chap, xii.] OPTIC ORGANS. 447 



The simplest condition of the final stage is to be 

 found in the Nautilus, where the eyes retain the primi- 

 tive condition of having their central cavity open to 

 the exterior ; the cells which line this cavity, and 

 which are the direct continuation of the epithelial 

 cells which invest the body, are converted into sensory 

 (retinal) cells, and are connected by nerve filaments 

 with the optic nerve which is given off from the cere- 

 bral ganglion. A. higher stage than this is to be seen 

 in the snail, for here the cup becomes closed up, and 

 there is developed in its cavity a spherical body which 

 has the function of a lens, while the outer wall of the 

 cavity plays also a part in refracting the rays of light, 

 owing to its having been converted into a cornea. 

 Peripatus has an eye which does not essentially differ 

 from that of the gastropodous Mollusca. 



The typical eye of a well-developed Polychaetous 

 Annelid presents an advance upon those of the just' 

 mentioned Mollusca by the following characters ; the 

 lens does not occupy the whole of the cavity of the 

 eye, but is placed anteriorly, while the rest is filled by 

 a vitreous humour ; the lens, therefore, is more 

 distinctly convex, and has a greater influence on the 

 impinging rays of light ; the layer of rods which 

 lines the cavity is bounded by a distinct and well- 

 marked layer of pigment. 



Though the physiology of the eye of a crayfish 

 offers some considerable difficulties which cannot as 

 yet be satisfactorily explained, the morphological series 

 is so complete that, from its point of view, much may 

 be made clear. 



The prime difficulty lies apparently in the large 

 number of lenses that seem to be present in a com. 

 pound eye physiologically, this arrangement is 

 preceded by what obtains in the Chsetognath Sagitta. 

 In this worm, the eye, which is completely covered by 

 the epidermis, consists of three biconvex lenses, each 



