CHAPTER XXIII 



Class AMPHIBIA 



Order I. Stegocephali (extinct). 



II. Gymnophiona or Apoda (a small order). 

 III. Urodela or Caudata, e.g. Newts and Salamanders. 

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



Amphibians 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 bodily size, the Amphibian race has dwindled since 

 the days of its prime, but it seems to have been progressive, 

 for some of its members show affinities with Reptiles. 



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. The unpaired fins, fre- 

 quently present both in larvce and adults, are without fin-rays. 

 In existing forms there is rarely any exoskeleton, but some 

 extinct forms had an armour of bony plates. The skull has 

 two occipital condyles. The heart is three-chambered, with 

 two auricles and a ventricle — and a conus arteriosus. The 

 gut ends in a cloaca, into which the ducts from kidneys and 

 reproductive organs also open. A bladder, growing out from 

 the hind region of the gut, is probably homologous with the 

 allantois of the embryos of higher Vertebrates. The ova are 

 small, numerous, usually pigmented, and with yolk towards 

 one pole. They are almost always laid in water ; the seg- 

 mentation is holoblastic, but unequal. There is usually a 



metamorphosis in development. 



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