CHAPTER XXIII. 



Class AMPHIBIA. 



Order I. Stegocehiali or Labyrinthodontia (extinct). 



. „ II. Gymnophiona or Apoda (a small order). 



„ III. Urodela or Caddata, e.g. Newts and Salamanders. 



,, IV. Anura or Ecaudata, e.g. Frogs and Toads. 



Amphibians are those Vertebrates which made the transition 

 from aquatic to terrestrial life. But almost all have lagged 

 near the water. Certain acquisitions, such as lungs and a 

 three-chambered heart, incipient in the Dipnoi, are here 

 firmly established. As regards the bodily size of its members, 

 the Amphibian race has dwindled since the days of its 

 beginning, but it seems to have been progressive, for 

 Amphibians show affinities with Reptiles and even with 

 Mammals. 



General Characters. 



Amphibia are Vertebrates in which the visceral arches of 

 the larva almost always bear gills, which may be retained 

 throughout life, though the adults normally possess functional 

 lungs. Whence it follows that the nostrils, through which the 

 air enters, must open into the mouth. When limbs are present, 

 they have distinct digits, and resemble those of higher Verte- 

 brates. The unpaired fins, frequently present both in larvce 

 and adults, are without fin-rays. I?i existing forms the?-e is 

 rarely any exoskeleton, but some extinct forms had an artnour 

 of bony plates. The heart is three-chambered, with two auricles 

 and a ventricle. The gut ends in a cloaca, into which the ducts 

 from kidneys and reproductive organs also open. A bladder, 

 which grows out from the hind region of the gut, is probably 

 homologous with the allantois of the embryos of higher Verte- 



