Exoskeletons are external, and are found in many invertebrates; they enclose and protect the soft tissues and organs of the body. Some kinds of exoskeletons undergo periodic moulting as the animal grows, as is the case in many arthropods including insects and crustaceans. Exoskeletons are made of different materials including chitin (in arthropods), calcium compounds (in corals and mollusks) and silicate (for diatoms and radiolarians).
The exoskeleton of insects is not only a protection but also serves as a surface for muscle attachment, as a watertight protection against drying and as a sense organ to interact with their environments. The shell of mollusks also performs all of the same functions, except that in most cases it does not contain sense organs.
An external skeleton can be quite heavy in relation to the overall mass of an animal, so on land, organisms that have an exoskeleton are mostly relatively small. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. The southern giant clam, a species of extremely large saltwater clam in the Pacific Ocean, has a shell that is massive in both size and weight. Syrinx aruanus is a species of sea snail with a very large shell.
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