FIG. 188. 



Comparative Anatomy of the Eye. 453 



blind spot. But now we have a curious compromise between these two 

 sorts of eyes, in another mollusk, a bivalve this time, like a scallop and 

 belonging to the oyster family. It is called the Pecten. In this eye, 



FIG. 188. Diagram of the Eye of Pecten, a bivalve 

 of the Oyster Family. ( After Hickson.) 



a. Cornea. 



6. Transparent Basement Membrane supporting 

 the epithelial cells of the Cornea. 



c. The Pig-mented Epthelium. 



e. Lens. /. Ligam en t supporting it. 



a Retina, h. The Tapetum. 



A;. Pigment, m. The Retinal Nerve. 



n. Complementary Nerve. 



The nerve goes around to the front of the retina in- 

 stead of the back as in most invertebrate eyes, or in- 

 stead of piercing the layers as in the vertebrate eye. 



fig. 188, the nerve passes around the edge 

 of the retina and spreads its fibres in front ; 

 so the light passes through them to reach 

 the retina as it does in the vertebrates. 

 The vertebrate eye in the lower orders is almost as rudimentary as in 

 lower invertebrates. In the Amphioxus and Hag-fish or Myxine it con- 

 sists of a rudimentary lens imbedded in the pigment which encloses the 

 termination of the optic nerve. ( Huxley. ) The eye of the Myxine is 

 overgrown with muscles and skin, which seems to imply that it has seen 

 better days and is falling into decay. 



There are some examples among the vertebrates of extraordinary eyes 

 in unusual places, as well as among the mollusks. Thus there are no 

 less than eight genera of the Salmon family in which such organs in 

 greater or less degree of perfection are found. The genera Chauliodus, 

 Astronesthes and Stomias possess eyes distributed on the belly and tail. 

 This is agreed to by both Leukart and Semper. Two genera, Sternoptyx- 



FIG. 189. Section through the Eye of Esox Lucius 

 (Pike Fish). 

 Co. Cornea. 

 L Lens. 



Pf. Falciform process a fold of the Choroid Coat. 

 C 11. Campanula Hailed attached to the Lens. 

 <Sc. Ossifications of the Sclerotic Coat. 

 No Optic Nerve. ( After Glaus.} 



and Argyropelicus, have organs in a stage of 

 development between pigment spots and true 

 eyes, while the Scopelus, Gonostoma and 

 Maurolicus have pigment spots in the corres- 

 ponding parts of the body, which are therefore 

 supposed to be the incipient stage of the same 

 organs. 



In the fishes the regular paired eyes usually have a rather flat cornea. 

 This is compensated by the -superior concentrating power of the lens, 

 which is almost spherical. They also have a folding of the choroid or 

 pigment coat called the falciform process, which appears to correspond 

 with the pecten in the eye of birds. (See. fig. 177.) There is also a 



CH 



FIG. 189. 



