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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.
Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks [a] (/ ˈ m ɒ l ə s k s /). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. [5]
2 – columellar plait 3 – aperture 4 – columella 5 – suture 6 – body whorl 7 – apex Four views of a shell of Arianta arbustorum: Apertural view (top left), lateral view (top right), apical view (bottom left), and umbilical view (bottom right). The gastropod shell is part of the body of many gastropods, including snails, a kind of ...
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 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, feeding, etc. The term aperture is used for the main opening in gastropod shells, scaphopod shells, and also for Nautilus and ammonite shells.
Among molluscs, it is primarily seen in snails and clams, i.e. in gastropods and bivalves, but it is also found in cephalopods such as Allonautilus scrobiculatus. The periostracum is an integral part of the shell, and it forms as the shell forms, along with the other shell layers. The periostracum is used to protect the organism from corrosion.
Diagram of the internal shell structure of the left valve of bivalve resembling a venerid in which the pallial sinus is shown.. The pallial sinus is an indentation or inward bending in the pallial line on the interior of a bivalve mollusk shell's valves that corresponds to the position of the siphons in those types of clams which have siphons (i.e. siphonate).