I.

Introduction

Mollusk, common name for members of a phylum of soft-bodied animals (Latin mollus, “soft”), usually with a hard external shell. Familiar mollusks include the clam, oyster, snail, slug, octopus, and squid. The mollusk phylum is the second largest in the animal kingdom, after the arthropods. Earlier estimates of the number of mollusk species sometimes exceeded 100,000, but more recently this figure has been reduced to less than 50,000; the new estimates are incorporated here.

Mollusks are highly successful in terms of ecology and adaptation, with representatives in virtually all habitats, but they are most diverse in the sea. Among them are some advanced animals, such as the octopus and squid. Giant squid are also the largest invertebrates, weighing up to 2,000 kg (4,000 lb). Most mollusks, however, are about 1 to 20 cm (about 0.4 to 8 in) long, and some are scarcely visible.

The first mollusk fossils appear in early Cambrian rocks, about 600 million years old. Seven of the phylum's classes have living representatives: the wormlike, shell-less aplacophorans, with 250 species; the chitons, with 600 species; the monoplacophorans, with 10 species; the bivalves, such as clams, with 7500 species; the scaphopods, or tusk shells, with 350 species; the gastropods, such as snails and slugs, with 37,500 species; and the cephalopods, such as octopuses and squid, with 600 species. Several fossil classes and thousands of fossil species are also known.