By Scott A. Rowan
A carapace is a dorsal (top) shell or exoskeleton that often protects vital organs in many animals. Shrimp, crabs, armadillos, turtles and tortoises are animals that have a carapace. The most notable feature of a horeshoe crab — it’s “shell” — is actually a carapace.
Technically, a horseshoe crab does not have a “shell” as most beachcombers are likely to say. A shell is comprised of two sections: a top and bottom half. The top (dorsal) half, is a carapace; the bottom (ventral) half is called a plastron.
Like with the horseshoe crab, the carapace of turtles and tortoises are the most distinguishing features for those animals. The carapace of species in the Order Testudines, which includes turtles and tortoises, are constructed of two materials: interlocking bone layers that are covered in protective scutes.
The outer layer of the carapace of most turtles is constructed of 13 main scutes that are ringed by many smaller scutes. Scutes are a form of dermal plates that are made of keratin, the same material that human fingernails and the horns of many cattle. In turtle and tortoise carapaces, the scutes fuse together through ossification, creating a hard, protective layer.
The Vertebrate Integument Volume 1, Theagarten Lingham-Soliar, (Springer, Verlag Berlin Heidelberg).