THE PERCEPTION OF SPACE 695 



845) ; in seeking and catching insects a high degree of ocular mobihty 

 is therefore essential; moreover, as in the sea-horse, the movements 

 of the body are sluggish and the eyes and the tongue are the only parts 

 of the animal to exhibit the activity necessary to maintam its livelihood. 

 As it sits motionless, the eyes constantly and diligently explore the 

 surrounding environment uniocularly, swivelling like turrets through 

 an angle of 180° horizontally and 90° vertically in complete incoordma- 

 tion, one eye, for example, looking straight forward while the other 

 looks backwards. When an insect is seen, however, the eyes suddenly 

 become coordinated in extreme convergence so that both central foveas 



Fig. 84."). — The Chameleon, Chameleon bilepis. 



The freely-motile eyes with the minute palpebral apertures look in 

 different directions (c./.. Fig. 420) (photograph by Michael Soley). 



are brought to bear upon the prey, and the long sticky tongue, impelled 

 by its own elasticity and by the forcing of blood into the hollow spaces 

 within it, shoots out as far as the length of its body with extreme 

 rapidity and infallible accuracy to catch any insect within its reach. 



Snakes show little ocular motility ; swinging the head from side 

 to side pendulum-like, they examine an object first with one eye and 

 then with the other and then binocularly, the head-movements taking 

 the place of ocular movements ; even those possessed of a temporal 

 fovea, such as the tree-snakes,^ do not require to converge their eyes 

 to achieve binocularity. 



BIRDS, because of the enormous size of their eyes filling the bony 

 orbits, have necessarily very restricted movements — if any. Slight 

 horizontal movements are often the only ones to be represented : the 

 eyes of the owl, for example, cannot be moved passively even with a 



1 p. .388. 



