mollusca

Mollusca is a large and diverse phylum of invertebrate animals that includes snails, slugs, clams, oysters, squid, octopuses, chitons, and tusk shells. They occupy marine, freshwater, and terrestrial habitats, from tides to the deep sea, and constitute one of the most speciose animal groups.

Most mollusks share a tripartite body plan: a mantle that often secretes a calcium carbonate shell, a muscular foot for locomotion, and a visceral mass containing internal organs. A mantle cavity houses gills or lungs. A radula is used for feeding in many groups, though bivalves lack one. Circulation is open in many mollusks, but cephalopods have a closed circulatory system. The nervous system ranges from simple to highly complex in cephalopods, which possess advanced brains and eyes. Reproduction is typically sexual; many species are hermaphroditic, and development ranges from free-swimming larvae to direct birth.

Major extant classes include Gastropoda (snails and slugs), Bivalvia (clams, oysters, mussels), Cephalopoda (octopuses, squids, nautiluses), Polyplacophora (chitons), and Scaphopoda (tusk shells). Monoplacophora and Aplacophora are smaller, less conspicuous groups. Gastropods are the largest and most diverse class by species count.

Mollusks play key ecological roles as grazers, filter feeders, scavengers, and prey for many animals. They are economically important for food (oysters, clams, mussels, scallops, snails) and for shells and other products in fisheries and trade.

Fossil mollusks date to the early Paleozoic, with a rich record documenting shell evolution and diversification, including ammonoids and nautiloids. The phylum shows notable evolutionary innovations and remains a central focus of invertebrate biology.