TWO KINDS OF EYES. 183 



an increase of the number of eyes would bring us to 

 the compound eye. 



On the other hand, it must be admitted that there 

 are reasons for considering the different kinds of eyes 

 to be of perfectly distinct origin. The eye of Limulus^ 

 according to Grenacher, is formed on a plan quite 

 unlike that of other Crustacea. Again, the develop- 

 ment of the eye in 3Iusca, to judge from Weismann's 

 observations, is very dissimiliar from that of other 

 insects. The varied position of the eye in different 

 groups, as, for instance, in Pecten, Spondylus, 

 Ewphausia, Onchidium, &c., point to the same con- 

 clusion. 



It seems clear that the image produced by the 

 ocelli must be altogether different from the picture 

 given by the compound eyes ; and we may therefore 

 reasonably conclude that the two organs have distinct 

 functions. It used formerly to be supposed that the 

 compound eyes were intended for distant, the ocelli for 

 near vision. Claparede, however, has maintained the 

 opposite theory, while Mr. Lowne regards the ocelli as 

 incapable of producing ' anything worthy the name of 

 an image,' and suspects that their function ' is the 

 perception of the intensity in the direction of light, 

 rather than vision.' 



The ocelli, or simple eyes, probably see in the same 

 manner as ours do. That is to say, the lens throws an 

 image on the back of the eye, which we call the retina. 

 In that case they would see everything really reversed, 



