622 William Patten 



forms of eyes: or, in other words. that the pigmeuted, as well as the 

 eolorless eells produce independeut rods, whose nerve fibres unite to 

 form one complex retinidial layer. His so-called »Stäbehenmantel« is 

 eomposed of the rods of the pigment eells, while the axial portion is the 

 rod of the eolorless one. It is true, that the rods in the eye of Haliotis 

 do not seem to have reached a very high degree of specializatiou , as 

 compared with those of Fissurella and Helix, but this is a difference in 

 degree not in kind; moreover it would be impossible to compare a rod, 

 as described by Hilger, with any similar structure known. The sharp, 

 diagrammatic outlines with which he separates the rods suggest that 

 possibly the material was not prepared in the best manner. Heis surely 

 in error in saying that the pigment eells end in one or more nerve fibres, 

 since they end abruptly in root-like fibres , resting upon the connec- 

 tive tissue membrane. The incorrectncss of his conception , that the 

 rods are formed by the united activities of the eolorless and pigmented 

 eells. is proved by the difficulties he encounters in attempting to com- 

 pare these rods with those of Heteropods, as described by Schultze : 

 for in the latter case, they are formed simply by the retinophorae, the 

 central nerve fibre of which constitutes the so-called axis of the rod; 

 while in the former iustance , the axis is not a single fibre , but the 

 double rod of the retinophorae, the sheath beiug formed by the rods of 

 the pigment eells; therefore, in the Heteropods, it is evident that the 

 rods of the pigment eells have entirely disappeared, while those of the 

 double eolorless ones with the central nerve fibre, have increased pro- 

 portionately. We have an exactly parallel case occurring in a single 

 genus of Molluscs; for in Area the optic cups contain a retinidial layer, 

 in which the rods of the pigment eells and the retinophorae play a 

 nearly equal part , while , by a graduai series of transformations that 

 may be foUowed step by Step, a much higher type of visual organs 

 is produced , in which the rods of the retinophorae, — or eolorless 

 eells, — are alone functioual, while the pigmented ones have become 

 subordinated to secondary purposes. It appears, from the description 

 given by Hensen of Pterotrachea, that the pigmented eells may be di- 

 vided into two rows, as in the faceted eyes oi Area. It seems, there- 

 fore, that in the majority of Gasteropods, essentially the same cou- 

 dition fouud in Haliotis prevails in the structure of the retinidial layer, 

 to which the pigmented, as well as the eolorless eells contribute their 

 more or less independent rods. In Haliotis. the ultimate fibrils of the 

 retina terminalia have assumed no special direction; this, I be- 

 lieve, may be accouuted for by the faet that no definite relation exists 



