Eyes of Mollusca and Arthropods. 675 



apical rod like that of Pecten. Area, or Phryganea, may be transformed 

 into an axial cone. like that of the posterior eye of Spiders, is sliown 

 in the diagram (PI. 32, tigs. 132—138). 



In the simpler eyes. whcre the retineum still exists, the retino- 

 phorae seem to be formed of double cells ; only oue nucleus is well 

 developed, the other is probably reduced in size and distinctness, as 

 in Molluses and in Worms. With the development of axial rods in 

 Spiders, one or two) nucleus may be found at the outer end of the cells, 

 above the rod, as in the compound eye : in other cases it seems to be 

 below. In spite of the interesting modification of the ommatidia in the 

 anterior and posterior ocelli, there can be no doubt that they represent 

 different phases of morphologically identical structures ; the posterior 

 eyes being simply a higher modification of the anterior ones. 



In the H exapoda, we have the most difficult question to deal 

 with ; the divergeuce of structure in the two kinds of eyes is so great 

 that doubt arises as to their morphological identity. The compound eyes 

 whether formed by one modified ocellus, or by the fusion of many, 

 have progressed wonderfully in structure and fuuctional powers. The 

 simple eyes, however, have remained nearly the same ; and the presence 

 of a vitreous layer, and of the terminal rods, shows at once their identity 

 with the anterior ocelli of Spiders, and their very slight departure from 

 the ancestral form. But what are the compound eyes? A group of 

 united ocelli, or a modification of a single ocellus? Three classes of 

 facts must be considered in answeriug the first of these questiona : (1) are 

 there any existing stages of such a fusion to be found in the possible, 

 existing ancestors of the Insecta? (2) are there any traces in the 

 perfeeted eye itself of any ancestral fusion? (3) does embryology 

 give any evidence on this point? On the other band, the same in- 

 quiries may be made concerning a supposed progressive modification 

 of a single ocellus into a compound eye. These two suppositions seem 

 to be the only permissible ones. 



First, then : are there any existing stages in the ancestors of the 

 Insects, showing a tendency for the ocelli to fuse into compound eyes? 

 First let US decide upon the possible ancestral forms. There are Limu- 

 lus, Scorpions, Myriapods and Spiders. The first two may be excluded, 

 since we have shown that they diverge from the ancestral forms in a 

 direction diametrically opposite to that of other Arthropods. There are 

 then remaining Myriapods and Spiders as possible ancestors of the 

 Insecta. 



What, now, would be the indications that a fusion of the ocelli, to 



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