160 AMPHIBIA 



slits close up during the metamorphosis and the lungs become 

 the chief organs of respiration, supplemented by the skin which 

 being moist and richly supplied with blood takes part in this 

 function ; and it is a most extraordinary and remarkable fact 

 that in some cases, both lungs and gills degenerate and the skin 

 is the only organ of respiration. Ribs being small or reduced to 

 mere vestiges, air is forced into the lungs by the compression of 

 the mouth cavity and expelled by the elasticity of the lungs and 

 body walls. The original single aperture of each nasal sac, as 

 seen in Elasmobranch fishes, is divided by the maxillae and pre- 

 maxillae into two, an external nostril on each side of the surface 

 of the snout and an internal nostril in the roof of the mouth cavity. 

 Air is taken in through the nostrils while the mouth is closed, and 

 the external nostril is closed by muscular action when the air is 

 forced into the lungs. 



In the larval or tadpole condition the heart and branchial, 

 or gill, arteries, have a structure and arrangement similar to those 

 of fishes. The heart has a single auricle and a single ventricle, 

 the former receiving the blood from the veins and passing it into 

 the ventricle, which pumps it through the gills, into all parts of 

 the body. After the metamorphosis, when the animal is adult 

 and possesses lungs, the auricle is divided into two, the right 

 receiving the blood from the body, and the left the blood from 

 the lungs. The ventricle, however, remains single, both auricles 

 opening into it, so that a certain amount of mixture of the 

 blood from the body and that from the lungs takes place, as in 

 reptiles. 



The existing members of the class consist of three obviously 

 distinct groups the characters of which are conspicuous, and be- 

 tween which there are practically no intermediate forms. These 

 three groups are the well-known and abundant tail-less jumping 

 forms like the frogs and toads, the tailed walking forms like the 

 newts and salamanders, and the small group of burrowing Am- 

 phibia which are entirely destitute of limbs and are confined to 

 the tropics. These groups form three Orders called the Anura 

 (meaning tail-less), the Urodela (meaning with persistent tails), 

 and the Apoda (meaning leg-less). Of these the Urodela or 

 newt-like forms are, if not in all respects the most primitive, 

 the least specialised, and will therefore be considered first. The 

 extinct Labyrinthodonts form a fourth group regarded by Dr. 



