CLASS IV. BATRACIIIA, 



407 



SKELETON or A FKOG. 



Class IV. BATRACIIIA. 



In descending from the higher to the lower forms of life, we now come to a class which, while 

 still preserving the vertebrate organization, has certain curious modifications of structure, show- 

 ing an approach to the fishes, and in some cases blending the fish and the quadruped, and even 

 presenting the same species as a fish, having a tail and breathing by means of gills in infancy, 

 and afterward rising at maturity to the dignity of four legs, and breathing the vital air by means 

 of lungs. Though many or most of these animals live in the slime of ponds, rivers, and ditches, 

 and are little esteemed by mankind, they present innumerable instances of admirable contrivance 

 in adapting them to their situations, and these are, perhaps, the more striking as they are fre- 

 quent deviations from the systems which nature has followed in the higher forms of existence 

 and those with which we arc most familiar. 



The Batrachia are divided into five very distinct orders : the Anura, of which the Frog is the 

 type, and in which the tail is wanting in the fully developed animal ; the Urodela, including the 

 Salamanders and Tritons, breathing by lungs alone, and retaining the tail in the perfect state ; 

 the Amphipneusta, including the Sirens, having two or four legs, and permanent gills ; the 

 Apoda, with a vermiform body and no legs ; and the Lepidota, having a fish-like, scaly body, 

 four simple limbs, and permanent gills. 



Though diff'ering in many important respects, all these animals agree in having a large 

 mouth, the tongue usually of large size, the intestinal canal short, the liver large ; they all 

 possess lungs, but during their young or larval condition they are furnished with branchiae, in 

 some cases these being persistent through life. The heart is composed of three chambers — a 

 single muscular ventricle, and two membraneous auricles ; but in some species the partition be- 

 tween the latter is imperfect. The arterial bulb is surrounded by a distinct muscular coat, as we 

 shall see in the Ganoid and Selachian fishes ; and from the continuation of this, the arteries 

 running to the branchiae and lungs are given off". 



The Batrachia are all strictly oviparous animals, although in some species the eggs are retained 

 in or upon the body of the parent until the young have attained a certain degree of development. 

 As a general rule, the ova arc impregnated by the male at the moment of their leaving the abdo- 

 men of the female ; the eggs are united by a glutinous matter into masses or long chains, whi(;h 

 may be constantly seen floating in the waters frequented by these animals. They are essentially 

 inhabitants of the warm parts of the earth, and abound particularly in tropical countries. 



