n. plOpisthobranchiata(Zoöl) A division of gastropod Mollusca, in which the breathing organs are usually situated behind the heart. It includes the tectibranchs and nudibranchs.
Century Dictionary and Cyclopedia
opisthobranchiataAn order of Gasteropoda having the gills behind the heart: opposed to Prosobranchiata. They have a relatively large foot and small visceral hump, with short mantle-flap, behind which is the anus. They are usually shell-less in the adult state, and many of them lose the ctenidial gills and mantle-flap, respiration being effected by very diversiform supplementary organs. Hence the equally various methods of subdivision of the order, and the application to its divisions of exceptionally numerous names ending in -branchia. The opisthobranchs are marine and littoral gastropods of more or less slug-like aspect, and many of them are known as sea-slugs, sea-hares, sea-lemons, etc. See Nudibranchiata, Tectibranchiata.