CEPHALOPODA. 



431 



The front wall of the vesicle at the point where it runs into the lens 

 is thickened, and constitutes the ciliary body (Ci). The outside skin 

 is thrown into folds over the eye : internally there is a fold which 

 covers over the ciliary body, and extends as far as the periphery of 

 the lens ; it is highly pigmented and constitutes the iris (<//<). 

 Outside this is another fold of much greater extent, which meets 

 over the front of the lens (C), and bounds a space the anterior 

 optic chamber the inner wall of which is formed by the lens, 

 iridean folds, and choroid. This fold constitutes the sclerotic and 

 cornea; it is in the Oegopsida perforated by a small pore over the 

 lens. Outside the skin may be again folded to form an eyelid. 



KK 



FIG. 346. Section through the eye of Sepia, diagrammatic after Hensen. Ac argentea externa ; 

 C cornea ; Ci ciliary body ; Go optic ganglion ; Jk iris cartilage ; K cartilage of optic bulb ; 

 KK cephalic cartilage ; L lens ; Opt optic nerve ; P pigment layer of retina ; Be outer layer of 

 retina ; Ri inner layer of retina ; W white body. 



The retina consists of two layers (Re and Ri}, as in Nautilus, 

 divided by a layer of pigment (P), and the sensitive layer is that 

 which lines the optic vesicle or posterior optic chamber (vitreous 

 humour). The optic nerve comes from the large optic ganglion (Go), 

 which is protected by the orbital expansion of the cephalic cartilage 

 (A'A'). The choroid is the internal continuation of the iris, and forms 

 the inner wall of the deeper parts of the anterior optic chamber ; it 



