INVE11TEBKATA. 



319 



_____ CORHCKL FACET 



CKYSfAL CUL 



--- flK$//U BOOt 



STRIKTCO 



SPINDLE 



NCRVE FIBKE 



Fig. 101. Two ELEMENTS OF THE 

 CRAYFISH'S EYE. 



(After lluxley.) 



f 



cejls, Whose nuclei lie close under the corueal facet. These 



cells are simply ordinary epidermal cells modified. (3) 



Next comes a group of cells called a retmula, surrounding 



a central rh.abd.om, which is 



spindle-shaped, and shows 



peculiar transverse striations 



(hence called striated spindle). 



This rhabdom is really made 



of four rod-like bodies closely 



apposed. The base of each 



rhabdom is surrounded by a 



process from a nerve-cell. These 



nerve-cells form a mass at the 



base of the eye (optic ganglion) 



and send their axis cylinders 



into the cerebral ganglion. 



Each nerve- element is separ- 

 ated from its neighbours by 

 tissue full of dense pigment 

 which effectively screens off all 

 light other than that falling directly normal to the surface 

 of each particular corneal facet. Each element thus sub- 

 tends a particular sector of the field of vision, and an imago 

 of each such fraction may be said to be separately focussed 

 by each particular crystalline cone on to each retinula. But 

 the "image" so projected might as well be a mere spot, for 

 it can give rise to only a simple sensation of varying in- 

 tensity. But the whole series of such spots put together 

 would make an image differing from the image on a 

 Vertebrate retina much as the familiar stippled "half- 

 tone" photo-engraving differs from a real photograph. 

 Moreover, the compound image would be erect, not inverted 

 as is that on the Vertebrate retina. In practice the cray- 

 fish's eye is quite as good as he needs, for it guides him iu 

 seizing an object with the greatest precision. 



24. Other Sense Organs. On the dorsal surface of the 

 basal joint of each antennule is the opening of a little sac, 

 lined by a continuation of the external cuticle. The cavity 

 of this sac contains a gelatinous fluid into which project setse, 



