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Description
Part 1 | Part 2 | Part 3 | Part 4

Although most of the exercise turned out to be about tube animals, I do have some examples of other multicellular life from that world.

1 - a variety of photosynthetic "bacteria" and "algae" may grow into simple floating bubbles.
2 - fungus-like organisms that look like spiderwebs (and some spider-like creatures may in fact live in symbiosis with them), may be both photosynthetic and predatory. Some of them also make bubbles to float instead of attaching to something (3).
4 - simple seaweed, related to the fungus-webs but their hyphae fused into a more plant-like shape. More advanced ones form a "stem" by twisting themselves (5).
6 - a wide variety of lichen-like organisms grow on the rocks.
7 - there are also stromatolite-like thingies, but rather than simple rock they have a variety of shapes.
The most prominent group of "plants" are balloon algae (8), which fill themselves with desalinated water to float and also prevent parasites from living in their hollow bodies (and be a source of fresh water for people stranded on this ocean world, of course). While most of them are more or less tubular or round (9-11), some are instead flattened, with their cavity becoming a network of vessels (12). Some of these algae may even become carnivorous, adapting to live in the lightless depths (13).

There`s also a group of "plants" that I thought about but then forgot to draw and only remembered now - armored algae, with a tough plastic exoskeleton. Their appearance could vary from something like horsetails to literal blade grass.
Also, the colors of the various flora would depend more on the depth they live at (from blue at the surface to red in the twilight zone) than their taxonomic position.
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And there is also another group of "animals" totally unrelated to the tube ones - they`re related to the balloon algae instead. They`re actually more similar to Earth animals in that their basic form (1) is a gastrula-like sack with a single orifice.
I`m not sure if the balloon algae are one-layered like a blastula and this sack evolved from them via gastrulation, or if they`re two-layered and instead evolved from the gastrula that lost its mouth.
Regardless, the simplest of these sack animals are exactly that, sacks (2). Today they only survive as symbiotic living nests for a variety of bugs.
A more interesting group is this world`s answer to cnidarians (3) - these things are like polyps, but with trap-like jaws instead of tentacles, supported by a very simple endoskeleton (though exoskeletal elements are also widespread among them). Some of these trapjaws take on a more wormy shape (4).
Like cnidarians, these thing form colonies by budding, though these colonies are often more bryozoan-like (5). Some of them are also floating (6).
7 shows a colonial organism that functions somewhat like a sponge - small heads inhale water that is filtered in the shared gut and exhaled through the big mounth on top.
Like their algal relatives, many trapjaws become photosynthetic (8). Turning jaws into a pair of leaves is the most common arrangement, but not the only one. The colonial forms of these leaf-jaws (9) are the only organisms of this world that may be considered "trees". This drawing also shows reproductive zooids on top (trapjaws usually release gametes through their mouth). There are also species that retain the ability to develop zoids with proper jaws to defend themselves and even supplement their diet with meat.
Another interesting group are clamjaws, whose jaws are armored and shell-like (10). They usually build tubes to hide their soft body, leaving only the "clam" part outside, and have a bunch of stalked eyes. While they don`t form large colonies, they can still bud. 11 shows a filter-feeding clamjaw with a smaller secondary head serving as an exhalant orifice/anus.
Growing another head is, in fact, the easiest way for these creatures to develop an anus, as demonstrated by the worm-like organism at 12.
However, some creatures aren`t content with just an anus. The hydras (13), the most advanced of the trapjaws, have not only feeding, excretory and reproductive zooids, but also the ones specialized for breathing and functioning as internal organs such as the heart (most trapjaws have a simple gastrovascular system, so of course the heart would be repurposed from a zooid`s gut).

Alright, I think that`s all I`ve got for now.

Description
Part 1 | Part 2 | Part 3 | Part 4

Although most of the exercise turned out to be about tube animals, I do have some examples of other multicellular life from that world.

1 - a variety of photosynthetic "bacteria" and "algae" may grow into simple floating bubbles.
2 - fungus-like organisms that look like spiderwebs (and some spider-like creatures may in fact live in symbiosis with them), may be both photosynthetic and predatory. Some of them also make bubbles to float instead of attaching to something (3).
4 - simple seaweed, related to the fungus-webs but their hyphae fused into a more plant-like shape. More advanced ones form a "stem" by twisting themselves (5).
6 - a wide variety of lichen-like organisms grow on the rocks.
7 - there are also stromatolite-like thingies, but rather than simple rock they have a variety of shapes.
The most prominent group of "plants" are balloon algae (8), which fill themselves with desalinated water to float and also prevent parasites from living in their hollow bodies (and be a source of fresh water for people stranded on this ocean world, of course). While most of them are more or less tubular or round (9-11), some are instead flattened, with their cavity becoming a network of vessels (12). Some of these algae may even become carnivorous, adapting to live in the lightless depths (13).

There`s also a group of "plants" that I thought about but then forgot to draw and only remembered now - armored algae, with a tough plastic exoskeleton. Their appearance could vary from something like horsetails to literal blade grass.
Also, the colors of the various flora would depend more on the depth they live at (from blue at the surface to red in the twilight zone) than their taxonomic position.
___________________________________________________________________________

And there is also another group of "animals" totally unrelated to the tube ones - they`re related to the balloon algae instead. They`re actually more similar to Earth animals in that their basic form (1) is a gastrula-like sack with a single orifice.
I`m not sure if the balloon algae are one-layered like a blastula and this sack evolved from them via gastrulation, or if they`re two-layered and instead evolved from the gastrula that lost its mouth.
Regardless, the simplest of these sack animals are exactly that, sacks (2). Today they only survive as symbiotic living nests for a variety of bugs.
A more interesting group is this world`s answer to cnidarians (3) - these things are like polyps, but with trap-like jaws instead of tentacles, supported by a very simple endoskeleton (though exoskeletal elements are also widespread among them). Some of these trapjaws take on a more wormy shape (4).
Like cnidarians, these thing form colonies by budding, though these colonies are often more bryozoan-like (5). Some of them are also floating (6).
7 shows a colonial organism that functions somewhat like a sponge - small heads inhale water that is filtered in the shared gut and exhaled through the big mounth on top.
Like their algal relatives, many trapjaws become photosynthetic (8). Turning jaws into a pair of leaves is the most common arrangement, but not the only one. The colonial forms of these leaf-jaws (9) are the only organisms of this world that may be considered "trees". This drawing also shows reproductive zooids on top (trapjaws usually release gametes through their mouth). There are also species that retain the ability to develop zoids with proper jaws to defend themselves and even supplement their diet with meat.
Another interesting group are clamjaws, whose jaws are armored and shell-like (10). They usually build tubes to hide their soft body, leaving only the "clam" part outside, and have a bunch of stalked eyes. While they don`t form large colonies, they can still bud. 11 shows a filter-feeding clamjaw with a smaller secondary head serving as an exhalant orifice/anus.
Growing another head is, in fact, the easiest way for these creatures to develop an anus, as demonstrated by the worm-like organism at 12.
However, some creatures aren`t content with just an anus. The hydras (13), the most advanced of the trapjaws, have not only feeding, excretory and reproductive zooids, but also the ones specialized for breathing and functioning as internal organs such as the heart (most trapjaws have a simple gastrovascular system, so of course the heart would be repurposed from a zooid`s gut).

Alright, I think that`s all I`ve got for now.