VISUAL OEGANS OF MOLLUSCA. 



355 



a kind of orbital cavity. In Nautilus the optic capsule is continuous 

 witli tie stalk ; in tke Dibrancliiata it is placed in the cartilaginous 

 orbits, where it encloses the ganglionic enlargement of the optic 

 nerve (Fig. 188, go), which is represented in Nautilus by a layer 

 which extends over a larger portion of the bulb. Anteriorly, the 

 optic capsule forms a thin investment (C), which is known as the 

 cornea ; and the refractive media of the bulb are placed behind it. 

 This cornea is absent from the eye of Nautilus, as is the lens. In 

 front of this cornea the optic capsule is therefore directly continuous 

 with a mem- 

 brane which is '^ P '^ 

 connected with 

 the integument 

 of the optic 

 stalk, and which 

 has a pupil-like 

 orifice which 

 leads into the 

 interior of 

 bulb. 



In the 

 branchiata 

 direct communi- 

 cation between 

 the internal 



cavity of the 

 bulb and the 

 surrounding me- 

 dium is broken 

 by a lens (Z/); 

 as, however, in 

 manyforms (Loli 

 gopsis, Histio- 



the 



Di- 



this 



n— 



Fig. 188. Horizontal section of the eye of Sepia (Diagram- 

 matic). KK Cephalic cartilages. C Cornea, li ,Leng. 

 ci Ciliary body of the lens. Td Internal layer of the retina. 

 He External layer. P Layer of pigment, o Optic nerve. 

 </o Ganglion. !<, Pupillary cartilage. i7i; Cartilage of the iris. 

 VI White body, ae Argentea externa (after Hensen). 



theutis, etc.), the 

 transparent por- 

 tion of the optic 

 capsule is alto- 

 gether absent or 



is perforated (Sepia, Loligo, Octopus), the anterior surface of the 

 bulb enclosed in the capsule is bathed by water. This space, which 

 communicates with the exterior, is not only continued through the 

 optic cleft as far as the lens, but also extends more or less around 

 the bulb. 



In many the integument is thrown into folds around the cornea ; 

 these folds form " eye-lids,'^ and are either limited in position or 

 extend around a larger portion of the bulb, when they form a defensive 

 oi'gan for the eye by the possession of occlusor muscles. 



The base of the bulb is formed by a cartilaginous capsule 

 (Fig. 188, li); around the pupil this becomes converted into the 



2 A 2 



