Mollusca

Abstract

Abstract Mollusca is a large and diverse phylum with many familiar representatives, from snails and clams to octopuses and squid, as well as several less-familiar representatives, such as chitons and aplacophorans. Despite this diversity in body plans, we can reconstruct a generalized mollusk and see how all these body plans are variations on a theme. The generalized body plan usually includes a shell that is secreted by a mantle. On the ventral side of the animal is a muscular foot, and dorsal to it is the visceral mass. In different lineages, each of these components has been expanded, reduced, or specialized as part of an adaptation to different lifestyles. Bivalves (oysters, clams, and relatives) have reduced the foot, the head, and sense organs as part of a transition to a sessile lifestyle. Gastropods (snails) maintain almost all of these structures, but have evolved a torsion of the visceral mass relative to the foot. Cephalopods (octopuses and squid) have lost or reduced the shell, evolved a closed circulatory system, and are adapted for an active and highly mobile lifestyle. Because of their hard shells, mollusks have an excellent fossil record, and they are important components of almost all fossil assemblages.