CHAPTER XLIII 

 CRUSTACEA 



Crustacea (krus ta' she a; L., crusta, a hard shell), to which the 

 crayfish belongs, is a class included in the phylum Arthropoda (ar throp' 

 o da; G., arthron, joint, and podos, foot). The animals of this class are 

 distinguished from the other arthropods by the fact that they carry on 

 respiration by means of gills, though some of them have become adapted 

 to terrestrial life. Many of the aquatic forms are found in fresh water, 

 but most of them are marine. Wherever found the individuals are 

 very numerous and frequently occur in vast numbers. The body is 

 usually divided into three regions, which are head, thorax, and abdomen; 

 in some cases, as in the crayfish, the first two divisions may be united to 

 form a cephalothorax. The head usually consists of five united met- 

 ameres and bears two pairs of antennae; a pair of mandibles, or jaws; and 

 two pairs of maxillae. The number of metameres in the thorax and 

 abdomen varies in different types. The thorax possesses a number of seg- 

 mented appendages, usually locomotor, while the abdomen may bear 

 appendages with other uses. The appendages exhibit homology, being 

 biramous and constructed on the same plan, but are specialized, each in 

 a manner fitting it for the function it performs. Not all crustaceans are 

 brightly colored, but among the marine shrimps are some of the most 

 brilliantly colored of animals. 



301. Malacostraca. — The many forms of Crustacea may be divided 

 between two subclasses. The first of these, or Malacostraca (mal a 

 k6s' tra ka; G., malakos, soft, and ostrakon, a hard shell), includes types 

 which in general are of large size, the largest being the largest of the 

 arthropods and so large as to be conspicuous among invertebrates 

 generally. 



The decapods, which agree in having five pairs of walking legs, include 

 crayfishes, lobsters, crabs, and shrimps. The more famihar of the crabs, 

 or those which may be termed typical, differ from crayfishes and lobsters 

 in the breadth of the cephalothorax, which frequently is broader than 

 long, and in the fact that the abdomen is brought forward under the 

 cephalothorax and closely applied to it (Fig. 169). The legs in the shore 

 crabs are particularly short and in consequence of the breadth of the 

 body are separated by a considerable interval. This explains the peculiar 

 sidling gait of the animal in rapid locomotion. Shore crabs occur in 

 abundance on the beach between tide marks, crawling under the rocks 

 and other objects for safety when the tide goes out. Other larger crabs, 

 including the edible ones, find it less easy to hide and follow the water 

 out with the tide. A soft-shelled crab is one which is caught in the process 



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