COLOR-SPOTS. 



131 



it may be possible to trace an almost complete series 

 from such a mere spot of colour in the skin up to a 

 complex organ of vision, such, for instance, as- that of 

 a snail ; indeed, in the development of the eye in tlie 

 individual animal may be traced some of the same stages 

 as have probably been passed through by the ancestral 

 forms of the animal itself in long bygone ages. 



We must not, however, suppose that all eyes can be 

 traced back to one and the same origin, or have been 

 developed in the same manner. There are even cases 

 in which an organ fulfilling a different function appears 

 to have been modified into an eye. 



Look, for instance, at the organ of touch of 

 Onchidium * (Fig. 16). The cuticle (see p. 14) is 

 thickened into a biconvex, almost lens-like body ; the 

 epithelial cells are elongated, and below is a mass of 

 cells, to which runs a nerve. A very little change 

 would make this an organ capable of distinguishing 

 light from darkness, and „ j 



some of the eyes of On- 

 chidium appear, indeed, 

 to have thus originated. 



Compare with this, for 

 instance, the ocellus of 

 the young larva of a 

 water-beetle (Fig. 84), 

 as figured by Grenacher. 



The eye-spots of Me- 

 dusae were first noticed 

 by Ehrenberg in 1836, 

 and the lens was discovered many years afterwards by 

 de Quatrefages. It is, in fact, by no means universally 

 • A slug-like genus of molluscs. 



Fig. 84 Section through the simple eye of 



a young Dytiacus larva (after Grenacher). 

 I, Corneal lens ; g, cells forming the vitreous 

 humor ; r, retina ; o, optic nerve ; A, hypo- 

 derm. 



