ARTHROPODA. 287 



Cray -fish (Astacus), Lobster (Homarus), and Crab (Can- 

 cer). Crabs differ from Lobsters chiefly in being formed 

 for creeping at the bottom of the sea instead of swim- 

 ming, and in the reduction of the abdomen or "tail" to a 

 rudiment, which folds into a groove under the enormous 

 thorax. They are the highest and largest of living Crus- 

 tacea: they have been found at Japan measuring fifteen 

 feet between the tips of the claws. 



CLASS II. Myriapoda. 



Myriapods differ from Crustaceans and Spiders in hav- 

 ing the thorax merged in the abdomen, while the head is 

 free. In other words, the body is divided into similar 

 segments, so that thorax and abdomen are scarcely distin- 

 guishable. They resemble Worms in form and in the 

 simplicity of their nervous and circulatory systems; but 

 the skin is stiffened with chitin, and the -legs (indefinite 

 in number) are articulated. The legs resemble those of 

 Insects, and the head appendages follow each other in the 

 same order as in Insects eyes, antennae, mandibles, max- 

 illae, and iabium. They breathe by tracheae, and have two 

 antennae and a variable number of eyes. 



There are two orders : 



1. Chilognatha, having a cylindrical body, each segment, 

 except the anterior, being furnished with two pairs of legs. 

 They are of slow locomotion, harmless, and vegetarian. 

 The Thousand-legged Worm (Julus) is a common repre- 

 sentative. 



2. Chttopoda, characterized by having a flattened body 

 composed of about twenty segments, each carrying one 

 pair of legs, of which the hindermost is converted into 

 spines. They have longer antennae than the preceding, 

 and the mouth is armed with two formidable fangs con- 

 nected with poisonous glands. They are carnivorous and 

 active. Such is the Centipede (Scolopendra, Fig. 259). 



