122 THE PHYSfOLOGY 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 eye, 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 
secondly, 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 Vertebrata, and the only alterna- 
tive appears to be the adoption of a modification of the 
theory of mosaic vision, propounded many years by 
Johannes Muller. 
