Eyes of Molluscs and Arthropods. 623 



between tlie course of the rocls. arrauged radiately aroiuid the wall of 

 the optic cup, and the direction of the rays of lig-ht cutering- it. In the 

 simpler forms of MoUusean eyes, the relation existing between the 

 rods of the pig-meuted and colorless cells has not been determined, 

 uor. whether. as I believe. the ultimate fibrillae of the rods assume a 

 direction more and more at right angles to the rays of lig-ht which 

 impinge upou them. It is, however, certain that in those more com- 

 plex eyes. in which the amount and direction of light is regulated by 

 accessory organs. as in Pcctcn, Heteropods and Cephalopods, the rods 

 are best developed at that point where the greatest uumber of nearly 

 parallel rays impinge upon the retina, and, also, that in these very 

 rods the transverse fibrillae attain the most perfect uniformity of direc- 

 tion. In Proportion as the accessory organs become more and more 

 complete, and consequently the functional power of the eyes, the 

 greater advantages possessed by the double rods and their centrai 

 fibres for uniform and economie distribution of parallel, transverse 

 fibres , are scized upon, and, in proportion as the double rods become 

 more and more developed , those of the pigment cells decrease and 

 finally disappear. Consequently one finds that, in the most speciali zed 

 eyes, only the true retinophorae bave retained their rods in which 

 the distribution of the transverse fibrillae has reached the highest 

 perfection. 



BüTSCHLi 41) finds reason to believe that the colorless cells are 

 the essential ones, and heuce doubts Grenacher's assertion that the 

 colorless cells or »Limitanszellen« of the Cephalopod eye are not sen- 

 sitive to light, and he would compare them with the colorless, sensitive 

 cells in Gasteropods^ The reasoning is most fallaceous. It is perfectly 

 well shown by the very article which he is discussing (Hilger's), 

 that the pigmented, as well as the colorless cells contribute to form 

 the rods, which are the homologues of the similarly named structures in 

 ali other eyes. It is equally evident that the so-called rods , and the 

 cells which bear them, are the essential elements of the retina ; hence 

 it iollows that the presence or absence of pigment canuot, in this case, 

 serve at ali as a criterion. Therefore, in attempting to discover the 

 homology between the retinal elements of Cephalopods and Gastero- 



' It does not appear as certain as it might, that the »Limitanszeilen« do not 

 contain anj' pigment. It would not be very surprising if the so-called »Sockel« of 

 the retinal cells should turn outto be composed of several minute, pigment-bearing 

 cells, and that the retinophorae, or retinal cells, were entirely colorless. 



