Pteropoda

Pteropoda (Gr., 'wing-footed'), a class or sub-class of molluscs, having two lobes of the 'foot' developed into wing-like swimming organs. They live in the open sea, and are carnivorous. Distributed in all seas, they often occur in immense shoals, and afford food to fishes and Cetaceans. The body is bilaterally symmetrical, but this is doubtless secondarily acquired. In some (Thecosomata) the viscera are covered with a delicate shell; the others (Gymnosomata) are naked, but all the larvæ have shells. The thin calcareous or gristle-like shells are abundant in the Ooze (q.v.) of some regions. It is very likely that the Pteropods should be ranked not as a separate class of molluscs, but as a sub-class of Gasteropods. Of the Thecosomata the genera Hyalea and Cymbula are representatives, as Clio and Pneumodermum are of Gymnosomata. Fossil Pteropods appear even in the Cambrian strata. See Pelseneer, Challenger Report (1889). Some Pteropods are sometimes called 'sea-butterflies.'