294 ELEMENTARY ZOOLOGY 



or tenth being the rod-shaped bone called the urostyle) 

 in the short, tailless frogs and toads. To any of the 

 vertebrae except the first (the single cervical vertebra) and 

 the last, ribs may be attached and the ccecilians have 

 about as many pairs of ribs as vertebrae. In the frogs 

 and toads, however, the ribs are lost. In any case they 

 are never fastened by their lower ends to the breast-bone. 



The alimentary canal is usually not much longer than 

 the body and is plainly divided into mouth, pharynx, 

 oesophagus, small intestine, large intestine or rectum, 

 and anal opening. The teeth when present occur on both 

 the jaws and the palate. They are small, sharp, point 

 backward and are fused to the bones. They are wholly 

 wanting in the toad and in some other allied forms. The 

 tongue may be wanting, or may be immovably fixed to 

 the floor of the mouth, or as in the frogs, fastened at its 

 front end but free behind, so that the hinder end can be 

 protruded far from the mouth for the purpose of catching 

 insects. 



The organs of respiration are gills, external and in- 

 ternal, lungs, trachea or windpipe, and the skin. In the 

 earliest larval stages all batrachians have gills; later, in 

 most cases, the gills become reduced and disappear, while 

 at the same time lungs are developing. In some sala- 

 manders the lungs never develop, but the animals, in their 

 adult stage, breathe wholly by means of the skin. In a 

 few cases, as in the siren and mud-puppies, gills are 

 retained through the whole life, although lungs are also 

 present in the adult stage. The lungs are two in number, 

 a right and a left lung, and are simple sacs with the walls 

 more or less folded or thrown into ridges and richly sup- 

 plied with blood-vessels. The front end of the lungs 

 opens directly into the pharynx or, in the more elongate 

 batrachians, is connected with it by a tubular trachea or 

 windpipe. In the frogs and toads there are vocal cords 



