• Webster's Revised Unabridged Dictionary
    • n. pl Opisthobranchiata (Zoöl) A division of gastropod Mollusca, in which the breathing organs are usually situated behind the heart. It includes the tectibranchs and nudibranchs.
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Century Dictionary and Cyclopedia
    • opisthobranchiata An 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.
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Webster's Revised Unabridged Dictionary
NL., from Gr. 'o`pisqen behind + gills