244 ZOOLOGY. 



VERTEBRATA. 



Class II. Reptilia. 



The Reptilia are cold-blooded vertebrates like fishes, but are distinguished 

 from these by the pulmonary respiration, the heart with three chambers, 

 the presence of organs of motion other than fins, and by various other 

 points of organization. 



The circulation of the blood is incomplete ; less complete even than in 

 fishes. The heart consists of two auricles and one ventricle. The 

 venous blood, collected from the various parts of the body, accumulates in 

 the vena cava, and thence passes into the right auricle. From the right 

 auricle it passes into the single ventricle, and by it is impelled through the 

 aorta into all parts of the body. A small branch leads to the lungs, and 

 the blood when purified is returned to the left auricle, which drives it into 

 the ventricle. This ventricle thus receives venous blood from one auricle, 

 arterial from the other, and it is a mixture of this kind which is distributed 

 through the body. The naked skin of the Batrachian reptiles serves a 

 good purpose in the decarbonization of the blood, owing to the extensive 

 distribution of bloodvessels immediately on the under side of the skin. 



The blood of Reptilia is characterized by the possession of the largest 

 globules to be found in the entire vertebrate sub-kingdom. These, in the 

 tailed batrachians, as Siren, &c., are distinctly visible to the naked eye. 

 As in fishes and birds these globules are elliptical in outline ; in mammalia, 

 with the single exception of the Camelidce, they are circular. 



The lungs lie free in the abdominal cavity ; these, with the heart, not 

 being separated from the other viscera by a diaphragm. The cells of the 

 lungs are of greater or less subdivision ; in man}' of the North American 

 SalamandrcB they are mere sacs. Reptiles are better able to sustain the de- 

 privation of oxygen than other vertebrates : this, however, depends greatly 

 upon the temperature and season. Thus a frog will bear the deprivation 

 of atmospheric air in summer for a space of time not much exceeding two 

 hours, while in winter it can sustain its absence for several days. 



A point of great physiological interest in the structure of reptiles consists 

 in the fact that some forms present, at different times of life, both fish-like 

 and reptilian features of respiration. Thus the salamanders and frogs, when 

 young, respire for a certain length of time, for years in some, by means of 

 external gills, the lungs being entirely rudimentary. In course of time the 

 lungs acquire a greater development, and the gills disappear. This fish-like 

 condition of things, transitory in some, is permanent in others, as in Meno- 

 hranchus, Siren, and Proteus, which throughout life possess external gills. 



So true is it that the skin in the naked reptilia is accessory to the 



function of respiration, that the experiment has been tried with perfect 



success as to how far respiration might be carried on entirely by means 



of the skin. Thus the lungs of the frog have been tied in such a manner 



448 



