282 COMPARATIVE ZOOLOGY. 



ganglia running along the ventral surface of the body 

 under the alimentary canal. The brain is in the form of 

 a ring encircling the gullet. The alimentary canal and 

 the circulatory apparatus are nearly straight tubes lying 

 lengthwise the one through the centre, and the other 

 along the back. The skeleton is composed of a horny 

 substance (chitine), or of this substance with carbonate of 

 lime. All the muscles are striated. 



There are four classes, of which the first is water-breath- 

 ing, 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 subkingdom, 

 armed with powerful claws and a hard cuirass bristling 

 with spines. Although constructed on a common type, 

 Crustaceans exhibit a wonderful diversity of external 

 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-one segments, 

 of which seven belong to the head, seven 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 tho- 

 rax 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 chitine 

 and calcareous matter. 147 



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

 pendages, feelers, jaws, claws, and legs beneath the ceph- 

 alo-thorax, and flat swimmerets under the abdomen. In 

 fact, as a rule, every segment carries a pair of movable 

 appendages. The seven segments of the head are com- 



