MICROSCOPICAL STUDIES. 71 



the fifth and perhaps the most interesting of the types of Molluscan 

 eye — for, standing as it does alone, it furnishes us with one of the 

 most remarkable instances of independent evolution that we are at 

 present aware of. 



And in this connection it is important to notice that no other 

 organ has had so many independent evolutions as has the eye. 

 Nothing could so strongly emphasize the supreme importance of 

 sight to the majority of creatures. Here, within the types referred 

 to above, four distinct evolutions of ocular organs undoubtedly took 

 place. No one can for a moment deny that the cephalic eyes of 

 Cephalopods and of Vertebrates have had independent and conse- 

 quently dissimilar origin. Embryology at once marks out this 

 divergence ; while the peripheral position of the eyes of Lamelli- 

 branchs is sufficient to separate them from each of the former. 

 Again one cannot gainsay divergence of origin to the dorsal eyes of 

 an aberrant Gastropod and the pallial eyes of the Lamellibranchs. 

 If only our knowledge of the homologies of the more obscure 

 organs were on a par with that of the eye — though much remains to be 

 done even here — our attempts at constructing phylogenetic tables or 

 trees of descent, would be infinitely simplified. Indeed, it may 'be 

 considered a biological axiom that no reliable phylogenetic tree can be 

 constructed till the homologies of individual organs have been exhaus- 

 tively made known. 



Explanation of Pl. VII. Figs. 1. to XI. 



The Gephalopod Eye. 



Fig. I. Section through eye of Sepia officinalis, ae. argentea 

 externa ; ey. eyelid ; c. cartilages of optic bulb ; cc. 

 cephalic cartilages ; ci. ciliary body ; co. cornea ; gn. 

 optic ganglion ; ir. iris, showing a thin plate of 

 strengthening cartilage ; I. lens ; on. optic nerve ; p. 

 retinal layer of pigment ; r' internal layer of retina ; 

 r" external layer of retina ; wb. white body. 



Figs. II. to V. Diagrams of the chief phases in the development of 

 the Cephalopod eye. III. represents that stage which 

 in Nautilus is the permanent condition ; IV. is prac- 

 tically a diagram of the eye in Helix ; e. unmodified 

 epidermis ; /. lens ; r. retinal layer ; a thick black layer 

 denotes the position of the distal margin of the layer of 

 rods and cones. 



Figs. VI. to VIII. Diagrams illustrative of the development of the 

 Vertebrate eye. b. represents a hollow outgrowth from 

 the brain which eventually form the retina and .the. 



