Phylum Mollusca

Abstract

The phylum Mollusca is the second largest in the animal kingdom including about 100,000 living species and with a long fossil record of some 35,000 more. The group is extremely diverse and includes such well known forms as the squid, octopus, slugs, snails and oysters. Like other large phyla, molluscs are not only represented by a large number of species but many of the species are very abundant. The vast majority are marine and the group originated in the sea. Of the four most abundant classes of living molluscs, Polyplacophora (chitons or coat-of-mail shells), Bivalvia (bivalves), Gastropoda (slugs and snails) and Cephalopoda (squids and octopuses), only two, the gastropods and bivalves, have invaded freshwater and in one, the gastropods, there are terrestrial forms. In addition to these important groups there is one class, Monoplacophora, believed for a long time to be extinct and now known from a single genus, and two classes, Aplacophora and Scaphopoda with only a few living species.