CHAPTER XXIII 
Ciass AMPHIBIA 
Order I. STEGOCEPHALI (extinct). 
», II. GYMNOPHIONA or APODA (a small order). 
», III, URODELA or CAUDATA, ¢.g. Newts and Salamanders. 
», IV. ANURA or EcauDATA, ¢.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 ave 
present, they have distinct digits. . The unpaired fins, fre- 
guently present both in larve and adults, are without fin-rays. 
In existing forms there ts 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, ts probably homologous with the 
allantots 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- 
