Comparative Anatomy of the Eye. 449 



some period of their lives, but as mentioned elsewhere, many of those 

 tli:it become fixed to a stationary object in the later and sedentary period 

 of their lives, lose the eyes they possessed while in active youth. Many 

 of the bivalves have simple eyes or pigment spots scattered along the 

 edge of the mantle. 



The cephalopods have two eyes. In the lower families these are quite 

 rudimentary, sometimes consisting of mere pigment spots situated over 

 the oesophagean ganglion, and at the base of the tentacles. But among 

 the higher families, as the octopus and cuttle-fishes, the eyes attain a 

 high degree of development. The eye of the Cuttle-fish ( Sepia ) is strik- 

 ingly like that of the higher vertebrates, in all essential features, and 

 differing only in details. The fig. 181 shows the lens resting on the 

 vitreous body behind which is the retina in two layers, with a choroid or 

 pigment layer between. The optic nerve does not pass through the coats 

 of the retina and spread its fibres on its inner side as in the vertebrate 



FIG. 182. Showing Optic Nerve of Vertebrate 

 Animals passing through the layers and spread- 

 ing in front of them. 

 n. Optic Nerve. 



/.Fibrous Layer. 

 *. Sensory Layer 



. Rods and Cones. 

 (Compare with fig. 176.) 



Fio. l83.0nchidium of Tonga. 



A land mollusk living on sand 

 beaches on the margins of seas or 

 marshes; and having eyes on its 

 back. (After Semper.) 



eye, but spreads out on the back part or outside layer of the retina, so 

 that the coat holding the sensitive crystalline rods is inside next the 

 vitreous humor. There is therefore no blind spot as in man, the rods 

 covering the whole of the retina. This would seem to be the direct orig- 

 inal construction of the eye, while the vertebrate plan is an innovation, 

 the optic nerve having pushed its way through the center of the layers 

 and formed the retina wrong side out. The eyes of the gasteropod uni- 

 valves ( snails, &c. , ), are at the tips of their tentacles or at their base. 

 They resemble those of the cephalopod in their essential features, and 

 differ from those of the vertebrates in the peculiar structure of the retina. 

 It is an exceedingly remarkable fact that both of these sorts of eyes 

 are developed in a single animal. That animal is the Onchidium, a 

 naked gasteropod mollusk. ( Semper. ) In its head are two eyes like 

 those of the snail with the direct retina without the blind spot. But on 



