150 THE CEAYFISH 



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 joints of 

 the anteimules, and open to the exterior on their 

 dorsal surfaces by three-cornered apertures, guarded 

 by fringes of plumose setse. 



Bemove an antennule entire. Holding it firmly between 

 the fingers, carefully remove with scissors the inner and outer 

 walls of the basal joint, taking care not to injure the auditory 

 sac itself. Clear away the muscles surrounding the sac, and 

 the setcB from the auditory 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 

 ' otolith ' has been given. 



Examine the sac in water or glycerine on a slide under 

 the 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 auditory setse are nearly straight, simple, 



pointed set® 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 



