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The body of a mollusc has a ventral muscular foot, which is adapted to different purposes (locomotion, grasping the substratum, burrowing or feeding) in different classes. [36] The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement.
In some mollusks the mantle cavity is a brood chamber, and in cephalopods and some bivalves such as scallops, it is a locomotory organ. The mantle is highly muscular. In cephalopods the contraction of the mantle is used to force water through a tubular siphon, the hyponome , and this propels the animal very rapidly through the water.
The mollusc (or mollusk [spelling 1]) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes. Not all shelled molluscs live in the sea; many live on the land and in freshwater.
A siphon is an anatomical structure which is part of the body of aquatic molluscs in three classes: Gastropoda, Bivalvia and Cephalopoda (members of these classes include saltwater and freshwater snails, clams, octopus, squid and relatives). Siphons in molluscs are tube-like structures in which water (or, more rarely, air) flows.
In all molluscs, the mantle forms a thin membrane that covers the animal's body and extends out from it in flaps or lobes. In bivalves, the mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of ligament , byssus threads (where present), and teeth . [ 16 ]
The gastropod shell is part of the body of many gastropods, including snails, a kind of mollusc. The shell is an exoskeleton , which protects from predators, mechanical damage, and dehydration, but also serves for muscle attachment and calcium storage.
Unlike most other molluscs, there is no intermediate stage, or veliger, between the trochophore and the adult. Instead, a segmented shell gland forms on one side of the larva, and a foot forms on the opposite side. When the larva is ready to become an adult, the body elongates, and the shell gland secretes the plates of the shell.
Molluscs have such diverse shapes that many textbooks base their descriptions of molluscan anatomy on a generalized or hypothetical ancestral mollusc. This generalized mollusc is unsegmented and bilaterally symmetrical with an underside consisting of a single muscular foot. [67] [68]: 484–628 Beyond that it has three further key features.