CHAPTER XXIII. 



CLASS AMPHIBIA. 



Order I. LABYRINTHODONTIA or STEGOCEPHALA (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 represent in the evolution of Vertebrates those 

 forms which made the transition from aquatic to terrestrial 

 life, but have lagged near the water. Certain acquisitions, 

 such as lungs and a three-chambered heart, gained by the 

 Dipnoi, are here firmly established. The race has dwindled 

 in size of body since the early days of its beginning, but it 

 seems to have been progressive, for Amphibians are not 

 awanting in affinities with Reptiles or 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 

 'always possess functional lungs. When limbs are present 

 they exhibit distinct digits, and conform to the same type as 

 those of higher Vertebrates. Although unpaired fins are 

 frequently present, both in larval and adult life, there are no 

 fin rays. In existing forms there is rarely any exoskeleton, 

 but some extinct forms were furnished with an armour of 

 bony plates. The heart is three chambered, having two 

 auricles and a ventricle. The gut ends in a cloacal chamber 

 into which the urinogenital ducts open. A bladder, which 

 grows out from the hind region of the gut, is probably homo- 

 logous 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 segmentation is holoblastic, but unequal. There 

 is often a marked metamorphosis in development. 



34 



