122 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



the crayfish are very widely met with among Crustacea 

 and Insecta, and are commonly known as compound eyes. 

 In many of these animals, in fact, when the cornea is re- 

 moved, each facet is found to act as a separate lens ; and 

 when proper arrangements are made, as many distinct 

 pictures of external objects are found behind it as there 

 are facets. Hence the notion suggested itself that each 

 visual pyramid is a separate eye, similar in principle of 

 construction to the human e3^e, and forming a picture of 

 so much of the external world as comes within the range 

 of its lens, upon a retina supposed to be spread out on 

 the surface of the crystalline cone, as the human retina is 

 spread over the- surface of the vitreous humour. 



But, in the first place, there is no evidence, nor any 

 probability, that there is anything corresponding to a 

 retina on the outer face of the crystalline cone ; and 

 secondh^, if there were, it is incredible that, with such an 

 arrangement of the refractive media as exists in the 

 cornea and crystalline cones, rays proceeding from points 

 in the external world should be brought to a focus in cor- 

 respondingly related points of the surface of the supposed 

 retina. But without this no picture could be formed, and 

 no distinct vision could take place. It is very probable, 

 therefore, that the visual pyramids do not play the part 

 of the simple eyes of the Vertehrata, and the only alterna- 

 tive appears to be the adoption of a modification of the 

 theory of mosaic vision, propounded many years by 

 Johannes Miiller. 



