CRUSTACEA. 



229 



other Crustacea molt or shed their skin in the same way, 

 the new skin rapidly growing hard again, but the blue 



FIG. 56. Shore-crab (Cancer irroratus}. 



crab is the only one taken in sufficient abundance at this 

 time to be of economic importance. 



ORDER U.--TETRADECAPODA. 



Contrasted to the Decapods are the fourteen-footed or 

 tetradecapodous forms, of which the sow-bug is one type. 

 In these we can distinguish clearly head, thorax, and 

 abdomen, the joints of the thorax being freely movable 

 on each other. The eyes are not placed upon movable 

 stalks, but are scarcely elevated above the general sur- 

 face of the head. Most of these forms are marine; a 

 few live in fresh water, and still fewer, like the sow-bugs 

 and pill-bugs, upon the land. All are small, those which 

 reach two inches in length being the veritable giants 

 among the group.* 



* An isopod from the greater depths of the ocean reaches a length 

 of six inches. 



