Eyes of Molluscs and Arthropods. 691 



tliose bodies, or rods , kuown to be the percipieut Clements in other 

 ani mais. 



(5) Aecording to Geenachers thcory, the retinulae and rhabdoms 

 must form acelllayer, distinct, in the strongest seuse of the word, from 

 that formed by the crystalline cone cells ; that this is not so, cau be 

 proved by the most conclusive evidence, showing- that the three circles 

 of retinulae and the crystalline cone cells form a single layer. 



(6) Aecording to this theory, no adequate explanation can be given 

 concerning the function of the corneal facets and crystalline cones. 



(7] Aecording to this theory, the crystalline cone cells must be 

 homologous with the vitreous layer of the simple eye. It is then im- 

 possible to account for the crystalline cones, the presence of pigment 

 cells , or the grouping of the crystalline cone cells in fours, — not the 

 slightest trace of any of these structures being found in the vitreous 

 layer of any simple eye. 



(8) Aecording to Grenacher, the rods, or rhabdoms, are secreted, 

 in the compound eye , by the retinulae , or pigmented cells ; this is 

 directly opposite to what is found in the immediate ancestors of Insects, 

 and. in fact, in all the higher Mollusca and Worms, and probably in 

 the Vertebrates as well, where it is the colorless cells alone which give 

 rise to the rods. 



(9) This theory entirely ignores the presence in all faceted eyes of 

 a corneal hypodermis, a fact which alone is sufficient to upset the 

 whole series of deductions upon which it is fouuded. 



If these facts are sufficient to overthrow the theory of Grenacher, 

 they are none the less important in their hearing upon a new Inter- 

 pretation of the structure , morphological siguification , and function, of 

 the dififerent Clements in the compound eye. The changes these facts 

 necessitated, concerning the origin and structure of the compound eye, 

 have already been alluded to. Aecording to what was then remarked, 

 it follows that the network of cross fibrillae, or retinidium, 

 contained in the calyx of the retinophorae, either with or 

 without a crystalline cone, is the essential dement of the 

 compound eye; without considering a great many minor points which 

 militate against all other suppositions, and in favor of this, we shall 

 briefly enumerate the following facts: (1) the crystalline cones, 

 and the cells which bear them, are homologous with the 

 rods and retinophorae in the eyes of Molluscs, and in the 

 ocelli of Arthropods; (2) a series of cross nerve fibrillae 

 can be traced in the crystalline cone, or in the place 



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