282 COMPARATIVE ZOOLOGY. 



ganglia running along the ventral surface of the body un- 

 der the alimentary canal. The brain is connected to the 

 ventral ganglia by a ring encircling the gullet. The ali- 

 mentary canal and the circulatory apparatus are nearly 

 straight tubes lying lengthwise the one through the cen- 

 tre, and the other along the back. The skeleton is com- 

 posed of a horny substance (cliitin), or of this substance 

 with carbonate of lime. All the muscles are striated. 



There are four principal classes, of which the first is 

 water-breathing, and the others air-breathing. 



CLASS I. Crustacea. 



The Crustacea 144 are water-breathing Arthropoda, usu- 

 ally with two pairs of antennae. 146 Among them are the 

 largest, strongest, and most voracious of the subking- 

 dom, armed with powerful claws and a hard cuirass bris- 

 tling with spines. Although constructed on a common 

 type, Crustaceans exhibit a wonderful diversity of ex- 

 ternal form : contrast, for example, a Barnacle and a Crab. 

 We will select the Lobster as illustrative of the entire 

 group. 



A typical Crustacean consists of twenty segments, of 

 which five belong to the head, eight to the thorax, and 

 seven to the abdomen. 146 In the Lobster, however, as in 

 all the higher forms, the joints of the head and thorax 

 are welded together into a single crust, called the cephalo- 

 thorax. On the front of this shield is a pointed process, 

 or rostrum; and attached to the last joint of the abdomen 

 (the so-called " tail ") is the sole representative of a tail 

 the telson. This skeleton is a mixture of chitin and 

 calcareous matter. 147 



On the under-side of the body we find numerous append- 

 ages, feelers, jaws, claws, and legs beneath the cephalo-tho- 

 rax, and flat swimmerets under the abdomen. In fact, as 

 a rule, every segment carries a pair of movable append- 



