AMPHIBIANS 



349 



Movable eyelids are found only in tlie Urodeles which have 

 adopted terrestrial life ; in aquatic forms the lids have receded to 

 immovable ridges or low folds, while in subterranean species the eyes 

 are completely covered by the skin.^ It is interesting that in terrestrial 

 salamanders the lacrimal glands are distributed along the lower lids 

 (Piersol, 1887 ; Maggiore, 1912 ; Engelhardt, 1924). 



The limicoline types f)f the Urodela which hve in mud, such as tlie North 

 American genera, Cryptobranchus, Amphluma, Necturus and Siren, have relatively 



Mega lohatrach ?<.s 



Amphiuma 



■J 0. 



cv 



rl' 



u^' 



Fig. 416. — The Eye of MEdALOBAiRAcuua maximum. 



A section of the ill-formed eye of this Urodele. C, cornea ; Ch, choroid ; 

 CO, optic canal ; H, skin ; K, the enormously lai'ge scleral cartilage with 

 its dense core, P; O, o[iti(' nerve ; R, retina ; S. fihrous tissue of sclera 

 {after Lauber). 



crude and ill-developed eyes which seem incapable of elaborate optical imagery. 

 In the related Japanese giant salamander, ]\Iegalobatrachns maximus, found also 

 in China and Tibet — incidentally the largest extant Ami:)hibian, 5 feet in length 

 — the monstrously hypertrophied scleral cartilage occupies more space than the 

 remainder of the eye ; indeed, this cartilage is the most massive seen among the 

 Vertebrates and occupies two-thirds of the section of the globe (Lauber, 1902 ; 

 Reese. 1905 ; Yano, 1926-28 ; Aoyama, 1928 ; Stadtmilller, 1929) (Fig. 416). 

 In this salamander also, as in some other Japanese types, the cornea is vascu- 

 larized (Tawara, 1933: Kurose, 1956). The visual elements are similarly sparse 



Necturus 



1-2G. 



