SECT, vi THE SENSORY ORGANS 93 



of these cones seems one of the simplest methods of 

 obtaining convex surfaces to act as lenses for the 

 reception of the light-rays from all directions. The 

 formation of corneal lenses over these cones is a 

 secondary and, we think, a much more complicated 

 specialisation. 



Another possible advantage gained in the separa- 

 tion of the cones from the cuticle is the slight 

 possibility of movement which the separate omma- 

 tidia or single eyes would thereby acquire. It 

 appeared clear to us, during our study of the eye of 

 Apus, that the separate elements were capable of 

 slight movement, brought about, no doubt, by the 

 layer of connective tissue, which is only indicated 

 by faint lines in the diagram (Fig. 23), but which in 

 reality is very highly developed. The slight attach- 

 ment of the crystal cone cells to the cuticle would 

 not altogether prevent such small movements as we 

 suppose. 



If the Crustacean eye is in this way to be referred 

 to the formation of the exoskeleton, it seems clear 

 that no special value can be attached to similarity of 

 eyes in establishing the relationships between animal 

 groups. The development of an exoskeleton is 

 common to the whole class of the Arthropoda, and 

 there is no reason why very different forms of cuticular 

 irregularity should not be utilised by the visual cells, 

 which would group themselves accordingly rn different 

 ways. We thus see no reason whatever for trying to 

 deduce the one form of Arthropodan eye from the 

 other, it being more probable that they are with a few 



