110 MICROSCOPICAL STUDIES. 



supporting tissues (cartilages, muscle fibres, &c.) of the wall of the 

 bulb, all parts of the eye have direct epidermal origin, whereas in 

 the Vertebrate eye the cornea and lens alone have direct epidermal 

 origin, as the retina is entirely derived from an outgrowth from the 

 embryonic brain. The heavy black lines in Figs. III. to VII. indicate 

 the external margin of the layer of retinal rods. They show how in 

 in the Cephalopoi eye this layer is turned towards the light, while in 

 the Vertebrate eye it is turned in the opposite direction. Thus 

 while the higher Cephalopod eye reaches what is practically the same 

 perfection and same plan of optical mechanism as the Vertebrate eye, 

 it does so by a different avenue of development. 



As already noticed, the majority of Molluscan eyes belong to one 

 or other of the three types so far described, and which may be 

 exemplified respectively by the the eyes of the Limpet, the Snail and 

 the Cuttlefish, ranked in order of development. These in structure 

 and development are in direct sequence. Their homologies are 

 definite and fixed. All are cephalic eyes, and in close relationship 

 to the central nervous mass. 



Hence, when among the Pectens and allied Lamellibranchs 

 we find visual organs that from their position are obviously not 

 homologous in origin, it is specially interesting to see what 

 differences in structure prevail. In such forms cephalic eyes are 

 wanting, their place being taken by small organs placed upon short, 

 deeply pigmented papillae, arranged at intervals along the edge of the 

 mantle or pallium, whence they derive their name of pallial eyes. 

 Stated briefly the structure is as follows. The ooular papilla is clothed 

 with an epithelial layer, deeply pigmened except at the summit, where 

 the cells are colourless and flattened. Beneath this layer lies the tiny 

 ocular sphere divided into two halves by a partition, the anterior 

 containing a transparent cellular lens, the posterior, a several-layered 

 retina of ordinary structure. The arrangement, however, of the rods 

 and cones is the reverse of that found in the cephalic eyes of 

 Molluscs, as they are here turned away from the light and are under- 

 laid by a pigmented layer. Again fibres from the optic nerve 

 form a layer anterior to the other retinal layer, so that light must 

 traverse this nervous layer before it can reach that of the rods and 

 cones. The plan of structure is therefore practically identical with 

 that found in the Vertebrate eye save that the optic nerve does not 

 pass through the retina, but attains its connection with the rods and 

 cones by passing around one side of this layer and thence spreading 

 out over the distal surface. 



Very curiously nearly similar eyes are found upon dorsal processes 

 in a peculiar Gastropod, Onchidium, and here even closer approxi- 

 mation is made to the plan of the Vertebrate eye. Instead of the 

 optic nerve turning the flank of the rods and cones, it passes through 

 at one point, thereby producing a blind spot exactly analogous to 

 that found in the Vertebrate eye. This eye of Onchidium constitutes 



