Pteropoda

Pteropods are free-swimming pelagic sea snails and sea slugs. They are marine opisthobranch gastropods. 
Are they a natural group? The monophyly of Pteropoda is the subject of debate. At present, by molecular studies, the group is monophyletic.
Pteropoda has two clades Thecosomata, the sea butterflies, and Gymnosomata, the sea angels. The Thecosomata have a shell, while the Gymnosomata ("naked body") do not. The two clades may or may not be sister taxa. If not, their similarity may reflect adaptation to their particular lifestyle.
All are pelagic, small, and transparent, and swim using wing-like flaps (parapodia) which protrude from their bodies.
The name "sea slug" is most often applied to nudibranchs, as well as to other marine gastropods without obvious shells.