304 



THE EYE IN EVOLUTION 



autonomously by the direct action of light (Brown-Sequard, 1847-59 ; 

 Magnus, 1899) (Figs. 337-9). 



Only in a few species, such as the flounders with upward-looking eyes, and 

 the eels, does much pupillary excursion occur ; in the pearl -fish, Encheliophis, 

 also with upward-looking eyes, the pupil is highly contractile. Some cat-fishes 

 have an opercvilum which reduces the pupillary aperture to a circular slit 



Figs. 337 to 341. — The Pupils of Teleosteans. 



® 



Fig. 338.— The 



sailfish, 



IstiopJwrus. 



Fig. 337.— The 



Moray eel, 

 Gymnothorax. 



Fig. 339.— The 



flounder, 



Pleuronectes. 



Fig. 340.— The 



serpent eel, 



Leptognofhus. 



@ 



Fig. 341. — The cat-fish, Plecostomus, showing the operculum in various 

 stages of closure of the pupil. 



{Plecostomus, Fig. 341), while the serpent-eel of New Zealand (Leptognathus) 

 has a secondary pupillary aperture in its lower part giving it a double effect 

 (Fig. 340).^ An interesting feature is the common presence of an aphakic area 

 in the pupillary aperture which the lens rarely entirely fills (Plate II). This 

 is sometimes situated below but is visually on the temporal or nasal side and 

 becomes particularly niarked when the lens is drawn sideways in accommoda- 

 tion (Beer, 1894). 



The lens of Teleosteans is usually spherical, approximating the 

 cornea, with a large spherical nucleus and a well-marked system of 

 sutures usually taking the form of a single Ime 

 as in Selachians but sometimes star-shaped 

 (Figs. 314, 342) (Rabl, 1898 ; Koch, 1950-52) ; 

 Yamasaki, 1953). The peripheral shell has a 

 refractive index approximating that of water ; 

 the central core, on the other hand, has the 

 high refractive index of 1-5 and is the effective 

 refractive constituent of the optical system 

 (Hogben and Landgrebe, 1940). The vitreous 

 is dense and filamentous (Koch, 1952-53). 



The teleostean retina is an advanced and 

 fully differentiated structure with, as we have 

 already seen, an open foetal fissure, nourished 



1 p. 325. 



Fig. 342. — Lenticular 



SUTURES OF TeLEOSTS. 



The usual system is 

 that of Selachians (Fig. 

 314) ; ;\ star-shaped sys- 

 tem is ■ !so relatively 

 common. 



