EYE OF CEPHALOPODA 



183 



which line the interior of the depression, and which communicate 

 directly with the branches of the optic nerve, there being no 

 iris or lens. This type of eye, t r 



it will be observed, corresponds 

 exactly with that which occurs 

 in Patella. It appears also to 

 correspond to a stage in the 

 development of eyes in the Di- 

 branchiata (e.g. Octopus^ Sepia^ 

 Loligo'). Lankester has shown ^ 

 that in Loligo the eye first ap- 

 pears as a ridge, enclosing an 

 oval area in the integument. 

 By degrees the walls of this 

 area close in, and eventually 

 join, enclosing the retinal cells 

 within the chamber in which 

 the lens is afterwards developed 

 (Fig. 89). It thus appears that 

 in some cases the development 

 of the eye is arrested at a point 

 which in other cases only forms 

 a temporary stage towards a higher type of organisation. 



The developed eye in the dibranchiate Cephalopods consists 

 of a transparent cornea, which may or may not be closed over 



Fig. 89.— Three stages in the develop- 

 ment of the eye of Loligo ; r, r, ridge, 

 enclosing p.o.c, primitive optic cham- 

 ber; or, orifice between the closing 

 ridges ; s.o.c, secondary optic chamber ; 

 ci, ci, ciliary body; I, rudimentary 

 lens; i?, retina. (After Lankester.) 



Z^ a.o.c 



Fig. 90.— Eye in 

 A, Loligo ; B, He- 

 lix or Limax; C, 

 Nautilus : a.o.c, 

 anterior optic 

 chamber; c, cor- 

 nea; int, integu- 

 ment ; ir, iris ; I, 

 lens; l', external 

 portion of lens ; 

 op.n, optic nerve; 

 op.g, optic gang- 

 lion; p.o.c, poste- 

 rior optic cham- 

 ber ; r, retina. 

 (After Grenacher.) 



the front of the lens. Behind the cornea is a narrow chamber 



(the anterior optic chamber) which is continued for three parts 



1 Quart. Journ. Micr. Sc. N.S. xv. p. 37. 



