502 The Animal Kingdom 



body. The related genus, Astaciis, which dwells on the west coast, has 

 an additional gill, the picurohranch, which is attached to the thorax 

 above the thirteenth appendage. 



The excretory system consists of a pair of green glands lying in 

 the head region anterior to the esophagus. Each organ possesses a 

 glandular region for waste removal, a bladder, and a duct which opens 

 ventrally at the base of the antennae. 



Fig. 165. — Longitudinal section through ommatidia of a crayfish. Note migration 

 of pigment. A, In bright light; B, in dim light. 



The nervous system is quite similar to that of the earthworm. 

 Above the esophagus is the supraesophageal ganglion, from which nerves 

 lead to supply the eyes, antennae, and antennules. A pair of circumesoph- 

 ageal commissures lead from this ganglion to the subesophageal gang- 

 lia. From these latter, a double ventral nerve cord passes posteriorly 

 for the length of the body. A series of ganglia along its length give ofif 

 lateral nerves. In the embryo there is a ganglion for each segment, but 

 in the adult, fusion occurs. Posterior to segment 13, however, each 

 segment does retain its individual ganglion. In addition to this central 

 nervous system there is a visceral nervous system which arises from the 

 ventral portion of the supraesophageal ganglion. 



