84 Descriptive Zoology. 



a carapace, and the head may consist of but one piece, formed by the 

 consolidation of several segments (usually five). The thorax has eight 

 segments and the abdomen usually seven. 



Without attempting to enumerate the orders, we may mention the 

 Amphipoda, and illustrate them by the beach fleas and sand hoppers, 



FIG. 56. WATER FLEA (CYCLOPS). 



Female with egg sacs. There is a single eye with two facets. 



which have a laterally compressed body, the anterior legs bearing gills, 

 and the posterior used for jumping. 



The Isopoda have a body flattened from above, and bear gills on the 

 abdominal appendages, as in the sow bug, shown in Fig. 57. 



The Decapoda, or ten-footed Crustacea, are so named from the five 



large pairs of thoracic appendages observed in the crayfish, which serves 



x . as an example of the group. The Decapods 



\ "iskffifellS^Es^. are sometimes ^ r ther divided into the 



^P| IreBBer Macrura, or long-tailed forms, such as the 



f^*yjK& : *" crayfish and lobster, and the Brachyura, or 



-y^sss^x^ short-tailed forms, such as the crabs. 



FIG. 57. Sow BUG 

 (Crustacean). 



Characters of Crustacea. i . The 



crustaceans have a hard cuticle, 

 formed by the underlying skin. The cuticle consists 

 largely of a substance called cJiitin, which is tough and 

 more or less elastic. The chitin becomes more or less 

 infiltrated by carbonate of lime, and is thus made harder. 



