molluscs

Molluscs are a diverse and widespread phylum of invertebrate animals. They vary greatly in form, from the shelled gastropods and bivalves to the highly cephalized cephalopods, and include less familiar groups such as chitons and tusk shells. Most possess a soft body and, in many species, one or more calcareous shells produced by the mantle. The defining body plan includes a muscular foot, a visceral mass containing most of the internal organs, and a mantle that often secretes the shell and encloses a mantle cavity housing the gills, nephridia, and other organs.

Major classes include Gastropoda (snails and slugs), Bivalvia (clams, mussels, oysters, scallops), Cephalopoda (octopuses, squids, cuttlefish, nautiluses), Polyplacophora (chitons), and Scaphopoda (tusk shells). They inhabit almost every environment, from deep oceans to freshwater to land. They feed by scraping algae and plant material with a radula, filtering suspended particles with gills, or capturing prey. Most molluscs have an open circulatory system, though cephalopods have a closed one. Reproduction is mostly sexual; many marine species release eggs and sperm into the water, while others are internal; development frequently includes free-swimming larval stages such as trochophore and veliger in marine forms.

Ecologically, molluscs perform important roles as grazers, filter feeders, and prey, and cephalopods are often top predators. Humans obtain substantial food from many shelled molluscs; shells and pigments are also exploited. Some species are pests or invasive. The phylum has a rich fossil record due to its shell-bearing members, illustrating long evolutionary history and diversification.