By way of a short introduction to the opisthobranch molluscs, it should be noted that not all opisthobranchs are nudibranchs, and that many types are frequently referred to as sea slugs. The opisthobranch molluscs are one of three groups (Subclasses) of gastropods (commonly called "snails"), the Opisthobranchia. The other two groups are the Pulmonata (air-breathing land snails and many freshwater as well as some marine forms) and the Prosobranchia (an enormous group of gastropods ranging from limpets and abalone to commonly recognized snails such as the murexes, cowries, and tritons). Hans Bertsch has an excellent presentation on opisthobranchs, found on Mike Miller's Slug Site, entitled Nudibranchs: Marine Slugs with Verve.
Opisthobranchs present a remarkable amount of variation in body form and they range from shelled individuals to those having complete absence of a shell. Although the opisthobranchs comprise a relatively small group of perhaps only several thousand species worldwide, they are conceivably the most stunning and attractive animals found in the ocean.
I've included a short list of opisthobranch-related books which may be of interest to some readers. In addition to the books I've also included a section of mollusc-related and other sites for additional investigation.
For those of you interested in more technical information on opisthobranchs or have specific needs, be aware of the "Mollusca List Server" at the University of California's Museum of Paleontology, and Gary McDonald's "Bibliographia Nudibranchia" at U.C. Santa Cruz. Gary's bibliography is found on-line as is Steve Long's Opisthobranch Site, which both feature bibliographic and taxonomic online search engines.
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