118 



ANIMAL BIOLOGY 





throughout life, though functional lungs are developed; others 

 resorb the gills but retain the gill slits; and still others lose all 

 traces of both gills and gill slits. In fact, some even go to the ex- 

 treme and lose their lungs, thus depending solely upon the moist 



skin to act as a respir- 

 atory membrane. Obvi- 

 ously the lung-breathing 

 method is not consistently 

 adopted. 



Common tailed Am- 

 phibians are Necturus (the 

 ' Mud-puppies ') , Cryp- 

 tobranchus (the 'Hell- 

 benders'), Ambly stoma 

 Fig. 76. -A, Amblystoma, Ambly stoma (the Blunt-nosed and 

 tigrinum; B, JNecturus, necturus macutosus. 1,. 

 (From Noble.) Tl g er Salamanders), and 



Triturus (the Newts). 

 Several species of Amblystoma have recently proved a boon to 

 biologists interested in fundamental problems of growth, vieing in 

 this field with the lowly Flatworms, such as Planaria. From 

 tadpole to adult they possess remarkable powers of regeneration: 

 they repair minor and major 

 mutilations, restoring excised 

 eyes and amputated limbs 

 and even appropriating the 

 limbs of other species that 

 are grafted. Ingenious experi- 

 ments have given an entirely 

 new conception of the marvel- 

 lous plasticity possessed by at 

 least some of the lower Verte- 

 brates. (Figs. 76, 77.) 



Fig. 77. — A Newt, Triturus cristata. 

 A, female; B, male during the breeding 

 season. (From Gadow.) 



2. Toads and Frogs 



The majority of Amphibians, some nine hundred species of the 

 order Salientia, are Toads and Frogs with a relatively clear-cut 

 metamorphosis from tadpole to limbed, lung-breathing, tailless 

 adult. The common Toads, such as Bufo americanus, hop about 

 chiefly after dusk devouring Worms, Snails, and Insects and so 

 render a considerable service. In fact, someone has estimated that 



