THE STRUCTURE OF THE EYEBALL 



503 



becomes connected with, the retina on the side nearest to the lens, an arrangement 

 similar to that found in vertebrates. The insect eye (G), which is also found in crus- 

 tacea, is arranged on an entirely different plan. It may be regarded as being formed 

 by packing an exceedingly large number of elongated ocelli together, with their lenses 



FIG. 251. Comparative anatomy of the eye. 



(A) Single cell as in amphibia and ccelenterata. (B) Mollusc Patella. (C) 

 Mollusc Nautilus. (D) Mollusc Helix. (E) Coslenterate Charybdea and ocellus 

 of insects. (F) Mollusc Pecten. (G) compound eye of insect. 



terior and their retinae posterior, to form a solid hemispherical body. It is this 

 rmation from a number of separate elements which gives the eye of the insect its 

 ted appearance. Exner and others have shown that the refracting media 

 of the separate elements cannot form a focussed image on the sensitive end organ 

 which each contains, but that vision must consist of a mosaic, as Johannes Miiller had 

 suggested. It is said that such vision is well adapted to observing movement. The 

 eyes of vertebrates may be considered to be of the same type as that found in man, for 

 the differences that are met with chiefly concern detail, except as regards the mechan- 

 ism used for accommodation. 



