ant colony. These infertile males are produced by separate specialised gametozoan glands, which are present in both male and female diploids.
Infertile gametozoans assist in numerous ways, such as cleaning the animal, distributing various oils across the body, and even serve a sensory function by probing the ground or objects. Gametozoans can also be used for defensive purposes, attacking other animals with toxins. Many live inside the parent animal as well as crawling along the skin, and early explorers initially believed spinoptilites were covered in unrelated bug-like organisms, presumed to be parasites of some kind.
Spinoptilites are oviparous, laying large hard-shelled eggs. These eggs are grown in the uterus and laid through the oral proboscis, which acts as an ovipositor. Each egg contains multiple larvae, and is lined with groves along the surface; these gaps in the shell allow air to reach the membrane below, where there are numerous stomata that are able to open and close depending on humidity. This provides the developing larvae with a steady source of oxygen. The egg’s groves gives the shell a cracked appearance, and coupled with its round shape this can lead it to resemble certain icy planets or moons like Europa or Oculia.
Spinoptilite larvae are tiny and less developed than the young of other sucoderms, lacking bones or the legs that will appear later in life. However, they do have numerous paired tube legs, supported entirely by hydrostatic pressure; these aren’t homologous to the legs of adults. Most spinoptilites care for their young.
Entomopterites
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| The entomopterite pictured is the field daymoth (Dimutili occasiensis), a fairly small fruit-eating animal - on the left is the top-down view, on the right is the underside |
Powered flight has evolved multiple times on Xenosulia, with the planet’s strong winds and high oxygen content making for ideal conditions for the development of flying animals, in spite of the thinner atmosphere. The most diverse clade of large flyers are the entomopterites – colloquially called bugbirds, batbugs, or sometimes just birds – exceeded only by the much smaller cardozoans in number of species.
This group is closely related to spinoptilites, likely a sister clade, and like spinoptilites they produce infertile gametozoans that aid in cleaning as well as other purposes. Where they differ, apart from the lack of spines, is mainly due to the presence of adaptations for flight.
Entomopterite wings
The most obvious characteristic of entomopterites is their wings, formed by the presence of a membrane of skin between each front limb and the side of the torso, with the finger of each wing greatly elongated to provide the animal with a larger wingspan. There are pads just at the base of the finger, used for walking on, with most entomopterites assuming a tripedal stance when walking on land. The wing membranes are supported by enlarged and partially stiffened blood vessels, often forming complex cross-linking patterns that divide the wing into various cells. These blood vessels can be made to become more rigid by increase their blood pressure, allowing bugbirds to stiffen their wings while in flight, and then make them more pliable to fold them up.
In most species there are horny processes on each hip supporting the wings. This allow the bugbirds to have wings with a greater variety of aspect ratios than they would if the membranes attached directly to the end of the rear leg. There are species lacking this feature that have narrower wings, but this results in the loss of the tail membrane, a trade-off that need not apply with the hip processes present to support more complex wing and tail membrane structures. Although they were initially believed to have originated as extensions of the hip, fossil evidence suggests they’re actually ossified wing veins. In many groups, development of the dermal muscle surrounding the hip spurs as well as more flexible attachments of the spurs to the skeleton allows them to actually move position, something that provides them with a greater ability to modify their wing shape.
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