286 



THE EYE IN EVOLUTION 



chamber there are ill -developed sinuses lined by endothelium (Rochon- 

 Duvigneaud, 1943) ; it may be that these allow the escape of aqueous 

 humour when the lens is pulled forward towards the cornea in 

 accommodation. 



The pufjiUary wperture varies and is largely determined by the 



Ftos. 303 TO 313.— The Pitils of Selachians. 



® 



Fro. 303.— The 



angel shark, 



Sqnntina. 



Fig. 304.— The 

 .school shark, 

 Oaleorhinns. 



Fig. 305.— The 



guinmy shark. 



Must el us antarctic us. 



Fig. 306.— The 



nurse shark, 

 Gin glym ostoma . 



Fig. 307.— The 



dogfish, 

 Mustelus canis. 



Fig. 308.— The 



carpet shark, 



Orectolobus. 



Fig. 309.— The 



leopard shark, 



Triakis. 



Fig. 310.— The 



white -tip shark, 



Carcharodon. 



Fig. 311. — The crested Port Jackson 

 shark, H eterodontus , pupil dilated 

 and contracted. 



Fig. 312. — The fiddler ray, Trygono- 

 rhina, pupil dilated and contracted. 



Fig. 313.- 



<::::> 



-The pupil of the dogfish, Scylliorhinus. 

 traction (after Franz). 



Showing stages of con- 



Heterodontus 



arrangement of the musculature of the iris ; when this forms a con- 

 tinuous sheet a round or oval pupil results ; where this is lacking in 

 certain areas an operculum is formed (Grynfeltt and Demelle, 1908). 

 Of the first type, some deep-sea species (the luminous shark, Etmopterus; 

 C'enirojjhorus calceus) have large, round, almost immobile pupils with 

 poorly developed muscles — a configuration to be expected in their dimly 

 lit habitat. Species which come to the surface and bask have contrac- 

 tile pupils, usually circular in dilatation and elliptical on contraction 

 (characteristically in the vertical direction but sometimes oblique or 



