148 THE CRAYFISH 



c. Minute structure of the eye. 



Carefully tease portions of the eye on a glass slip in 

 glycerine with fine needles, and examine with the microscope. 



i, The crystalline pyramids readily divide into 

 inner and outer portions. The outer portion 

 is more highly refractive, and the outer end of 

 it may sometimes be separated as a quadran- 

 gular concavo-convex plate, the convex surface 

 being turned outwards. The inner portion 

 is softer, and sometimes, especially after boil- 

 ing, can be made to split longitudinally into 

 four. The crystalline pyramids are probably 

 refractive bodies, which aid the cornea in 

 focussing the rays of light on the sensitive 

 elements of the eye. 



ii. The superficial cells of the pigmented layer are 

 pear-shaped. The body of each cell is colour- 

 less, its nucleus containing a few granules of 

 black pigment. These cells lie, in the natural 

 condition, between the pointed inner ends of 

 the pyramids. 



iii. The fusiform bodies, or * striated spindles,' are 

 transparent spindle-shaped bodies, marked by 

 a series of constrictions running round them, 

 and giving them a wavy outline. They are 

 placed radially and opposite the inner ends of 

 the pyramids. The finely granular substance 

 surrounding them appears almost black by 

 transmitted, but pale pink by reflected light. 

 It is probable that these are the elements 

 directly sensitive to light, and corresponding, 

 therefore, physiologically to the rods and cones 

 of the retina of a vertebrate eye. 



iv. The crystalline rods are delicate transparent 

 rods ending in highly refractive spheres, and 

 lying between the fusiform bodies. The rods 

 are surrounded by very fine granules of very 



