Pteropoda

Pteropoda (common namepteropods, from Ancient Greek πτερόν (pterón), meaning "wing", and πούς (poús), meaning "foot") are specialized free-swimming pelagicsea snails and sea slugs, marineopisthobranchgastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The monophyly of Pteropoda is the subject of a lengthy debate; they have even been considered as paraphyletic with respect to cephalopods. Current consensus, guided by molecular studies, leans towards interpreting the group as monophyletic.

Pteropoda encompasses the two cladesThecosomata, the sea butterflies, and Gymnosomata, the sea angels. The Thecosomata (lit. "case-body") have a shell, while the Gymnosomata (lit. "naked body") do not. The two clades may or may not be sister taxa; if not, their similarity (in that they are both pelagic, small, and transparent, and both groups swim using wing-like flaps (parapodia) which protrude from their bodies) may reflect convergentadaptation to their particular lifestyle.