AMPHIBIA. 355 



The Urodela are the Tailed Amphibia, the larval tail being 

 retained throughout life : they have a smooth naked skin 

 and a compressed or cylindrical tail, and their vortebrs are 

 both amphiccelous and opisthoooelous. To this division belong 

 the Tritons or Water Salamanders or Newts, and the true 

 Land Salamanders, The Anoura are the Frogs and Toads, 

 or the Tailless Amphibia, which are destitute of gills in the 

 adult state, and always devoid of a tail ; both structures, how- 

 ever, are present in the larva. Anoura have always two pairs 

 of limbs, procoelous dorsal vertebrffl with large transverse pro- 

 cesses, serving the functions of ribs, which are absent. The 

 radius and ulna and the tibia and fibula are anchylosed together. 

 The posterior limbs, which are the larger, have usually the feet 

 webbed and adapted for swimming. The heart of the Amphib- 

 ian is a stage higher than that of fishes. Although it is com- 

 posed of three chambers, two auricles and one ventricle, in the 

 adult, the larval heart is the same as in fishes. The blood in the 

 adult is received from the lungs in the left auricle and from the 

 body in the right auricle, both emptying into the single ven- 

 tricle, which therefore contains mixed arterial and venous blood, 

 and this mixed blood is pumped to both the lungs and system. 

 The larva has only a two-chambered heart, the blood being 

 driven to the gills, through them, and on to the system ; but as 

 the gills begin to give place to lungs, little branches pass off 

 from the branchial vessels, which unite as the lungs increase 

 and carry part of the blood to the pulmonary sacs. Event- 

 ually the gill supply may cease, the entire blood going to the 

 lungs, when we get a third chamber formed in the heart in the 

 form of another auricle. Other Amjjhibia have both branchial 

 and pulmonary supplies permanent. The blood - supply then 

 being mixed, oxygenation is comparatively slow, and thus little 

 heat is generated, the Amphibia, like fishes, being cold-blooded 

 vertebrates. 



The development of an amphibian is best seen in the frog. 

 The spawn, which is deposited in masses in the water, is sur- 



