KLEDONE. 549 
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. 83 and 84, et. 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 (Pl. X, figs. 83 
and 84, ps. eat.). 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 (Pl. 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 deepiy pigmented iris is 
exposed; this iris bounding an oval pupil (fig. 78, Zrs) 
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, 1e., it is noticed 
when examining Hledone early in the morning. Stage (e) 
shows the eye opened much more widely, as 1t is when 
the animal is disturbed, and (f) shows the condition- 
during periods of great fright or agitation. 
NN 
