164 THE CRAYFISH. 



the fourth and fifth ganglionic masses diverge 

 slightly, and the sternal artery (p. 167) passes 

 between them. 

 The thoracic nerve-chain lies beneath the endqphragmal 

 skeleton, which must be removed to expose it. 



e. The abdominal nerve-chain consists of six ganglionic 

 masses, joined by connectives, and supplying 

 the abdominal segments and their appendages. 

 The hindmost ganglionic mass is larger than 

 the others, and supplies both the nineteenth 

 segment and the telson. 



2. The visceral nervous system. 

 This can only be dissected in a specimen in which the ali- 

 mentary canal has been left undisturbed. 



a. The anterior visceral nerve is a median nerve 



formed, just in front of the oesophagus, by the 

 union of a median nerve from the ' brain ' with 

 two pairs of nerves arising from ganglionic 

 swellings on the para-cesophageal connectives. 

 From the union of these, the nerve runs up in 

 front of the stomach to branch out on its dorsal 

 wall. 



b. The posterior visceral nerve arises from the hind- 



most ganglionic mass of the abdominal nerve- 

 chain, and runs forwards along the ventral 

 surface of ihe intestine. 



G. The Organs of Special Sense. 



Besides tactile organs, i.e. the antennules, antenna?, and 

 the palps of the oral appendages, the crayfish has eyes, and 

 1 auditory ' and olfactory organs. 



1. The eyes, on movable stalks, have already been seen at 

 the sides of the rostrum. 



a. External characters of the eye. 



Remove one of the eye-stalks, and examine it carefully 

 with a lens. 



