Gracile alien fauna brainstorm

This is a brainstorming page exploring the four-fold symmetry of the alien’s native fauna. I hadn’t had a clue what it’d be like when starting out, and just built it up as I went, starting with the cross-section concepts at the top of the page.

The 4-fold radial symmetry does not exist on Earth, which is why I picked it. Even the radially symmetrical coelenterates do seem to have elements of bilateral symmetry, especially evident in the internal structures of coral polyps. The closest Earth’s fauna gets to it are the ctenophores which have a double-reflected bilateral symmetry: they are bilateral, but their “back” and “belly” are exactly the same, and their quarters divide further to give them eight longitudinal combs of ciliae.

It’s ctenophores I had been thinking of when I toyed with the two “egg” critters, giving them four ciliate bands on the outside. Besides, the combs gave a reason for the four “gastric” channels to exist. I have also given them a structure that’d be analogous to Earth chordates’ notochord, only in this case it is a skeletal spike keeping the sessile animal above the sediment, like in some of Earth’s sponges. These early designs have a central “ampule” in the ir gastric system, which I later retired in favor of a secondary atrium, the ex-“gut” repurposed as embryologic source of the notochord. But they already have a “gill” in their radial channels. There are also some ideas for further evolution, like ciliate bands becoming grooves and a four-tentacled “polyp”.

The bottom half of the page explores the possible ways to break this four-fold symmetry to get a more or less bilateral animal. It later proved to be a very rich idea I relied upon heavily.
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