The filterworm evolved from the mudworm. It has gone upstream from the swamp and lives in the fast-rushing rapids. Its front flipper-claws have evolved into filtering bristles. These bristles collect small organic particles which it will lick off and eat. The body has become smaller, and its legs have evolved into anchoring hooks. Even the tail-flipper is a hook, for they need to keep attached to the bottom or else they risk getting pulled downstream. It still needs to breathe air, so it will crawl to the edge and take a breath, since it no longer swims.