Phylum Mollusca

Mollusca (Latin molluscus, soft-bodied) is the second-largest animal phylum, found in terrestrial and aquatic (marine or freshwater) habitats and showing organ-system organisation. Molluscs are bilaterally symmetrical (except the univalves), triploblastic and coelomate.

The body is unsegmented, typically protected by a calcareous shell, and organised into a distinct head, a muscular foot, and a visceral hump (visceral mass). A soft sheet of skin, the mantle, drapes over the visceral hump, and the space between mantle and visceral mass — the mantle cavity — houses feather-like respiratory gills called ctenidia. The digestive system is complete, and the mouth typically contains a unique rasping feeding organ called the radula, armed with rows of chitinous teeth (absent in bivalves). Sense organs include tentacles, eyes, and an osphradium that tests the chemical purity of incoming water (present in bivalves and gastropods). …