Eyes of Molluscs aucl Arthropods. 559 



uot tu be foimd in the other Clements of tbe retina, uamely, a central 

 nerve fibre, or bündle of nerve fibres, which cxtends, through tbe axis 

 of the retinophora, into the rod. Now, if we suppose that each cell was 

 supplied witb a number of externa! nerve fibres, like those described 

 for the pigmented ones, and that a special Cluster of such fibres was 

 formed between the juxtaposed walls of two retinophorae, it is evident 

 that. with the fusion of the two cells, and the disappearance of the 

 apposed walls, the bunch of nerve fibres will come to lie in the centre 

 of the retinophorae ; and since, according to our suppositions, the nerve 

 fibres extend to the summit of the cell, — as we have always found to 

 be the case, — they will therefore penetrate the centre of the rod. It 

 is difficult to follow the nerves through the retinophorae, on account of 

 the mass of refractive globules, but here, as in other similar cases, it is 

 only necessary to find favorable examples, where one may distiuguish 

 a bündle of at least two or three nerve fibres extending upwards from 

 the base of the cell, beyond the refractive granules ; near the nuclei, 

 they become varicose and irregularly distributed. I bave not succeeded 

 in tracing them directly into the rods, although I do not doubt that they 

 terminate there. The inner ends of the retinophorae are always pointed. 

 lacking the root-like fibres of the pigment cells ; their transformation into 

 the issuing nerve fibres is so graduai, that it is impossible to say where 

 the one begins and the other ends. This nerve fibre, which is really a 

 nerve bündle composed of several smaller fibres, is larger than those 

 Seen connected with the pigment cells. In some cases, the varicosities 

 of the axial nerve, at a short distance from the cell, become so large as 

 to form one or two vesicular swellings, which I thought might contain 

 nuclei, but I have never been able to determine this point with certainty. 

 The very small size of the rods has rendered the Observation of the ulti- 

 mate nerve endings there a matter of extraordinary difficulty. That the 

 outer fibres send their ultimate fibrillae into the substance of the rod is 

 shown by the persistency with which they invariably adhere to its sur- 

 face, while, on the other band, they are always torn away from the sur- 

 face of the retinophorae. If we accept the supposition that the retino- 

 phora is formed by the fusion of two cells, — which, in fact, can hardly 

 be called a supposition, since we have incontestible proof of it in the 

 double nature of the rod, and the presence of two nuclei, — then the 

 central bündle of nerve fibres belongs to the same category as the ex- 

 ternal ones, and we may presume that they, like the outer nerve fibres, 

 also extend to the outer ends of the rods, that is. they form a system of 

 axial nerve fibres in the centre of the rod, just as is the case in the rods 



