334 



THE EYE IN EVOLUTION 



Caecilian 



represented by the c^cilians {ccecus, blind) and a number of related species 

 in all of which the eyes are degenerate ; they will therefore be discussed 

 subsequently.^ 



2. URODELA (ovpd, tail ; StjAo?, visible) (or caudata, cauda, a tail), tailed 

 Amphibians, typified in the salamanders and the newts, are generally divided 

 into 7 families. Of these several are cavernicolous in habit, having little use 

 for eyes ; these are therefore often degenerate and are discussed at a later stage 

 {Proteus, etc.).^ Others, such as the newts {Triturus), Ambystoma, and the 

 terrestrial salamanders, have relatively simple but well-formed eyes. 



3. ANURA (d, privative ; ovpd, tail) or tail-less Amphibians, consisting of 

 nearly 1,000 different types including the common frog (Kana), the toad (Bufo), 

 the tree-frog (Hyla), the obstetric toad (Alytes), the Surinam toad (Pipa), 

 the African clawed toad (Xenoptis) and some other species, have well-developed 

 eyes. 



The general characteristics of the amphibian eye as seen in the 

 last two orders are as follows : 



The transition from water to air and the consequent lack of the 

 necessity for streamlining the globe, allow it to assume a spherical shape. 

 Moreover, the difference in refractivity between the air and the cornea 

 allows this structure to assume a 7iew role in the dioptrics of the eye ; it 

 therefore becomes highly arched and its optical properties are good. The 

 lens can therefore fall backwards from the cornea ; it still, however, 

 remains large and is moved as a whole, thus retaining an accommodative 

 mechanism somewhat resembling although 7iot analogous with that seen in 

 fishes. 



The visual elements are complex and relatively gross — two types of 

 rods and single arid double cones rerniniscent of those occurring in 

 Holosteans and Dipnoans. 



To protect and moisten the cornea, lids are provided, together with a 

 harderian gland and a naso-lacrimal duct. 



THE ANURAN EYE 



THE EYE OF THE FROG [Rana) has probably received more 

 detailed study than that of any Vertebrate other than man (Figs. 

 397-8).^ The globe is almost spherical, the cornea and the sclera main- 

 taining the same curvature. The latter, after metamorphosis from the 

 tadpole stage, develops on its inner aspect a cup of hyaline cartilage, 

 thickest at the posterior pole and extending anteriorly to beyond the 

 insertions of the rectus nmscles ; it is pierced by the foramen for the 

 optic nerve as well as by small canals which allow the passage of the 

 ciliary vessels and nerves (Caso, 1931 ; Yamasaki, 1952). In some 

 tree-frogs (Hylidse) the scleral cartilage is discontinuous or lacking ; 



1 p. 730. 2 p. 728. 



^ Dating from the description of Petit (1737) and Soemmerring (1818). See Gaupp 

 (19C •), Tretjakoff (1906), Walls (1942), Rochon-Duvigneaud (1943). For development, 

 see r: -riep (1906), Studnicka (1913), Jokl (1918-20). 



