XII.] THE FKESH-WATER CRAYFISH. 149 



spindle. If fresh eyes be treated with osmic 

 acid and then teased out, each of these rods 

 can be split up into four fibres. 

 B. The striated body is fusiform and presents 

 well-marked transverse striations. Besides 

 these coarse striations, however, much finer 

 ones can be seen by careful examination with a 

 high power. The outer ends of these spindles 

 correspond in position to the second of the pig- 

 mented layers seen with the low power (b. a.) : 

 they are best seen in specimens treated with 

 dilute caustic potash. 



e. Beneath the striated spindles is a perforated 

 membrane through which the spindles pass to 

 become continuous with the optic ganglion. 

 From their ends pass nerve-fibres which run 

 inwards in a converging manner and among 

 which nerve-cells are here and there scattered. 

 Within the ganglion are several concentric 

 pigmented bands. 



f. If the section has passed back along the 

 optic nerve two obliquely placed lenticular 

 masses will be seen among its fibres. 



97. Passing back from the cornea to the optic 

 ganglion is a membrane investing each cone, 

 rod, and spindle. It is on this that most of the 

 pigment lies which causes the two outer dark 

 bands. Over the rods the pigment is wanting 

 and there the membrane is seen to possess oval 

 nuclei. 



24. The Auditory organ. 



This lies in the basal joint of the antennule and is 

 best examined in the lobster. The upper surface of 



