250 



HISTOLOGY 



thelium, forms the other part of the lens as a distally directed and supple- 

 mentary portion of the whole outline of the lens, which is almost spherical. 

 Thus the surface epithelium, which in other mollusks, as Strombus, is 

 used to form the cornea, in this case forms the outer part of the lens; 

 while the inner surface, which forms the entire lens of Strombus and 

 other higher mollusks, in Loligo forms the proximal two thirds of the 

 lens. 



It will be remembered how, in most of the previously described 

 mollusks' eyes, an area of pigment was placed about the cornea to keep 



FIG. 221. Five outline sketches to illustrate the histogenesis of the eye in a dibranchiate 

 cephalopod. ret., retinal epithelium; /./., fold which forms the lens; /., lens; ir. t iris fold; 

 cor., corneal fold. (After LANG.) 



too much light or too oblique rays of light from entering the eye-sac. 

 This same region of pigmented epithelium is found in Loligo, but it has 

 been raised into a circular ridge, and then this ridge is drawn centrally 

 as a circular septum, with a central aperture for the light to pass through. 

 The amount of light to be admitted is thus determined by the septum, 

 which can enlarge or diminish, the central opening. It thus becomes a 

 very perfect form of iris. 



Still a third ridge of integument, lying outside of the iris, is now 

 developed and closed in centrally until it forms an external covering for 

 all the other structures. This is the cornea which, it can be seen, is not 

 homologous with that of any other mollusk. The tissues over the eye 

 are transparent, and are composed of an outer and inner epithelium with 



