THE ANURAN EYE 593 



visual field. But once the cornea and lens became able to embrace a 

 wider cone of light-rays than the lens alone, there was no longer any 

 need of having the lens placed as far forward in the eye as possible. 



The Amphibia have never felt fully the penalties, nor completely 

 realized the possibilities, in this situation. Their palpebral and glandular 

 complexes have not had to be brought to the perfection demanded of 

 the dry-skinned vertebrates; and they have clung to a lens-moving meth- 

 od of accommodation — indeed, one which they developed themselves — 

 without having ever developed the ciliary body to such a degree that it 

 could bear upon the lens and directly squeeze it. 



The three living orders of amphibians are not closely inter-related. 

 The origin of the caecilians is quite unknown. The anurans and urodeles 

 are usually held to have had separate origins from stegocephalians; but 

 a modern theory, for which support is slowly growing, holds that the 

 urodeles were derived directly from lungfishes. We shall find no ophthal- 

 mological reasons for considering the urodeles any closer to the lung- 

 fishes than the anurans; and we shall see that since the two groups share 

 a number of new features — among them, such things as 'green' rods, 

 retractor bulbi and protractor lentis muscles, discontinuous ciliary mus- 

 cles, and fibrous zonules — there are good reasons for considering the 

 tailed and tailless amphibians to have had common ancestry after all. 

 Neither group can be called more primitive than the other; but the 

 Anura are treated first here because their eyes are a little more complex, 

 making it easy to describe the salamander eye largely by saying what 

 anuran features it lacks. 



(A) Anurans 



According to Noble, the tailless amphibians comprise ten families in 

 four suborders. The ocular structure of only two families — the Ranidae 

 (common frogs) in the highest suborder (Diplasiocoela) , and the Bufon- 

 idas (common toads) in the next highest (Proccela) — can be considered 

 well worked out. Future researches on other families may alter some of 

 the generalizations below. 



The Eye as a Whole — At the time of metamorphosis from tadpole into 

 adult, the lids and 'nictitans' develop; and the aquatic, benthonic tad- 

 pole's dermal spectacle then fuses with the primary dural cornea, except 

 in the tongueless toads of the primitive family Pipidas* and in one or 



*Xenopus, however, has a niaitans-Iike lower lid— chough no upper. 



