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A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer usually created by an animal or organism that lives in the sea. Most seashells are made by mollusks, such as snails, clams, and oysters to protect their soft insides. [1] Empty seashells are often found washed up on beaches by beachcombers.
The BMC shell appears icosahedral [26] or quasi-icosahedral, and is formed by (pseudo)hexameric and pentameric protein subunits. [27] Structures of intact shells have been determined for three functionally distinct: BMC types, carboxysomes, [28] the GRM2 organelles involved in choline catabolism [29] and a metabolosome of unknown function.
The shells have small pores that allow for gas exchange and nutrient uptake. Coccolithophores and foraminifera also have hard protective shells, but the shells are made of calcium carbonate. These shells can help with buoyancy, allowing the organisms to float in the water column and move around more easily.
The shell-less aplacophora have a chitinous cuticle that has been likened to the shell framework; it has been suggested that tanning of this cuticle, in conjunction with the expression of additional proteins, could have set the evolutionary stage for the secretion of a calcareous shell in an aplacophoran-like ancestral mollusc.
The gastropod shell has three major layers secreted by the mantle. The calcareous central layer, ostracum, is typically made of calcium carbonate (CaCO 3) precipitated into an organic matrix known as conchiolin. The outermost layer is the periostracum which is resistant to abrasion and provides most shell coloration.
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.
A Greek tortoise shell opened to show the skeleton from below Main article: Turtle shell The carapace is the dorsal (back) convex part of the shell structure of a turtle , consisting primarily of the animal's rib cage, dermal armor, and scutes .
The bones of the shell are named for standard vertebrate elements. As such the carapace is made up of eight pleurals on each side, these are a combination of the ribs and fused dermal bone. Outside of this at the anterior of the shell is the single nuchal bone, a series of twelve paired periphals then extend along each side.