158 THE CRAYFISH. 



connected with the optic ganglion. Bach of these 

 component elements is isolated optically from its 

 neighbours by pigment. 



i. The cornea is thin, 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 to one 

 corneal facet. They are separated from one 

 another by layers of pigment near their outer 

 ends. 



iii. The inner ends of the pyramids are imbedded in 

 a black pigmented layer, in which lie a series 

 of radially arranged rods and fusiform bodies, 

 each of the latter being surrounded by a- 

 layer of brilliant pigment, either white or 

 pink. 



iv. The optic ganglion occupies the centre of the 

 eye. Its surface is deeply pigmented. 



V. The optic nerve lies in the axis of the eye-stalk, 

 and connects the optic ganglion with the 

 ' brain.' 



2. The ' auditory ' organs are lodged in the basal segments 

 of the antennules, and open to the exterior on their 

 dorsal surfaces by three-cornered apertures, guarded 

 by fringes of plumose setae. 



Bemove an antennule entire. Holding it firmly between 

 the fingers, carefully remove with scissors the inner and outer 

 walls of the basal segment, taking care not to injtire the' audi- 

 tory ' sac itself. Clear away the muscles surrounding the sac, 

 and the setce from the a'perture. 



With a fine-pointed syringe wash out the contents of t]ie 

 ' auditory ' sac — a collection of sand and other foreign matter 

 from the water in luhich the animal lives, to which the name 

 ' otolith ' has been given. 



