Crustacean

A crustacean is an invertebrate (an animal without a backbone) with several pairs of jointed legs and two pairs of antennae. It is covered by a tough exoskeleton (a hard outer support structure) with overlapping plates that thin out at the joints to allow maximum movement. Its body is divided into two main regions that are fused together. Most crustaceans live in water.

A crustacean is a member of the phylum Arthropoda, the largest and most successful phylum in the kingdom Animalia. It is also a member of the class Crustacea which is one of the three major groupings of arthropods (the other two are Arachnida and Insecta). The name crustacean is derived from the Latin word cursta meaning "crust," and refers to the hard outer shell that this class of invertebrate wears. There are about 40,000 species of crustaceans, including the better-known animals like shrimps, lobsters, crayfishes, and crabs as well as barnacles, water fleas, and isopods like the wood louse. Some are predators and eat other invertebrates, while others are herbivores and eat only plant material. While there are simple crustaceans, most usually have a large diversity (and a large number) of paired appendages (like legs, arms, or pincers). To be classified as a crustacean, an animal must have two joined body parts—a cephalothorax (a head and middle region) and an abdomen (the lower part of the body). The head has two compound eyes that are located on the ends of retractable and flexible stalks. A compound eye is made up of many separate compartments, each having its own lens. A crustacean must also have two pairs of antennae with which it feels and receives chemical stimuli. It must also have at least four pairs of walking legs, and often has more. Shrimp, lobster, crabs, and crayfish are called decapods because they have ten legs. All have a broad, paddle-like tail used for swimming.

As with a representative crustacean species like the crayfish, its first set of legs are adapted as claws. It uses these claws or pincers to obtain food and to defend itself. The other four pairs of smaller legs are used for walking. Behind these walking legs and attached to the lower half of its body called the abdomen are tiny appendages called swimmerets used for swimming and during reproduction. The respiratory system of a crayfish consists of gills over which water passes as the animal moves. The gills are really feathery outgrowths located on both sides of its body. The crayfish has a heart that moves its blood through arteries. Other crustaceans have variations of these systems. For example, a crab has an especially strong claw used for tearing up seaweed and attacking another animal. The barnacle, which attaches itself to a rock by a long stalk, has appendages almost like feathers that are used to comb or sift the water for microscopic food.

The smallest crustacean might be the 7,500 species of copepods, some of which are no more than a few millimeters long. These free-swimming herbivores play an important role in the diet of many fish. All crustaceans reproduce sexually and develop through a series of larval stages.