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 (Pl. XXXVIIL., 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 (Pl. XXXVIII., Fig. 4, 72) 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 
