THE EAR 149 



high refractive power, which appear black by 

 transmitted, white by reflected light. 



v. The pigment in which these rods and fusiform 

 bodies are imbedded is a dense black substance 

 in which no structure can be seen. 



vi. The optic ganglion shows the usual ganglion- 

 cells and nerve-fibres, and its outer part is 

 deeply pigmented. 



2. The auditory organs are lodged in the basal joints 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 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 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 : 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 auditory nerve supplying these rows of setae 



may have been removed with the muscle, etc., 

 but should be looked for. 



