Crustacean, a member of a large class of animals with segmented bodies. Crustaceans belong to the same major division, or phylum, of the animal kingdom as insects and spiders. Included among them are lobsters, shrimp, crabs, wood lice, barnacles, water fleas, and many other animals. All told, there are about 25,000 species of crustaceans. Most of them live in the water; a few, such as wood lice, live on land.

Wood liceWood lice are small crustaceans.

The crustacean's body is made up of as many as 50 segments, but most of the higher crustaceans have only 19. The body usually has three regions—head, thorax, and abdomen. The head and thorax are often fused into a single unit called the cephalothorax. A hard substance, chitin (kī'tīn), covers the body, forming an external skeleton like a coat of armor. This exoskeleton does not grow with the body of the animal. Periodically it is shed and a new one is formed.

Crustaceans have compound eyes (consisting of many lenses) that are sometimes on stalks. Extending from the head are two pairs of jointed antennae (feelers) that act as sense organs and sometimes as swimming organs. Usually the two outer antennae are much longer than the inner pair. Armlike appendages on the thorax, usually including a pair of large, pincerlike claws, are used for walking, grasping, tearing, and eating. Appendages on the tail are swimming structures.

Crustaceans reproduce by means of eggs. The eggs of saltwater crustaceans hatch into tiny larvae that are unlike the adults. Young freshwater crustaceans resemble adults except that they are smaller.

The smaller crustaceans are important mainly as food for fish. Lobsters, crabs, and shrimp are excellent human food, and cray-fish are also eaten by many people. Crustaceans also serve as scavengers, helping to keep beaches and streams clean. On the other hand, some crustaceans are serious pests, destroying crops or boring into and destroying wharves and sea walls.

The class Crustacea of the phylum Arthropoda contains two subclasses. The Entomostraca are mostly small, without distinct body regions. They include the fairy shrimp, water fleas, cyclops, fish lice, barnacles, and copepods. The Malacostraca include forms with the typical 19 segments. They include crayfish, lobsters, shrimp, sow bugs, pill bugs, sandhoppers, and wood lice.