370 ZOOLOGY. 



develop and establish a pulmonary circulation. Finally, the 

 branchial vessels are obliterated, and then the circulation takes 

 place nearly as in other reptiles. The venous blood returning 

 from all parts of the body, is poured into the ventricle by one 

 of the auricles, and is therein mingled with the arterial blood 

 arriving from the lungs, and poured into the ventricle by the 

 other auricle. This mixture penetrates into the aorta, and 

 through it proceeds in small quantities to the lungs, and 

 hence to the other organs of the body. 



The lungs of the adult batrachia have but a few incomplete 

 cells ; they receive the blood from two small branches of the 

 aorta, these performing the office of a pulmonary artery. 

 Thus the pulmonary respiration is feeble, but the cutaneous 

 makes up for it by its activity. When the temperature is low 

 it is sufficient to maintain lii'e. Frogs breathe by the action 



Fig. 380. Aquatic Salamander (Newt, Eft, or Evvet). 



of deglutition, so that to suffocate the frog it is only necessary 

 to hold its mouth open for a time. This mode of breathing 

 is necessitated by the incomplete state of the skeleton of 

 the adult frog; the ribs are wanting, and thus the thorax can 

 uu longer be dilated, as in mammals, birds, and ordinary rep- 

 tiles. Finally, the nervous system in the^e animals is but 

 little developed ; the brain is small, and the cerebellum is 

 scarcely visible. '* 



473. Though the class batrachia be not numerous, it 

 has, notwithstanding, been divided into four orders. 



The Arioures, which undergo complete metamorphoses, and 

 which in the adult state have no tail ; these are the frogs, 

 toads, rainettes, tree frogs, pipas, Surinam toad, &c. 



The Urodeles, which preserve the tail, but which in the 

 adult state have four limbs, but no branchiae : the aquatic 

 salamanders or tritons, are examples of this order (Fig. 380). 



The Perenniata, which preserve the branchiae throughout 

 life, and which also have lungs : these are the proteus, the 



