THE THEORY OF ARTHROPOD VISION. 565 



the transmission of oblique pencils, and such rods transmit only 

 axial pencils. It is theoretically possible to obtain an image by 

 the light transmitted when only parts of the rods are blackened, 

 even when the rod is curved. As part of the surface of each 

 great rod at least is pigmented in all Arthropods, it is possible 

 that only such light passes through them as is suitable for the 

 production of an image, and such light, even in the remarkable 

 eyes of Phronimids, may be concentrated by a second refrac- 

 tion upon one or more retinal elements. 



The most remarkable indication of the truth of the view 

 advocated by Exner and myself is afforded by the structure of 

 the eye in Copilia. In this remarkable Crustacean the cornea 

 has a true lens beneath it separated from a lenticular cone by 

 a space filled with a vitreous-like body. Gegenbauer described 

 a muscular apparatus for effecting the requisite accommodation 

 in these eyes, and although the presence of muscles was sub- 

 sequently denied by Claus, it seems hardly possible that a 

 means of accommodation is absent (PI. XXXVIII. , Fig. 4). 



Grenacher saw that the eye of Copilia represents one of the 

 ommatea of a compound eye [222, p. 73]. But he compares 

 the second refractive body (PI. XXXVIII., Fig. 4, rh) with the 

 crystalline cone, and the pigmented portion of the eye with 

 the great rod. If we compare the vitreous space of Copilia, 

 however, with the pseudo-cone and the second refractive lens 

 with the rhabdome, then the whole structure becomes in- 

 telligible, and I cannot see on what grounds Grenacher 

 arrives at the conclusiou that the second refractive body of 

 Copilia represents the crystalline cone. It is not difficult to 

 see that the second refractive body in Copilia cannot be 

 regarded as a cone, and that it should rather be compared 

 with a solid lenticular rhabdome. 



Grenacher concludes that the complex and beautiful eye of 

 Copilia is useless, and cannot perceive an image. I agree with 

 Exner that this conclusion is scarcely probable, and I am more 

 inclined to accept Exner's conclusion, which is, that the second 

 refractive body and retina are capable of moving over the 

 picture produced by the lens, which Copilia explores as it were 



