12 BATEACHIANS. 



Nevertheless, they do not altogether forget their native 

 element ; thanks to their webbed feet, they can still traverse the 

 waters which sheltered their infancy ; and when alarmed by any 

 unusual noise, they rush into the water as a place of safety, where 

 they swim about in apparent enjoyment. In some of them, as 

 Proteus and the amphibious Sirens, where the limbs are confined 

 to the pectoral region, swimming seems to be the state most natural 

 to them. They are truly amphibious, and they owe this double 

 existence to the persistence of their gills ; for in these perenni- 

 branchiate Batrachians, arteries are developed from the last pair 

 of branchial arches which convey blood to the lungs : while, 

 in those having external deciduous gills, the office being dis- 

 charged, they lose their ciliate and vascular structure and disap- 

 pear altogether. The skull in Reptiles generally consists of the 

 same parts as in the Mammalia, though the proportions are dif- 

 ferent. The skull is flat, and the cerebral cavity, small as it is, is 

 not filled with brain. The vertebral column commences at the 

 posterior part of the head, two condyles occupying each side of the 

 vertebral hole (Fig. 2). The anterior limbs are mostly shorter than 

 the posterior, as might be expected of animals whose progression 

 is effected by leaps. Eibs there are none. The sternum is highly 

 developed, and a large portion of it is cartilaginous ; it moves in 

 its mesial portions the two clavicles and two coracoid bones, 

 which fit on to the scapula, the whole making a sort of hand which 

 supports the anterior extremities, and an elongated disk which 

 supports the throat, and assists in deglutition and respiration. The 

 bone of the arm (humerus) is single, and long in proportion to the 

 fore arm. In the Frogs [Rana), the ilic bone is much elongated, 

 and is articulated in a movable manner on the sacrum, so that the 

 two heads of the thigh bones seem to be in contact. The femur, 

 or thigh, is much lengthened and slightly curved, and the bones 

 of the leg so soldered together as to form a single much elongated 

 bone. 



The respiration of Reptiles and some of the Batrachians, like that 

 of Birds and Mammals, is aerial and pulmonary, but it is much 

 less active. Batrachians have, in addition, a very considerable 

 cutaneous respiration. Some of them, such as Toads, absorb more 

 oxygen through the skin than by the lungs. Their circulation is 



