The opisthobranchs include the sea slugs and their relatives the sea hares, sea butterflies, canoe shells, and others. The bodies of most member of this group show evidence of detorsion. Shells may or may not be present; if present they may be reduced and/or internally located. Opisthobranchs usually lack a mantle cavity, and they also usually have no operculum. Many species lack gills and respiration takes place through the skin, which may bear numerous projections and folds that increase the area for gas exchange. The head bears 1-2 pairs of tentacles.
This diverse group of organisms is divided into around nine orders. There is some disagreement about whether these form a monophyletic group.
Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.
Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor, John B. Burch (author), Mollusk Division, Museum of Zoology, University of Michigan-Ann Arbor.