188 INSECTS AND CRUSTACEA DISTINGUISHED. 



sometimes forty-eight inches. When irritated it emits 

 a bluish light. 



The great class of CRUSTACEA, of which Crabs and 

 Lobsters are familiar examples, demands some notice, 

 but is so varied in its aspect and in the numerous types 

 which its orders, families, and genera pi'esent to us, 

 that I cannot attempt to give, in the few pages to be 

 devoted to the subject, even an outline sketch of its clas- 

 sification. I can scarcely do more than notice in this 

 place a few of the commoner Crabs which one meets 

 with in dredging. 



The class Crustacea of modern authors was included 

 by Linnaeus among his Insects, and formed a part of the 

 Aptera or wingless insects of that author. The struc- 

 ture of a Crab or Lobster, or a Wood-louse, which are all 

 members of the class, does indeed in many ways resem- 

 ble that of the true Insects. The body is cased with 

 hard materials, it is divided by articulations into several 

 rings, it is furnished with jointed legs and with those 

 curious organs called antennae or feelers, and it possesses 

 a mouth constructed on a very similar type. There is 

 one peculiarity, however, in which there is a remarkable 

 difference between the Crustacea and both the Insect and 

 Spider Classes. These latter groups of animals are des- 

 tined to live in the ah*, and their respiration is conse- 

 quently performed by lungs. Even such species of them 

 as are aquatic carry with them under water the quantity 

 of air necessary to their existence, just as a diver inflates 

 his lungs before he leaves the surface ; though it is 

 quite true that some breathe through the medium of 

 gills while they continue in a rudimentary state. The 



