AMPHIBIANS, CROCODILIANS, TURTLES 



vertebrates and exceeds that of even the lizards, whose fresh lenses will 

 drool through one's fingers if one attempts to hold them in the hand. As 

 might be expected, the range of accommodation is rather less in the com- 

 pletely terrestrial tortoises and in the thoroughly aquatic sea turtles than 

 it is in the in-and-out pond-dwelling majority. In Emys, a pond genus, 

 for example, the ciliary processes bear on the lens (Fig. 110, p. 277), and 

 during accommodation the lens is squeezed equatorially, its diameter 

 reduced. In the terrestrial Testudo, the ciliary processes touch the lens 

 but the deformation of the latter in accommodation is much less than in 

 Emys, and its diameter is not affected. In the marine Thalassochelys, the 

 ciliary processes do not reach the lens, which is relatively small and is 

 much more nearly spherical than that of other turtles. Konig found a 

 transversalis muscle (p. 269) in Emys and Thalassochelys, but not in 

 Testudo, whose embryonic fissure is entirely closed. 



I 



Fig. 148 — Accommodation in turtles. 



a, b, relaxed and accommodated conditions in Emys orbicularis. x4/4. From Franz, after 

 Beer, c, diagram showing roles of sphincter iridis and ciliary muscles in produrtion of 

 anterior lenticonus. c- ciliary muscle; s- sphinrter iridis muscle; o- scleral ossicle. 



The female marine turtle is in a bad way visually when she comes 

 ashore at night to lay her eggs, for her aerial vision must be hazy and 

 dim even if the moon is bright; and though she closes her eyes tightly 

 when digging her nest, the reduced lids are inadequate to prevent the 

 eyes' getting clogged with sand. All in all, she must be very glad to get 

 back in the water, toward morning! 



Since the lens of a turtle always projects through the pupil, to let the 

 iris get a grip on it during accommodation (Fig. 148c), the pupil can 

 actually close but little if at all. But the turtles have obtained immunity 

 from dazzlement by eliminating nearly all of their rods, though they 

 might perhaps have kept a well-balanced duplex retina if they had also 

 retained efficient photomechanical changes. They are thus under some 

 handicap in seeing under dim underwater conditions, and undoubtedly 

 such bottom forms as the snappers and musk-turtles hunt chiefly by 



