OCELLI OF SPIDEKS— MYKIAPODS. 155 



The spiders have simple ocelli only, the higher 

 Crustacea compound eyes only, while many of the 

 lower Crustacea and of the great class of the insects 

 possess both eyes and ocelli. It would seem probable, 

 therefore, that the ancestral stock must have possessed 

 both, though not perhaps in so perfect a form as that 

 which has now been attained, and that the spiders have 

 lost the compound eyes, while, on the contrary, in the 

 higher Crustacea the ocelli have disappeared. 



Moreover, though the ocellus of a spider at first 

 sight closely resembles the eye of a Scolopendra, the 

 internal structure is, according to Grenacher, altogether 

 different. In the ocellus of a spider or an insect we 

 find, at a greater or less distance behind the lens, a 

 retina consisting of a receptive surface, extended con- 

 centrically with that of the lens, and consisting of a 

 number of more or less rod-like perceptive elements so 

 arranged that the light falls on their ends. 



On the contrary, in the eyes of Myriapods there is, 

 he says, either a single element behind the cornea, or 

 where there are many such elements, they are arranged 

 with their longer axes perpendicular to the direction 

 of light ; so that any separate perception of the rays 

 of liglit coming from different points seems to be an 

 impossibility. In the eye of Lithobius, behind the 

 biconvex lens, he states that the cells lining what I 

 may call the tube of each separate eye, terminate in 

 filaments, between the free ends of which is left a narrow 

 passage, down which the light must pass to reach the 

 end of the optic nerve. Such a structure is certainly 

 very remarkable, and seems entirely to preclude the 

 possibility of the formation of a true image. Altogether 

 the account given by Grenacher, both as to the mode 



