522 



AMPHIBIA 



[CH. 



an untwisted position. This movement is known as supination, 



the reverse movement being known aspronation. 



The hind-limb in the higher Amphibia and 

 other Vertebrata is likewise rotated forward so 

 that the knee points forward and the first digit 

 is internal, but this does not occur in the lower 

 Amphibia, such as Molge. 



The pectoral girdle is not essentially dif- 

 ferent in the lower Amphibia and the more 

 primitive Osteichthyes, but the pelvic girdle is 

 firmly joined to the transverse process of one of 

 the vertebrae, which is called the sacral. This 

 is one of the most distinctive features of all 

 pentadactyle animals; it is a consequence of 

 the adaptation of the pentadactyle limb to raise 

 the body from the ground (Fig. 252). It is 

 necessary for this purpose that the limb should 

 have a firm purchase on the axial skeleton. 

 Consequently when we find some Amphibia 

 which never use their limbs for crawling but 

 only for swimming, we assume that this is a 

 secondary degenerate condition. 



Next to the character of the limb one of 

 the most distinctive features of Amphibia is 

 the nature of the skin. Indeed the five great 

 classes of Gnathostomata Fishes, Amphibia, 

 Reptiles, Birds, and Mammals are each per- 

 fectly characterised by the nature of their skin. 

 In a typical Amphibian the skin is soft and 

 moist and devoid altogether of any ossifications 

 like the scales of fishes. The skin is a most 

 important breathing organ, since the lung alone 

 cannot meet the demand for oxygen, and if the 

 skin becomes dry and consequently incapable 

 of absorbing oxygen the animal dies. The 

 necessary moisture is supplied from a series of 

 pockets, to form which the ectoderm is pouched 

 inwards or to use a more convenient term 

 " invaginated " at various points, and the cells 

 lining these pouches have the power of secreting 

 great quantities of mucus. As the cells become 



FIG. 252. Skeleton of 

 Triton, Molge cris- 

 tata x 1. 



