166 THE CRAYFISH. 



layer of brilliant tapetal pigment, either white 

 or pink, 

 iv. The optic ganglion (really ganglia) occupies the 

 eye-stalk. Its surface is deeply pigmented. 



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

 and connects the optic ganglion with the 

 1 brain.' 



2. The ' auditory > organs or * statocysts ' 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. 



Remove 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 injure the ' auditory ' 

 sac itself Clear away the muscles surrounding the sac, and 

 the setos from the aperture. 



With a fine-pointed syringe wash out the contents of the 

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

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

 1 otolith ' has been given. 



Examine the sac in water or glycerin on a slide under a 

 low power of the microscope. 



a. The * auditory * sac is a transparent ovoid chitinous 



sac, continuous at its opening with the cuticle of 

 the antennule. 



b. The sensory setae are nearly straight simple-pointed 



setae varying greatly in size, the outer ones being 

 the largest. The larger ones are arranged in 

 parallel curved lines along the floor and inner 

 wall of the sac ; of these there are two rows 

 behind, the number increasing to five or more 

 in front. Parallel to these runs a row of very 

 minute setae on the inner side. 



c. The sensory nerve supplying these rows of setae 



may have been removed with the muscles, but 

 should be looked for. 



