81 



the Cephalopod the fibres of the optic nerve enter the 

 retinal cells from behind and do not pierce the retina. 



The circular external orifice of the eye is small, only 

 about 6'5 mm. in diameter, and is surrounded by the skin 

 of the head, and the muscular sheet which binds the 

 cephalopedal mass together and to the mantle, super- 

 ficially (PL X, figs. 80 and 84, ext. or.). This circular 

 rim round the eye forms an eyelid which can completely 

 close over that organ by radial contraction. Dorsally 

 this eyelid is continued over the aperture of the eye as a 

 membranous semilunar transparent fold (PI. X, figs. 88 

 and 84, ps. ext.). Ventrally another fold is continuous 

 with the eyelid, this fold also being transparent, but lying 

 below the dorsal one mentioned above, and extending 

 further over the eye (figs. 83 and 84, ps. int.). These 

 two membranes appear to be only slightly if at all 

 moveable, and through them may be seen the pupil of 

 the eye (PI. X, fig. 78, ps. mem.). Possibly water may 

 penetrate between them and so bathe the lens directly, as 

 in the Oegopsida. These two membranes may be called 

 the external and internal pseudocorneal membranes, the 

 internal being the thinner. Cutting away these two 

 membranes, the metallic deeply pigmented iris is 

 exposed ; this iris bounding an oval pupil (fig. 78, Iris) 

 which it opens and closes by a dorso-ventral expansion or 

 contraction. Text fig. V shows the various stages of 

 contraction and expansion which occur in the eye of 

 Eledone. When resting, the eye seems to vary from (a) 

 to (d) without apparent cause. Stage (a), however, seems 

 to occur after a rest of long duration, i.e., it is noticed 

 when examining Eledone early in the morning. Stage (e) 

 shows the eye opened much more widely, as it is when 

 the animal is disturbed, and (/) shows the condition 

 during periods of great fright or agitation. 



