The name nudibranch means "naked gill", the name refers to the external respiratory organs, such as the branchial plume of dorid nudibranchs, the club-like processes found in the aeolids and dendronotids and the club-like, leaf-like or arborescent processes of arminaceans located along the sides of the body or in a lateral groove.
Nudibranchs belong to a class of molluscs called the gastropods, this class is divided into three subclasses; the prosobranchs (eg. the common whelk, the periwinkle and the limpet); the pulmonates (air breathing snails and slugs, eg. garden and pond snails and slugs) and the opisthobranchs (commonly know as sea-slugs). Both the opisthobranchs and the pulmonates are descendants of the prosobranchs.
Nudibranchs are opisthobranch molluscs in which the adult stage has completely lost both the shell and operculum. They share this character with the plant-eating sacoglossa or ascoglossa, which are not covered here. The loss of the shell has allowed a diverse array of body forms within this order. The Nudibranchia is divided into the following four suborders; the Dendronotacea, Doridacea, Arminacea and Aeolidacea. Approximately 108 described species occur around the British Isles.
The technical terms used in this book to describe the main morphological features of nudibranchs are described in the section on anatomy and listed in the glossary.