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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.
Scaphopoda / s k æ ˈ f ɒ p ə d ə / (plural scaphopods / ˈ s k æ f ə p ɒ d z /, from Ancient Greek σκᾰ́φης skáphē "boat" and πούς poús "foot"), whose members are also known as tusk shells or tooth shells, are a class of shelled marine invertebrates belonging to the phylum Mollusca with worldwide distribution and are the only class of exclusively infaunal marine molluscs.
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
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.
A bivalve shell is the enveloping exoskeleton or shell of a bivalve mollusc, composed of two hinged halves or valves. The two half-shells, called the "right valve" and "left valve", are joined by a ligament and usually articulate with one another using structures known as "teeth" which are situated along the hinge line .
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]
A fasciole in mollusks refers to a band or ribbon-like structure that is often seen as a flattened, slightly depressed area on the shell, formed by the successive growth lines on the edges of a canal. It is typically found on the surface of the shell near the siphonal canal or around the aperture. The fasciole can play a role in the shell's ...
The crescent shaped shell of C. paula facilitates entry into the narrow space inside snail shells where they attach to the columellae and siphonal canal via byssal threads. The position and location of attachment anteriorly near the final whorls of the shell is an evolutionary behavior of adult individuals which prevents removal by the host. [5]