nearly black pigment, which gives the color already referred to, 

 so that in sections of large eyes, where the pigmentation is 

 deepest, the nuclei of the cells are not easily found. The pigmen- 

 tation becomes less dense toward the base of the tentacle and 

 gradually disappears. Muscle fibers that extend back into the 

 eye stalk are attached to the edges of the cornea so in preserved 

 specimens it is not uncommon to find the cornea pulled back 

 so the pigmented portion extends around it as a ridge. These 

 muscles may be of use in changing the focus of the eye. So 

 far as I know, there is no other provision for focussing. 



The lens (le.) is cellular in structure, and except for a thin 

 layer of muscle and connective tissue fibers that cover its outer 

 surface and are continuous with the muscle fibers at the edge 

 of. the cornea, it is in contact with the inner surface of the cor- 

 nea. There is no space between them so the cornea, muscles 

 and lens form a single optical lens. The inner surface of the 

 lens is applied to the retina, (r.) but as part of the nerve (on.) 

 that supplies the retina enters the eye from one side, the nerve 

 is continued between the retina and the lens. The edges of the 

 lens are bounded by a blood space. 



In sections the lens varies greatly in shape. It may be nearly 

 circular, indicating that the lens is nearly globular, or either its 

 anterior or its posterior face may be greatly flattened. The 

 shape shown in figure 26 is not uncommon, but sections in which 

 the posterior face is drawn out and is very convex are not at 

 all rare. It is a question whether these shapes indicate a pos- 

 sible focal range, or whether they are to a considerable extent 

 distortions due to preservation. 



The flattening of the outer face of the lens may be accom- 

 plished by the muscles that are attached to the margin of the 

 cornea, and that are continued over the surface of the lens 

 between it and the cornea. It is possible that injecting the blood 

 space with blood and contracting muscles in the eye stalk which 

 surround this blood space, may lengthen the lens that is, make 

 it more convex. The mechanism is not very complete, but it is 

 hardly to be expected that focal changes take place with great 

 rapidity. 



The retina is rather thick, and is slightly concave toward the 

 lens, with which it is always in contact. The exceedingly con- 



4 49 



