694 



Triturus (female) 



THE EYE IN EVOLUTION 



the protruding eyes, well raised above the water to search for a meal, are 

 freely motile, particularly antero -posteriorly; this motility is matched 

 only by the chameleon-like movement seen in the turretted eyes of the 

 mud-skipper, Periophthalmus, as it skips about on land upon its fins 

 seeking its insect food in the search for which the eyes move about in 

 all directions, even downwards, as if set upon universal joints (Fig. 844). 



The eyes of amphibians, whether Anurans such as the frog or toad 

 or Urodeles such as Triturus or Salainandra, have never been observed 

 to exhibit voluntary movements, although the lizard-like insect- 

 catching habits of many would suggest that these would be biologically 

 useful. 



Fig. 844. — The Mud-skippek, Periophtbalmvs 



The fish is on land and the prominent, freely-motile turreted eyes are 

 well seen (c./., Fig. 386) (photograph by Michael Soley). 



Heloderma 



REPTILES with some marked exceptions are not characterized by 

 active ocular motility ; most of them when quiescent maintain com- 

 plete immobility of the eyes. Rochon-Duvigneaud (1943) divided 

 them into two types : the first — crocodiles, geckos and snakes — with a 

 wide palpebral fissure and an extensive field, in which the eyes appear 

 to be immobile ; the second — Chelonians and most lizards, particularly 

 the chameleon — with a small palpebral aperture and with mobile eyes. 

 The turtles and tortoises, like some teleostean fishes, can coordinate 

 their eyes in lateral movements for binocular vision, but all vertical 

 movements are independent. The more active lizards show a con- 

 siderable ocular motility — in Lacerta viridis, the excursion is 40° — but 

 all voluntary movements are independent and incoordinated ; in the 

 more sluggish and many nocturnal forms the eyes are relatively 

 immobile ; in some forms, such as the Gila monster, Heloderma, 

 ocular movements are apparently absent ; but the chameleon is 

 notorious in the animal kingdom for the extraordinary excursion and 

 rapidity of the movements of its eyes (Rochon-Duvigneaud, 1933). 



The eyes of this animal bulge from the head while the small 

 circular palpebral aperture considerably restricts the visual field ^ (Fig. 



1 p. 671. 



