This, the first of two volumes on the Opisthobranchia, gives a general account of the sea-slugs and bubble-shells of the world's seas and concludes with a systematic account of the British representatives of the orders Bullomorpha, Aplysiomorpha, Pleurobranchomorpha, Acochlidiacea, and Sacoglossa. The biology of the British Nudibranchia (the largest order of the Opisthobranchia) are the subject of Volume II.
The Opisthobranchia are popular with marine biologists and with general zoologists because of their common occurrence and their often vivid colours and elegant behaviour. They are a subclass of the gastropod molluscs, and all are hermaphrodite, marine, and macroscopic. Worldwide there are about 3000 species, and around 150 of these have been recorded from the shallow waters around the British Isles. Of the British species, 30% are infaunal, the remainder epifaunal. It is chiefly the epifaunal type, exemplified by the aplysiids or the nudibranchs, which has attracted the attention of naturalists for the last century and now, in the nineteen-seventies, has captured the interest of the growing body of aqualung enthusiasts.