Eyes of Molluscs and Arthropods. 679 



a lateral eye of Spiders into a compound one , woiild be to reduce the 

 thick lens to a thin cuticular layer, and decrease the depth of the 

 ommateal cup; or. viceversa, to change an eye like that of Gammarus 

 into au ocellus. we have only to increase the thin corneal cuticula 

 into a thickened cuticular lens. 



To recapitulate • we have answered, as nearly as possible, the six 

 questions proposed above. To the question: Does 1) comparative 

 anatomy. (2) histology or (3) embryology give any evidence of a present, 

 or past, fusion of ocelli to form a compound eye, we are obliged to 

 answer, no; even a very thorough study has proved that just those 

 features are absent that ought to be present, provided any fusion had 

 taken place! (l) We find no traces in the possible ancestors of the 

 Insects and Crustacea of a union of ocelli, that could lead up to the 

 present compound eye. [2) We find no evidence in embryology, or 

 in the simple forms of compound eyes, of any fusion of Clements; the 

 faceted cornea, which on this supposition, i. e. that of fusion, should 

 represent the primitive condition and should therefore be found in the 

 lowest types, is on the contrary a highly specialized condition only found 

 in the higher forms, and is entirely absent in the lowest. (3) Careful 

 study has failed to reveal the presence of any structural peculiarity, 

 such as envelopes for the ommatidia, arrangement of their cells, struc- 

 ture of the basal membrane, or of Innervation, which could in any way 

 be interpreted in favor of such a supposition. In fact the evidence is 

 all against such a mode of thinking. 



On the other band, our questions concerning the supposition that 

 a compound eye is formed by the progressive modification of a single 

 ocellus have received the following answers: (1) we do find stages in 

 the Spiders where a single ocellus has developed into an organ that 

 only insignificant changes would suffice to convert into a compound 

 eye like that of Amphipods and Isopods ; (2) embryology shows that 

 the very thin and degenerate (shown by faint staining of nuclei) corneal 

 hypodermis of the more specialized compound eye , assumes , in the 

 larvai stages, a greater proportional development, rendering its homo- 

 logy with the important corneal, or vitreous layer of the ocellus more 

 intelligible ; (3 we find a complete identity in all essential parts be- 

 tween the ommatidia of a compound eye and those of an ocellus. 



When to all this evidence against the first supposition we add 

 that obtained by comparing all the fundamental changes necessary 

 for the conversion of a group of ocelli like those of the Myriapods into 

 a compound eye like that of Insects and Crustacea, and the disappearr 



