The Segmented Animals of Amthalassa

Segment structure

As well as plant life, Amthalassa is inhabited by hydrogen breathing animal life. The physically largest and most prominent group of animals are those belonging to the phylum Plurafistulata.

They are characterised by the basic arrangement of their bodies, with multiple tube-shaped segments lined up together. Early in their evolution these tubes were almost identical to each other, but over time they became more specialised. Plurafistulates have radial symmetry, and each layer of tube segments starting from the central segment serves a specific role.

The basic form that each segment used to have is still found in many simple, single segmented animals alive today, descended from the same ancestor. In the centre of such tubes is a digestive tract, with nerves and two major blood vessels further out running across the length of the tube. These blood vesicles are surrounded in muscle that allows it to contract in a motion similar to that of a digestive tract to draw blood across, and as it does so some of this blood is forced into smaller capillaries. Around all of this is a layer of muscle tissue, and along the outside of the segment is epithelial tissue.

As segments have become specialised in more complex life, however, most of these features have become reduced, although the needed features have increased in complexity. The digestive tract, for example, is only present in the central segment of most organisms within the phylum Plurafistulata, and muscular sections of the veins and arteries of the circulatory segments have developed to the point that they have become hearts. In simpler organisms consisting of a single tubular segment, there is very little distinction between the cross section of one part of a tube and another.

Development

One of the simplest forms of plurafistulate are boneless fish-like animals that can be found in the ammonia oceans of the planet. Most are limbless and serpentine in appearance, and possess a ring of eyes radially surrounding the mouth at the front of their head. Many species have developed an extension of the muscle segments at the front of their mouths that have eventually formed tentacles for use in the manipulation of food. Some have moved away from radial symmetry by developing fins along the tops and bottoms of their bodies, with some even possessing tail fins. However, it is the more snake-like, radially symmetrical fish that segmented land animals evolved from.

The first plurafistulates on land, having evolved from limbless fish, moved in a snakelike fashion, and many similar amphibious animals still exist today. Without oxygen, Amthalassa lacks an ozone layer, and as such the surface is exposed to high amounts of UV radiation in modern times. However, since the hazy tholin layer in the upper atmosphere was very thick at this point in the planet’s past – a product of the interaction of atmospheric methane with sunlight – these creatures needed very little protection against solar radiation. This layer blocked out not only UV light but also a great deal of the visible spectrum, making the planet a fairly dim place at this point in history. Weak against sunlight, these animals tended to retreat to darker areas at points in history when this tholin layer was thinner, and even today their most basal descendants still prefer darkness.  

Since Amthalassa has usually been quite forested, and was especially forested during the damp period of history when plurafistulates first moved to land, many animals took to the trees (plants during this period mainly used infrared radiation for photosynthesis (although they moved toward the visible spectrum later on), so the tholin layer wasn’t as much of an issue for them as it would otherwise be). While many tree “snakes” evolved during this period, some species of the early snake-like amphibians adapted to use their facial tentacles for locomotion. As their tentacles elongated, these tree dwelling amphibians became more octopus-like in appearance.

Most large land animals evolved from a group of these tree-octopuses that developed a hard exoskeleton as a response to a much drier and brighter point in the planet’s history. This clade is called Exoskeletida. With this exoskeleton, they were able to walk on land using what were once their tentacles as reasonably sturdy legs.

These exoskeletons allowed this group to quickly diversify since, in addition to the support it gave their bodies, their exoskeletons also allowed them to survive in a wide range of conditions, regardless of the amount of light they were exposed to or the level of humidity. They were able to survive in arid steppes just as they could survive in the now shrinking jungles their ancestors were previously limited to; the lack of tree cover no longer posed an issue and their exoskeletons protected them from losing moisture.

Exoskeletida anatomy

Vertical cross section of a typical exoskeletid

Digestive system

The single innermost tube of an exoskeletid serves the function of digestion. At the bottom of the tube is an oral opening. Many species have developed inner tongues, a series of tentacle-like extensions of the muscles of the central tube, for assistance in the swallowing of food. There is a short oesophagus, with sphincters at either end, leading to a stomach where food is broken down. Since ammonia is more alkaline than water, the stomach acids don’t have to have as low a pH and even a pH of 7 is considered acidic to life on Amthalassa. Above this is an intestine that stretches across the tail present at the top of the animal’s body. The intestine eventually leads to an anus or cloaca at the end of the tail.

The digestive organs are surrounded by a layer of energy storing tissue which primarily contains alkynes, nitriles, and various unsaturated hydrocarbons. There are two primary blood vessels running through this, one vein and one artery, often with a number of tiny “heartlets” to assist in the pumping of blood. There is one primary nerve in this segment, which branches out into numerous smaller nerves. Outside of the alkyne tissue is a thin layer of muscle, which is surrounded by epithelial tissue.

 

Cardiorespiratory system

Surrounding this innermost segment are a series of cardiorespiratory segments, varying in number depending on species. At the bottom of each segment is a lung, capable of drawing in and expelling air from the contraction and relaxing of muscles in the muscular layer of the segment. There are two entrances to the lungs; openings leading to the mouth and a “nostril”. Each opening is connected to a different side of the lung, the nostrils to the top of the lung and the one in the mouth to the bottom. The nostril actually originally evolved from the gills at the sides of their fish-like ancestors bodies, with ammonia entering through mouths of these fish as they swim and flowing out through what is now their nostrils. Certain species developed a pumping method to draw air into their mouths and out of their gills in order to respire more effectively, some even going as far as using these gill pumps as a means of locomotion via jet propulsion. It is these gill pumps that later became the lungs of land animals, who can now breathe either in or out from both openings.

Above the lungs are a series of hearts running along the tube segment. The one closest to the lung is the primary heart, responsible for pumping dehydrogenated blood towards the lungs and hydrogenated blood to the legs and brain. The other hearts pump blood to the various organs and to muscle outside the legs, such as the tail, and also pump venous blood back to the primary heart.

This segment also contains a large, primary artery, as well as a larger primary vein, both of which run through each heart. These are the largest blood vessels of the body. Most larger organisms cannot obtain enough energy purely from hydrogen dissolved in the blood plasma, and as such make use of hydrogen carrying substances. In the case of most plurafistulates, this is an iridium based compound similar to a substance called Vaska’s complex known to Earth scientists. It is yellow or orange in colour and carried by blood cells.

Like all segments, there is a layer of muscle and then epithelial tissue on the outside.

After hydrogen is breathed in, a mixture of methane, ethane, and ammonia is exhaled as by-products of respiration.

Nervous system

Surrounding the cardiorespiratory segments are segments containing the central nervous system. While all segments have one primary nerve running through them, these segments have a much more developed one, the plurafistulate equivalent of a spinal cord. Early in the evolutionary history of Plurafistulata, the nerves at the bottom of these segments bunched together into ganglia and then eventually evolved into a brain. In spite of the fact there are multiple nervous segments, in the exoskeletid body there is only a single brain; the brain tissue of each segment is connected to the brain tissue of every other segment, so that it forms a single, ring shaped multi-lobed brain.

The epithelial tissue on the outside of the part of the segment surrounding the brain is very hard and chitinous – and in some cases has undergone ossification – in order to protect the brain. While not as hard as the outer exoskeleton, this still serves to defend it against damage.

Horizontal cross section of an exoskeletid

Muscular system

The outermost segments of exoskeletids are the muscular segments. The muscle that originally surrounded each segment has become so developed so as to fill the entire tube. This allows for movement of the tail, and is especially important in non-exoskeletid plurafistulates that need it for structural support. The snake-like amphibians depend on this tail muscle for locomotion.

Like all segments, these muscle segments contain a major vein and artery and a number of small heartlets. In the case of the muscle segment heartlets, their pumping can be assisted by the movement of muscle during times of intense activity.

These segments extend further than all other segments in exoskeletids and many other plurafistulates,  beyond the mouth, forming legs (or tentacles in the case of non-exoskeletids). In exoskeletids these legs are surrounded by jointed exoskeleton, providing support and allowing the organism to walk. Since these legs are an extension of the muscle segment, not only are they muscle filled, but they are also equal in number to the amount of such segments the organism possesses. Many species possess toes at the ends of these legs, which initially evolved when the tentacles of their ancestors split and branched at the end for better grasping of trees or finer manipulation of food.

On the outside is a second layer of alkyne, alkene and nitrile tissue, surrounded by skin and then the exoskeleton. The exoskeleton is composed primarily of a substance close to chitin as well as, in hardened areas, calcium carbonate – although this calcium carbonate is part of a tissue with a specific structure that allows it to become much harder.

Sensory organs

Structure of a typical plurafistulate eye

The surface of their exoskeletons (as well as the skin of non-exoskeletids) possesses numerous sensory hairs, allowing them to feel touch even on the most armoured parts of their bodies. In many amphibious and fish species some of the sensory hairs on their tentacles, where these hairs are most concentrated (as the tentacles are used as feelers), have become sensitive to vibrations. In exoskeletids the area around these hairs have become increasingly concave for protection to the point that they are located within an internal cavity. These holes, which are located on their feet, serve as ears.

A line of eyes surrounds the bodies of exoskeletids, giving them 360 degree peripheral vision in a horizontal plane. Of course, objects can still be out of their sight above or below them, and the amount of vertical peripheral vision varies from species to species. For example, those living in flat, mossy plains have very little need for vertical peripheral vision, perfectly comfortable with their ability to see clearly in every horizontal direction. However, vertical peripheral vision tends to be high in species inhabiting dense jungles, where tree dwelling organisms may be lurking above.

The number of eyes an animal possesses is usually equal to the number of limbs, with one eye per outer segment. 

The actual structure of the eye shares a degree of similarity to the camera eyes of Earth and many other planets due to convergent evolution. However, due to their independent development, there are a number of significant differences. The lens, for example, is made of calcite, much like the eyes of trilobites. There is not just one of these lenses, but two; one outer and one inner. While human eyes focus light by changing the shape of the lens, this is not possible to do with the harder calcite lenses of plurafistulates. Instead, the eye focuses by changing the position of the lens, much like cephalopod eyes. However, it functions differently to a cephalopod eye in that it is a second lens inside the eye that moves. As this inner lens changes position relative to the outer lens, the image changes focus. The inner lens is attached to a membrane separating two halves of the eye, and its position is changed by a series of muscles attached to it, in addition to openings in the membrane that allow fluid inside the eye to move from one half to the other. Not only can the inner lens move towards and away from the outer lens, but it can move slightly side to side and change angle.

In most species, the actual eye itself isn’t attached to muscle and cannot change position, but with 360 degrees peripheral vision many species don’t need to move their eyes that much. Those that need to see above and below themselves, for example species living in jungles, have to move their bodies slightly to look around. Slight movement may also be done by mossland species just to bring already visible objects into better focus, especially if they’re nearby.

Mouths

Most species of exoskeletids have a sheet of skin and muscle underneath their mouths to prevent food from falling out while it is being chewed. This extends from their muscular segments. There is a hole in the middle of this sheet of skin held shut by a muscular sphincter.

Inside the mouth are many tentacle-like tongues. The inner tongues, mentioned earlier, are part of the digestive segment, and are used primarily to assist in the swallowing of food. The outer tongues are radulae attached to the muscular segment and perform a wider range of functions; namely the manipulation of food and for chewing. As the oral sphincter opens they grasp food and bring it into the mouth, then begin grinding the food up with the mouth closed using the many chitinous teeth on each tongue. The radulae also usually possess sensory hairs capable of distinguishing taste. Food is often tasted before bringing it into the mouth in order to assess it for nutritional value and to ensure what’s being eaten is edible.

Reproductive system

Most groups of exoskeletid are hermaphroditic and reproduce sexually. Their sexual organs are located in their tail, within the central digestive segment, although the uterus is lower down where the body is thicker and there’s more room for it. There is a canal connecting the uterus to the mouth which eggs pass through.

Many species mate by rubbing their cloaca together, although there are some species that have developed penises in their tails. Reproduction can only usually take place after mating; although hermaphroditic, very few exoskeletid species can self-fertilise.