196 BACKBONED ANIMALS. 
the jaws enveloped in horny béaks. 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 
fish 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 aération, 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 (.Sivenide).—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. /acertinz) 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 
