196 BACKBONED ANIMALS. 



the jaws enveloped in horny beaks. The liver is two- 

 lobed, and the intestine short and straight, not enlarging 

 into a stomach proper. Frogs and toads feed upon in- 

 sects as a rule. 



Respiration. At first Batrachians with few exceptions 

 breathe by means of gills, as the fishes, the breathing- 

 organs being external, as in the young of the curious 

 ash Polypterus (Fig. 207). Frogs, toads, and newts have 

 at first two sets of gills, an external and an internal 

 pair. The former disappear first. As growth progress- 

 es, they lose the latter also, then breathing by perfect 

 lungs. 



Circulation. When young and possessing the red gills, 

 blood is pumped to them as in the fishes, the heart then 

 possessing two chambers, an auricle and a ventricle. 

 Later, when the lungs appear, the auricle divides and the 

 heart becomes three-chambered ; the blood, on account of 

 its incomplete aeration, is cold. 



Development. The Batrachians pass through a distinct 

 metamorphosis. The eggs are generally placed in or 

 near the water, enveloped in some cases in a jelly-like 

 mass, the young first being water-animals, breathing by 

 external gills (Fig. 243), finally changing to the adult 

 form. 



Order I. Trachystomata. Sirens (Siremfa). The 

 sirens are long, slender creatures, with permanent gills. 

 They have no hind-limbs ; even the fore pair, which are 

 either three- or four-toed, are weak and almost useless. 

 The great siren (S. lacertina] attains a length of three feet, 

 is nearly black in color, dotted with light spots, the abdo- 

 men pink or purple. It has four toes on each fore-limb, 

 and is found in the muddy ditches and swamps of the 

 Southern States. A small siren (Pseudobranchus striatus), 

 with three toes, is found in the rice-field streams of 

 Georgia. 



Order II. Proteida. The Proteus (Fig. 239) is a 



