xii. THE FRESH-WATER CRAYFISH. 155 



y. Behind the crystalline cone comes the con- 

 nective rod. It is widest in front where it 

 joins the cone, but narrows posteriorly where 

 it is continuous with the striated spindle. If 

 fresh eyes be treated with osmic acid and 

 then teased out, each of these rods can be 

 split up into four fibres. 



8. 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 pigmented layers seen with the low 

 power (b. a.) : they are best seen in specimens 

 treated with dilute caustic potash. 



c. 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. 



. If the section has passed back along the optic 

 nerve two obliquely placed lenticular masses 

 will be seen among its fibres. 



17. 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 



