414 



MANUAL OF ZOOLOGY. 



pairs of limbs are always present. The skin is soft, and there 

 are rarely any traces of an exoskeleton. The dorsal vertebrae 

 are " procoelous," or concave in front, and are furnished with 

 long transverse processes, which take the place of ribs, which 

 are only present in a rudimentary form. The radius and ulna 

 in the fore-limb, and the tibia and fibula in the hind-limb, are 

 anchylosed to form single bones (fig. 159). The mouth is 

 sometimes edentulous, but the upper jaw has usually small 

 teeth, and the lower jaw sometimes. The hind-limbs usually 

 have the digits webbed for swimming, and are generally much 

 larger and longer than the fore-limbs. The vertebral column 

 is short (of ten vertebrae in the Frogs, but only eight in Pipa). 

 The tongue is soft and fleshy, not supported by an os hyoides, 

 but fixed to the symphysis of the lower jaw in front. Pipa has 

 a sort of valve over the tympanum ; Hyla and Rana have the 

 tympanum shown externally; and Bufo has the tympanum 

 concealed. 



Fii'. IS9— Skeleton of the common Frog {^Rann iemfarnrin). d Dorsal Vertebra;, 

 with long transverse processes. 



_ In the adult Anntra, respiration is purely aerial, and is car- 

 ried on by means of lungs, which ar^ comparatively speaking, 

 well developed. As there are no movable ribs by which the 

 thoracic cavity can be expanded, the process of respiration is 

 somewhat peculiar. The animal first closes its mouth, and 

 fills the whole buccal cavity with air taken in through the 



