310 



THE EYE IN EVOLUTION 



Blennius 



Bathylagus 



Blennius — Verrier, 1933) and on occasion is deep and highly organized, 

 as in the sea-bream, 6VreZZa (Verrier, 1935).i With few exceptions such as 

 the sea-horse, Hippocampus, where it is nearly central (J. Carriere, 1885), 

 it is typically situated temporally in the region of the retina which could 

 be used for binocular vision. In this region rods and twin cones are 

 excluded and the single cones are densely packed, long and rod-like, 

 while the other retinal layers, including the ganglion cells, become 

 attenuated but do not disappear. It is interesting that in some deep- 

 sea Teleosts {Bathyfrocfes, Bafhylagus) with a pure-rod retina, the 

 rare occurrence of a temporal fovea populated with rods is found 

 (Brauer, 1908)-; in Bafhylagus there are 6 superimposed rows of rods 

 in this region instead of the usual 3 found elsewhere in the retina 



Fig. 354. — The Optic Xerve of Teleosts. 



Cross-section of the optic nerve of Serranus cahrilla showing the folded 

 ribbon structure (after tStudnicka). 



Hippocampus 



(Vilter, 1954), an arrangement which may act by increasing the 

 sensitivity to light. 



The optic yierve, even in Teleosteans, is relatively primitive (Ucke, 

 1891 ; Deyl, 1895 ; Lumbroso, 1935). In many species the disc is 

 narrow and oblong, for the nerve fibres leave the retina not only at this 

 point but for some distance along the open foetal fissure. The nerve 

 thus emerges from the eye as a tape rather than a cord assuming a 

 circular cross -section in the orbit, and on section the nerve fibres appear 

 as a broad pleated ribbon folded concertina-like to accommodate itself 

 into its tubular sheath (Fig. 354). In a few species on approaching the 

 globe the nerve divides into as many as a dozen strands so that it enters 

 the eye in multiple rootlets with a corresponding number of optic discs 

 (the bull-head catfish, Ameiurus, the loach, Misgurnus, and the 

 deep-sea Polyipnus).^ A septal system may be absent or represented 



1 Other foveate Teleosts are the butter-fish, Pholis, the puffer-fish, Tetraodon, the 

 sea-bass, Serranus, the trigger-fish, Batistes, and the weever, Trachinus. 



- See, also, pp. 365, 382, 486. 



^ This peculiar arrangement is also seen in Pohjpterus, some salamanders and some 

 meiiSfirs of the deer family. 



