Flatbuns
Flatbuns split from their ancestor. They are flat in shape, with a somewhat triradial appearance like a clover when viewed from above—though from below and internally, they are still bilateral. Their filter-baits are replaced with hooks made of biogenic silica, which serve to pull them along the sediment, make them unpleasant or potentially dangerous to eat, and to ensnare potential food. The original bait follicle has been totally repurposed to produce this instead of the bristle. Hooks on the undersides of Flatbuns’ counter-arms are arranged so that struggling prey is drawn towards the mouth; otherwise, the hooks can be moved to simply pull food towards the mouth. Only the hooks along the edges of the arms have corresponding gonads. They have developed photoreceptive patches, ten in total running in a row of 5 along each counter-arm.
Flatbuns have blood which makes use of an oxygen carrier not found in any Terran fauna, but in Terran flora: leghemoglobin. This gives them red iron-based blood. When conditions are unfavorable, Flatbuns will curl into tight little balls and stay like that until conditions improve. Most polar species do this over the winter, when there would otherwise not be enough food for them. Deep sea species may also do this when there is not enough food. There are far too many species to individually describe, present in nearly all benthic marine environments. Despite their benthic habits, however, Flatbuns are actually capable of swimming in short bursts by rapidly flapping their anal arm.
Like their ancestor, Flatbuns can reproduce sexually by broadcast spawning or asexually through binary fission. Chemoreceptors at the tips of each arm help them to coordinate spawning with others of their kind using pheromones. Deep sea species will still spawn even if there are only two individuals in the area as long as they are different sexes, as they may not get any other chance to breed.