A phylum of soft-bodied invertebrates characterized by an unsegmented body differentiated into a head, a ventral muscular foot used in locomotion, and a dorsal visceral hump covered by a fold of skin – the mantle – which secretes a protective shell in many species. Respiration is by means of gills (ctenidia) or a lunglike organ and the feeding organ is a radula. Molluscs occur in marine, freshwater, and terrestrial habitats and there are six classes, including the Gastropoda (snails, slugs, limpets, etc.), Bivalvia (bivalves, e.g. mussels, oysters), and Cephalopoda (squids and octopuses).
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