676 William Patten 



form a compound eye like that of Insecta, was taking place? We should 

 expect an increase in the uumber of ocelli, accompanìed by a reduction 

 of each retineum to a single ommatidium with axial rods. There 

 does not appear to be any evideuce of this kind in either of these 

 groups. The ocelli are notnumerous; they remain perfectly distiuct, 

 each one surrounded by its basai membrane, and provided with a 

 single optic nerve ; the retinenm shows no traces of reduction to a 

 single ommatidium; and the rods are apical. In Spiders there is much 

 less evidence of such changes than in Myriapods. In the latter group, 

 Scutigera promises to give a solution to this problem. Whether its 

 systematic position is beyond doubt, remains to be seen. The obser- 

 vations of Grenacher alone are too uncertain to decide the question. 

 and besides it would be exactly as difficult to suppose, with our pre- 

 sent knowledge of the subject, that the eye oi Scutigera was formed 

 by the union of many ocelli, as by the modification of a single one. 

 Therefore, in spite of the interest that must centre upou a more accurate 

 knowledge of its structure, we must leave it for the present, and be 

 satisfied with more definite evidence, 



(2) Is there any evidence in the compound eye itself, that it was 

 formed by the fusion of ocelli? If such had been the case, we should 

 expect to find around each ommatidium some trace of the counective 

 tissue capsule which formerly surrounded the ocellus. In some of th« 

 simpler compound eyes, where we may suppose that the modifications 

 of the ocelli bave been less extensive, we should surely expect to find 

 some indication of this membrauous capsule, a structure which, by its 

 almost universal presence in the isolated eyes, could not be so quickly 

 disposed of in the aggregate condition. Around each ommatidium 

 there is, however, not the slightest trace of any capsule 

 arisingfrom the basai membrane, noristhereanyinfolding 

 of the same, which might be interpreted as a remnant of the 

 basai membrane belonging to a primitive ocellus ! But even 

 if we suppose that such infoldings once existed and bave now disap- 

 peared, we surely ought to find some trace of the originai arrange- 

 ment of the ommatidial cells in isolated groups. The circular arrange- 

 ment of the retinulae around the retinophorae is a very ancient 

 one; and, as we bave seen, the position of the cell-ends upou the 

 basai membrane is very Constant, and furnishes a valuable key to the 

 relation of the circles of retinulae to the retinophorae. If, then, each 

 ommatidium is to be regarded as the remnant of au ocellus, the position 

 of the cell-ends, which furnish us with the safest evidence, should show 



