Crustacean, common name for any of a group of mainly aquatic arthropod invertebrates, including the crab, lobster, and shrimp. Crustaceans are also successful in fresh water, and a few, such as sow bugs, are common in moist land environments. Although most crustaceans are small, they vary widely in form and include such large members as lobsters up to 60 cm (24 in) long and a spider crab with a leg span of 3.6 m (12 ft). The subphylum contains about 26,000 known species.
             Like all arthropods, crustaceans have an external skeleton (exoskeleton) and a body made up of a series of segments. Each of these segments generally bears a pair of branched limbs. In the course of evolution, the segments and other parts of the body have become specialized. The limbs, used in respiration, swimming, crawling, and feeding, may be highly modified as jaws, reproductive organs, and other structures, or they may be simplified or lost
             The head is usually fused with several segments behind it, forming a region called the cephalothorax, which is followed by the abdomen. Commonly an outgrowth of the head, called the carapace, covers much or even all of the body. On the head are two pairs of sensory organs (antennae) and a pair of jaws (mandibles), behind which are two other pairs of mouthparts (maxillae). The head is usually equipped with a pair of compound eyes, an unpaired eye, or both
             The cephalothorax generally bears limbs used in locomotion and respiration. Often, the carapace provides a protective cover for the gills, which are part of the limbs. Some of the limbs may have pincers (chelae). Abdominal appendages may be used in locomotion but frequently have other functions, such as respiration, and they tend to be reduced in size. A tail portion (telson) that bears the anus is occasionally used in swimming
             The main body cavity is an expanded circulatory system through which blood is pumped by a dorsal heart. The gut is basically a strai
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