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Warmbuns split from their ancestor and pursued a tiny benthic scavenger and detritivore niche. These translucent worm-like creatures are externally bilateral, having reduced their counter-arms to a pair of small brush-like appendages. They have lost the baits lining the sides of their anal arm, replacing them with fleshy flaps which cover and protect their reproductive openings. The baits on their counter-arms remain, being used to brush food particles into their mouths. Their counter-arms lack any gonads. They have a hydrostatic skeleton, and like a terran worm they move by extending and contracting their muscular bodies. They have a circulatory system which circulates colorless hemolymph, but they depend heavily on direct diffusion from the surrounding water to obtain oxygen.

With their niche being largely unexploited, Warmbuns were able to swiftly spread far and wide even into freshwater habitats. They evolved brackish and freshwater adaptations several times independently, so their adaptations to such are difficult to generalize. They are generally active year-round in most of their range with a minor drop in activity during cold winters, though freshwater polar species had to find a way to avoid mass die-offs from ice; they typically do this by hibernating deep enough underwater that the ice does not reach them.

Like their ancestor, Warmbuns are broadcast spawners. Their bodies have 6 gonad-bearing segments, making 12 gonads per individual. When they have coordinated with others in their environment using pheromones, the flaps protecting the reproductive openings fly open and they spawn into the surrounding water. Like their ancestor and like a terran flatworm, they are also capable of reproducing asexually using binary fission.