600 AMPHIBIANS 



pedatids) differ from the arhythmic ranids in their total lack of oil- 

 droplets. 



The anuran (and urodele) visual-cell patterns share features in com- 

 mon with the holostean and with the dipnoan (see Fig. 170b, p. 587; 

 Fig. 171, p. 591; and Plate I). The single and double cones of amphib- 

 ians and those of Amia and of Protopterus are clearly homologous, 

 respectively. It seems highly signficant however that the immature frog 

 (tadpole) rod has been claimed by several investigators to have an oil- 

 droplet, which it loses before or during metamorphosis. If this can be 

 confirmed, the implication is that the frog's rods were once single cones. 

 If their derivation was a recent one, then there may have been no rods 

 at all in the stegocephalians (which, we may be sure, were diurnal — see 

 pp. 164, 208, 274, 518-9). And, the cone-like characteristics of the 

 amphibians' peculiar 'green' rod (see p. 58) make this element in a sense 

 structurally, perhaps therefore genetically, intermediate between the 

 single cone and the ordinary or red rod (Plate I). It is possible how- 

 ever that the Stegocephali did have a rod type, homologous with that 

 of the Chondrostei and Protopterus, and that likewise it contained an 

 oil-droplet — which the frog, in its infancy, still remembers. The presence 

 of oil-droplets in the cones of both anuran amphibians and modem rep- 

 tiles is proof enough that the common ancestors (Stegocephali) had such 

 droplets — ^presumably, colored ones, else they would have gotten lost. 



(B) Urodeles 



The tailed amphibians compose eight families in five suborders. The 

 members of four of these families (Crytobranchidae, Amphiumidas, Pro- 

 teidae, Sirenidse) are 'larval' or 'partly metamorphosed' forms which are 

 permanently aquatic, and whose eyes are in a state of degeneracy or on 

 the ragged edge of it. Even among the other families — the primitive 

 Hynobiidae, their offshoots the Ambystomidae and their cousins the 

 Salamandridas, and the latter 's American derivatives the Plethodontidae 

 — there are scattered genera with greatly reduced or (cave-dwellers) 

 wholly degenerate eyes. At its very best, as in newts and especially in 

 terrestrial forms, the urodele eye is relatively small as compared with 

 the anuran, and its importance to the animal is relatively less. It is also 

 comparatively simple; but it cannot be too strongly emphasized that the 

 urodele pattern is not to be looked upon either as directly ancestral to 

 the anuran ocular plan, or as a descendant simplification thereof. The 



