The Crayfish. 



47 



six in the abdominal. It will be observed that in the abdominal 

 region there is an exact correspondence between gangHa and 

 segments, but not in the cephalothoracic. As, however, there 

 is some fusion between the originally separate segments of the 

 anterior region of the body, it is hardly surprising to find that 

 this characteristic has extended itself to the ganglia of the 

 nervous system. From the circum-cesophageal commissure 

 and from the cerebral (supra-oesophageal) ganglia arise three 



Fig. 24.— Green gland of Crayfish. (After Marschall.) 



The upper figure represents the apparatus dissected out. v, vesicle ; /.i, terminalsac ; 



between them lies the glandular part. 



nerves which unite to form a trunk, which supplies the anterior 

 part of the alimentary tract, and is known as the visceral nervous 

 system. 



The crayfish is well off for sense organs. The eyes have 

 been already mentioned. They have a complex structure, 

 equally complicated with that of the eye of a man, but of a 

 different character. It will not be possible to describe them 



