ARTHROPODA. 227 



Usually these segments can be grouped in regions, of 

 which, at most, three can be distinguished: in front the 

 head; next, the thorax; and behind, the abdomen. The 

 head is largely concerned in the taking of food, and is the 

 seat of the special senses. The thorax is the locomotor 



FIG. 104. Diagram of grasshopper showing the body divided into the three 

 regions : head, thorax, and abdomen. 



region, while in the abdomen the primitive segmentation 

 is most marked. 



Through the body as an axis runs the alimentary canal, 

 the mouth being on the under surface of the head, while 

 the vent is at the tip of the abdomen. Above the digestive 

 tract lies the heart, which in some forms has a chamber in 

 each of several somites of the body; that is, the heart is 

 segmented. On the floor of the body, below the alimentary 

 canal, is the nervous system, which exhibits this segmen- 

 tation in a more marked degree. In each segment there is 

 a paired enlargement or ganglion from which nerves go to 

 the various organs of the segment. These ganglia of the 

 successive segments are connected with each other by a 

 double nerve-cord, so that all are in communication with 

 each other. At the front end of the body one of these 

 nerve-cords passes on one side of the oesophagus, the other 

 on the other, and above it they unite with a large compound 

 ganglion, the so-called brain. In this way a part of the 

 nervous system is brought above the alimentary canal, 

 while the rest lies below. In other words, the digestive 

 tract passes through the nervous system, a condition which 



