CHAPTER XXII. 



AMPHIBIA. 



Among living Amphibians there are three distinct orders : — 

 the frogs and toads in which the tail is absorbed before adult 

 life is attained, the newts and salamanders in which the tail 

 persists, and the worm-like limb-less Csecilians. The first 

 two orders have some extinct representatives, but most of 

 the Amphibian fossils belong to a distinct set, and are 

 usually united in the order Labyrinthodontia. Thus the 

 class includes at least four orders : — 



Labyrinthodontia or Stegocephala, wholly extinct, such 



as Mastodonsaurus and Dendrerpttum. 

 Gymnophiona or Apoda, tailless worm-like forms, sub- 

 terranean in habit, such as Ccecilia ; 

 Urodela or Caudata, with persistent tails, such as newts 



and salamanders ; 

 Anura or Ecaudata, tailless in adult life, such as frog 

 and toad ; 

 General Characters. — The Amphibians are almost without 

 exception aquatic in their youth, but the adults become 

 more or less terrestrial. The young forms always breathe by 

 gills, and in some cases this mode of respiration is retained 

 throughout life. All, however, acquire functional lungs- The 

 heart is three-chambered, having two auricles and a ventricle. 

 The limbs have distinct digits ; unpaired fins which are 

 almost always present ' in the larvae, and sometimes in the 

 adults as well, have no fin rays ; the skin is soft and gland- 

 ular without any hint of exoskeleton except in a few 

 exceptional cases. 



The gut ends in a cloacal chamber into which the urino- 

 genital ducts open, and from the hind end of the gut there 



