Ichthyopsida 



145 



fishes only in some details of structure and arrangement. These organs, 

 at least in part, persist in those tailed amphibians which are wholly 

 aquatic in adult life and also in some aquatic adult frogs and toads. 

 They are organs which can function only in water. 



The larval period passes over into a period of metamorphosis. 

 In amphibians whose adults are permanently aquatic, the changes are 

 relatively small (Fig. 341), but in frogs and toads the metamorphosis 

 involves absorption of tail and gills, closing of gill-clefts, and various 

 internal changes. Meanwhile, legs develop, the lungs become fully 

 functional, and the bodily proportions 

 are altered to those of the adult. 



The immediate stimulus which 

 brings about metamorphosis is pro- 

 vided by the secretion of the thyroid 

 gland. The chemical peculiarity of the 

 secreted substance is its high content 

 of iodine. If thyroid substance, or even 

 pure iodine, is fed or injected into a 

 tadpole, metamorphosis may be accel- 

 erated or induced prematurely. 



In some amphibians the larval 

 stage may be indefinitely prolonged, 

 the larva becoming sexually mature 

 and reproducing while otherwise in the 

 larval form. The Mexican "axolotl" 

 and the axolotls of the Rocky Moun- 

 tain region are sexually-mature per- 

 sistent aquatic larvae of the tailed amphibian (salamander) Ambys- 

 toma tigrinum which, in other parts of the country, regularly meta- 

 morphoses into a gill-less adult. Under appropriate environmental con- 

 ditions, temperature perhaps being a factor, or if thyroid material is 

 administered, the axolotl may undergo metamorphosis. Reproduction 

 by an animal which is still in a larval form is called neoteny or 

 paedogenesis. 



Fig. 344. Gastrotheca marsu- 

 piata, the purselike brood-pouch 

 cut open on the side to show the 

 eggs within. (By permission from 

 " Biology of the Amphibia," by 

 G. K. Noble, copyrighted 1931. 

 McGraw-Hill Book Co., Inc.) 



Respiration in adult amphibians may be carried on by means of 

 gills, lungs, skin, and the lining of the cavities of the mouth and 

 pharynx (buccopharyngeal surfaces). In some cases the respiratory 

 function may extend back even into the esophagus. Respiration may 

 take place in all four regions at once in an individual amphibian. The 

 relative importance of the several regions differs greatly in various 

 amphibians. In all cases the nasal cavities, as in liingfishes, open into 



