THE EYE 149 



G. The Organs of Special Sense. 



Besides tactile organs, i.e. the antennules, antennae, 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. 



Remove one of the eye-stalks, and examine it ca/refull/y 

 with a lens. 



The ' cornea ' is the transparent, apparently 

 black, patch of uncaleified 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 halves. The calcified portion of the cuticle is best cut 

 with scissors, the remainder with a sha/rp knife. Examme 

 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 connected with the optic ganglion. Each of 



these component elements is isolated optically 



from its neighbours by pigment. 



i. The cornea is thin and semicircular in section, 



and the strips of cuticle separating its facets 



scarcely differ from the facets themselves. 



ii. The crystalline pyramids are transparent 



conical bodies arranged radially and abutting 



upon the cornea, each pyramid corresponding 



