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Family  Limacinidae

Members of the Limacinidae, one of the families commonly known as sea butterflies, occur in the plankton, remaining in the water column throughout their entire life. They have a minute sinistral shell, up to 2 mm in size. As with all sea butterflies, the foot has been modified into two wings (called parapodia) used in swimming, but the animal can withdraw its body and parapodia completely into its shell. Once withdrawn, the aperture can be sealed by an operculum. Sea butterflies trap phytoplankton on the parapodia, bind the plankton together in mucus, and move it to the mouth by cilia. Limacinids occur in the tropical and subtropical regions of all oceans, where schools of them can be extremely dense. They are usually collected in plankton trawls.

Four species of this family are recorded from NSW. 

Heliconoides inflatus (d'Orbigny, 1835) 

Limacina bulimoides (d'Orbigny, 1834) 

Limacina lesueurii (d'Orbigny, 1835) 

Thielea helicoides (Jeffreys, 1877)