Carapace

Carapace, (1) a term applied to the hard covering of the Arthropoda. It is composed of a series of layers containing hard bands of phosphate of lime, chitin, etc. ; in some cases, as in the crab, the carapace is composed only of one piece (the cephalothorax), formed of the skeletal elements of many somites fused together; or it may be bi-valved, as in some Entomostraca. The carapace is usually thrown off periodically by a process of moulting, known as "ecdysis." The carapace differs from shell in microscopic structure as well as in composition. (2) The dorsal or upper half of the "shell" of a Turtle or Tortoise. The lower half is called the Plastron.