108 MICROSCOPICAL STUDIES. 



a horizontal external fold of skin extending along the lower side of 

 the eye. In Octopus the eyelid is sphincter-shaped. 



The retina, bounded internally by a thin transparent or hyaline 

 membrane, consists of two distinct portions, the outer and the inner, 

 separated by a sharply denned layer of black pigment. The inner 

 layer is made up of rols, the outer of nerve cells and nerve ramifica- 

 tions. The optic nerve in Cephalopods is very short and stout. On 

 entering the optic capsule it forms an immense ganglion whence 

 arise very numerous nerve fibres. These gain access to the interior 

 of the optic bulb through sieve-like openings in the cartilaginous 

 layer of the wall of the bulb. Thence they pass to the external 

 surface of the retina. In front, and partly at the sides, of the gan- 

 glion lies a peculiar soft whitish organ, the white body (w.b.). 



Examined superficially, the general structure of such an eye 

 seems partially identical with that of a Vertebrate eye, except in the 

 absence of an anterior chamber. In reality, there are some very 

 radical divergences ; thus in the Cephalopoda the retina has its layer 

 of rods turned inwards, i.e., pointing towards the interior of the eye ; 

 in Vertebrates, this layer of rods is external, directed outwards ; in 

 Cephalopods the pigment layer divides the retina into two regions ; 

 in the Vertebrates it lies external to the rods and cones. Most im- 

 portant difference of all, the optic fibres proceeding from the large 

 optic ganglion pass into the retina from the exterior in Cephalopod 

 eyes, whereas in Vertebrates the optic nerve forms no gang] ionic 

 mass, but passes through the wall of the optic bull) by a single 

 opening and then breaks through the retina in the same way, 

 spreading a network of fibres over the infcrtifi/ surface of the retina. 

 Hence light entering the eye of Cephalopods, impinges first upon the 

 retina and pas-338 directly downwards to the nervous layer beyond. 

 In the Vertebrate eye, the impressions of sight fall first upon the 

 nervous layer, are transmitted thence through the various layers till 

 the rods and cones are reached and thence returned by them through 

 the same layers to the nerve fibres upon which they first impinged. 

 This fundamental divergence is clearly diagrammatised in figs. X. 

 and XL, Plate VII. 



A less important difference is that of the cornea being part of the 

 optic bulb in the Vertebrates, separate and part of the optic capsule in 

 the Cephalopods. 



In the latter the bulb represents the Vertebrate eyeball, minus the 

 cornea and sclerotic, the equivalents of these being here possessed 

 by the optic capsule, which here functions as an orbit. 



The development of the Cephalopod eye is very instructive, both 

 as throwing light on its origin and relationship with the other forms 

 of molluscan eyes, and also in regard to the origin of its divergences 

 from the Vertebrate type of eye. Figs. I. to V., Plate VII. graphically 

 describe the stages. 



The earliest stage (Fig. II.) is the equivalent of the optical stage 



