THE EYE. 157 



union of a median nerve from the ' brain ' with 

 two pairs of nerves arising from ganglionic swell- 

 ings on the para-oesophageal 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 the intestine. 



G. The Organs of Special Sense. 



Besides tactile organs, i.e. the antennules, antennEB, and 

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

 ' 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. 



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

 with a lens. 



The ' cornea ' is the transparent, apparently 

 black, cap of uncalcified cuticle covering the end 

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

 Its surface is divided into quadrangular facets by 

 lines crossing each other nearly at right angles, 

 each facet corresponding to one of the elements of 

 which the compound eye consists. 



b. Structure of the eye as seen in a longitudinal 



section. 



Carefully, divide the eye-stalk and eye longitudinally into 

 two holmes. The calcified portion of the cuticle is best cut 

 with scissors, the remainder with a sharp knife. Examine one 

 of the cut surfaces with a strong lens. 



The eye consists of a number of conical bodies 

 arranged in a radiate fashion, their bases being 

 turned outwards and formed by the corneal facets, 

 and their apices being directed inwards and 



