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Crustaceans 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 36 m 12 ft The subphylum contains about 26000 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
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