314 



THE EYE IN EVOLUTION 



Figs. 363 and 364. — Thk Retinal Elements of Protopterus 

 (Gordon Walls). 



C C 



Fig. 363. — A Pigment Cell. 

 Showing a mass of filamentous pro- 

 cesses laden with pigment sharply 

 differentiated froni the body of the 

 cell, r and c represent the spaces 

 occupied by the rods and cones respec- 

 tively ( X 500). 



Fig. 364. — A Single Cone, a Double 

 Cone and a Rod. 



Members of the double cone are 

 unusually loosely associated. There 

 is an oil -droplet in the single cone and 

 one member of the double cone. The 

 rods are large and have an oil -droplet 

 as well as a paraboloid ( X 1,000). 



the nerve divides into a number of bundles each with a similar core, as 

 if the primitive optic nerve of the lamprey had reduplicated itself 

 several times and all the nerve -cords had been gathered in one sheath ^ 

 (Prince, 1955). 



THE CCELACANTH EYE 



THERE IS ONLY ONE SPECIES of this ancicnt order of fishes known 

 to be extant — Latimeria (Fig. 365) — lately and surprisingly discovered 

 in the Indian Ocean off the coasts of South-East Africa. The eye of 

 this species is of great interest, showing characteristics closely resem- 

 bling those of Selachians on the one hand and Chondrosteans on the 

 other, clearly demonstrating the remarkable unity of this organ through- 

 out the vertebrate phylum. ^ In general its structure shows adaptation 

 for vision in the ocean depths where little light is available. For this 

 reason trie eye is unusually large and takes the general form of a flat- 



1 See also snakes, p. 392. 



- p. 234. 



