Molluscan

Molluscan refers to mollusks, members of the phylum Mollusca, a diverse group of soft-bodied invertebrates that occur in marine, freshwater, and terrestrial environments. The group includes snails, slugs, clams, oysters, octopuses, squids, and chitons. Many molluscs secrete a calcium carbonate shell, though some are shell-less. Typical features include a mantle, a muscular foot, and a visceral mass, with a mantle cavity that houses gills or lungs.

A defining organization is a three-part body plan: head-foot, visceral mass, and mantle. Most molluscs have a radula for feeding, though bivalves lack this organ. They are triploblastic, bilaterally symmetric, and coelomate. Many have open circulatory systems, while cephalopods possess a closed circulation; nervous systems are often well developed in cephalopods.

Diversity and habitats: The major classes are Gastropoda (snails and slugs), Bivalvia (clams, oysters, mussels), Cephalopoda (octopuses, squids, cuttlefish, nautiluses), and Polyplacophora (chitons). They inhabit oceans, freshwater, and on land, from intertidal zones to deep sea, performing ecological roles as grazers, filter feeders, and predators.

Reproduction and development: Most molluscs reproduce sexually with separate sexes; some are hermaphroditic. Fertilization is often external in marine species and internal in others. Early development commonly includes trochophore and veliger larval stages, though some lineages show direct development.

Economic and scientific significance: Molluscs are important as food and sources of shells and pearls; they underpin studies in evolution, development, and biomineralization. The fossil record is prolific due to durable shells. Conservation concerns include habitat loss and ocean acidification, which threaten shell formation and survival in many species.