320 



THE EYE IN EVOLUTION 



Polypterus 



definite layer. Both rods and cones are present ; the rods, long and 

 thick, the cones single and containing colourless oil -droplets — the most 

 primitive Vertebrate species in which these appear. There is no area 

 centralis (Fig. 373). 



In general it would appear that the eye of the sturgeon represents a 

 transitional phase between the selachian and teleostean eye with more 

 affinities for the former than the latter. The scleral 

 cartilage is of the selachian type, but the subcon- 

 junctival bony plaques are an innovation. The 

 argentea, present in Selachians as a rucUmentary 

 layer in the iris, is continued throughout the uveal 

 tract. The foetal fissure persists but the retractor 

 lentis muscle of Teleosts has not yet evolved. The 

 immobility of the iris is more primitive even than in 

 Selachians. The general architecture of the retina is 

 selachian in its simplicity rather than teleostean in 

 its perfection ; but the appearance of oil-droplets at 

 an early stage among Vertebrates in the cones is an 

 interesting phylogenetic innovation. 



The POLYPTERiNi are represented only by two 

 archaic types both inhabiting African waters — 

 Polypterus and Calamoichthys. The eyes of the 

 former were studied by Ley dig (1854) and Rochon- 

 Duvigneaud (1943), and the latter also by Roclion- 

 Duvigneaud (1943). In Polypterus the eye appears 

 to resemble that of Amia^ and is of the teleostean 

 type. The cornea is not divided and Bowman's 

 membrane is lacking ; there is a continuous scleral 

 cartilage without bony enforcement, an argentea 

 lining the choroid but poorly represented in the iris, and a spherical 

 lens. There is no open foetal fissure, no choroidal gland but an 

 extensive membrana vasculosa retinse, no pupillary musculature, no 

 tensor choroideae, and a poorly developed annular ligament. The rods 

 are large, the cones are single and contain oil-droplets, the ganglion 

 cells are scanty and do not form a definite layer, and the optic nerve 

 which has the lamellar structure of the teleostean type, is branched 

 with multiple optic discs (Studnicka, 1898). 



Fig. 373. — The 

 Visual Ele- 



ments OF Aci- 



PESSER FULVES- 



CEN.S. 



A cone (contain- 

 ing a colourless oil- 

 droplet) and a long 

 thick rod ( X 1,000) 

 (Gordon Walls). 



Calamoichthys 



The eye of Calamoichthys is of the same general structure but, according to 

 Rochon-Duvigneaud (1943), the retina is exceedingly thin, with few cellular 

 elements of any kind, the short and thick visual cells being of one type only 

 having the morphological characteristics of cones some of which are provided 

 with an oil-droplet. 



» p. 321. 



