68 



DIVISION OF BATRACHIANS. 



SKELETON OF A FROG. 



and establish a pulmonary circulation. Finally, the branchial 

 vessels are obliterated, and then the circulation is carried on 

 nearly in the same manner as it is in other reptiles. The venous 

 blood, returning from all parts of the body, is poured into the 

 ventricle by one of the auricles, and there mixed with the arterial 

 blood, coming from the lungs and poured into the same ventricle, 

 by the other auricle. This mixture enters the aorta; a small 

 portion goes to the lungs, but the largest part is distributed to 

 the different organs of the animal. 



4. The skeleton of Batrachians presents remarkable peculiari- 

 ties ; in general, the ribs are entirely wanting, or are merely 

 rudimentary, for which reason pulmonary respiration cannot be 



carried on by the 



Fig. 29. ordinary mechanism ; 



and in fact, the ani- 

 mal introduces air 

 into its lungs by a 

 species of deglutition. 

 It is to be observed, 

 also, that the skin of 

 these reptiles is not 

 covered with scales 

 like that of Saurians, 

 Ophidians, and most Chelonians, but is naked. Almost all Batra- 

 chians are without nails. Their eggs are enveloped only in a 

 gelatinous mass which swells very much in water, and they are 

 not generally fecundated until after they are laid. 



5. This order is divided into four families, namely : 

 1st. The ANOURA, which, in their perfect state, have no tail, 

 and do not preserve their branchias, and are provided with four 

 extremities. 



2nd. The URODELA, which also lose their branchia?, and acquire 

 extremities, but preserve the tail. 



3rd. The BRANCHIFERA, which always preserve their branchiae ; 

 they resemble those Urodela in which developement had been 

 arrested while yet in the tadpole state. 



4th. The APODA, or CECILIA, which also lose their branchiaB, but 

 never acquire extremities : until lately they were classed amongst 

 the Ophidians, under the name of Naked Serpents, or Nuda. 



4. What are the peculiarities of the skeleton of Batrachians ? What 

 is the character of their skin ? 



5. How is the order of Batrachia divided ? What are the characters of 

 the family of Anoura, when perfect? What are the characters of the 

 family of Urodela when perfect? How is the family of Apoda charac- 

 terised ? What are naked serpents ? What are the characters of the 

 family of Branchifera ? 



