146 THE CRAYFISH 



The thoracic nerve-chain lies beneath the endophragmal 

 skeleton, which must he 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 

 alimentary canal has been left undisturbed. 



a. The anterior visceral nerve is a median nerve 



formed, just in front of the oesophagus, by the 

 imion of a median nerve from the ' brain ' with 

 a pair of nerves arising from the para-cesophageal 

 connectives. Prom the point of union of these 

 three 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 the 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, ears, 

 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. 



Bemove one of the eye-stalks, and examme it carefully 

 with a lens. 



The 'cornea' is the transparent, apparently 

 black, patch of uncalcified cuticle covering the end 

 of the eye-stalk, and bounded by an oval outline. 

 Its surface is divided into quadrangular facets by- 



