92 W. J. DAKIN. 



present in Area, Liinn, Spoiidylus (34), and Cardium (42) ; 

 in the Cephalopoda it is probably of general occurrence. It 

 is very definite in the rods of the Alciopiden, and has been 

 found in the Polychtetes Nereis and Lysidice l)y Hesse (33). 

 In Gastropods a definite bundle of neurofibrillar has been 

 found in the visual cells of Liinax (Smith [40] ). In other 

 forms there occur, instead of one thick axial fibril, a nuuiber 

 of fibrilho Avhich terminate in a comb-like margin ("Stiftchen- 

 saum" of Hesse). This is a feature of the distal cells of the 

 Pecten eye, and according to Hesse is practically universal, the 

 fibrilh\3 occurring also in the rods and cones of vertebrates. 



The rods or analogous structures are also of widespread 

 occurrence in optic sense-organs, though it would be difficult 

 to homologise many of the rod-like structures with one another. 

 Hensen, and later Grenacher, looked upon all the rods as 

 cuticular structures, but 1 doubt now if any rod can be shown 

 to be cuticular, not even the rhabdome of the Arthropods, 

 a differentiated part of the reticular cells. Hesse regards 

 the neurofibril las then as the universal actual recipient ele- 

 ments of the visual cell and the plasmatic part of the rod as 

 a support for the fibrils. Experimentally it is impossible to 

 determine whether the neurofibrillar are the recipient elements 

 or not, but from the constancy of their presence I believe 

 they })lay a great part in this process. I have shown how in 

 macerations the rod-cell may break up, leaving the axial fibre 

 (PI. 7, fig. 14). It does not appear from this as if the rod 

 could give much support to the latter, but the true state of 

 things in the living eye may possibly be different. I am 

 rather inclined to believe, however, that the plasmatic poilion 

 of the rod acts conjointly as a recipient oi-gan, and that the 

 stimulus is passed on to the neurofibrillar which conduct the 

 nerve impulse wider. 



I consider Hesse's estimation of the number of rods in a 

 retina to be rather low for the large eyes of P. jacobaeus 

 or P. maximus. In the latter species there were about 

 ten thousand in the retina of one specimen examined, and the 

 number of rod-cells therefore exceeded this number, since the 

 marginal ones do not bear rods. 



