Molluscs are a major group of animals that include shellfish, snails and slugs. They have soft, limbless bodies with no skeleton-most of them are protected by a hard shell. Squids, octopuses and cuttlefish have internal skeletons.
Most molluscs live in the sea, but some live in fresh water or on land. They are cold blooded.
The mollusks have adapted to all habitats except air. Although basically marine, bivalves and gastropods include freshwater species. Gastropods have also adapted to land, with thousands of species living a fully terrestrial existence. Found on rocky, sandy, and muddy substrata, mollusks burrow, crawl, become cemented to the surface, or are free-swimming.
Mollusks are found worldwide, but there is a preponderance of some groups in certain areas of the world. The close association of many molluscan groups with their food source—whether by direct dependence on a specific food supply (e.g., plant-eating, or herbivores) or by involvement in food chains—limits their geographic distribution; for example, bivalves of the family Teredinidae (shipworms) are associated with wood. In general, cold-water regions support fewer species.
Credit : Britannica
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