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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.
Three views of a shell of Norelona pyrenaica with the apertural view in the center A shell of Semicassis pyrum, which has a large aperture and a pronounced parietal callus The aperture is an opening in certain kinds of mollusc shells : it is the main opening of the shell, where the head-foot part of the body of the animal emerges for locomotion ...
Anatomical diagram of a hypothetical ancestral mollusc The evolution of the molluscs is the way in which the Mollusca , one of the largest groups of invertebrate animals, evolved . This phylum includes gastropods , bivalves , scaphopods , cephalopods , and several other groups.
Umbilicus: in shells where the whorls move apart as they grow, on the underside of the shell there is a deep depression reaching up towards the spire; this is the umbilicus; Varix: on some mollusk shells, spaced raised and thickened vertical ribs mark the end of a period of rapid growth; these are varices
The generalized mollusc is an unsegmented, bilaterally symmetrical animal and has a single, "limpet-like" shell on top. The shell is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot". [19] The visceral mass, or visceropallium, is the soft, nonmuscular metabolic region of the mollusc.
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. Unlike the fully grown adult, the larva has a pair of simple eyes, although these may remain for some time in the immature ...
The existence of this pocket shows even in an empty shell, as a visible indentation in the pallial line, a line which runs along parallel to the ventral margin of the shell. [10] The bivalve's two siphons are situated at the posterior edge of the mantle cavity. [11] There is an inhalant or incurrent siphon, and an exhalant or excurrent siphon. [12]
The body whorl shows in this side view as the last complete turn of the shell spiral, the part of the shell that leads up to the aperture. The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk.