Pteropoda

Pteropoda [tə′räp·ə·də]

(invertebrate zoology)

The sea butterflies, an order of pelagic gastropod mollusks in the subclass Opisthobranchia in which the foot is modified into a pair of large fins and the shell, when present, is thin and glasslike.

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Pteropoda 

a group comprising two orders of hermaphroditic opisthobranch planktonic marine mollusks.

The leg in Pteropoda is rudimentary: only its two lateral lobes (the winglike parapodia, by means of which the mollusks swim) are highly developed. In representatives of the order Thecosomata there is a spiral or bilaterally symmetrical shell or, sometimes, a pseudoshell. In representatives of the order Gymnosomata, the shell is absent. The body is semitransparent and often brightly colored. Pteropods are widely distributed. In the seas of the USSR, the most commonly encountered of the Thecosomata are Limacina; the most widespread of the Gym-nosomata are Clione. Pteropods serve as food for certain fishes and whales. The detritus of the shells makes up the mass of what is called the pteropod ooze in the South Atlantic. Remains of extinct pteropods date from as early as the Paleogene. The extinct Paleozoic Coniconchia are sometimes included among the Pteropoda.