Unique characters: Scavengers and carnivores that actively feed by rasping at prey with a bony tongue. They tie themselves into a knot to lever a chunk out of prey. They can coat themselves with mucous for defense. They contain no vertebrae and no bone.
Shared novelties:
A head (cartilage brain case); Sense organs on the head (weak eyes); A true heart; True gills for efficient oxygen retrieval from water; Cartilage gill
supports to hold up these flimsy sheets.Range: Carboniferous-Recent
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