THfi BATRACHIANS. II 



feet and fins, but with large heads. In stagnant ponds they are 

 frequently found in numbers, where they live and breathe after the 

 manner of fishes. By degrees, however, they are transformed, their 

 limbs and air-breathing lungs commence to develop, when they dis- 

 appear, till the day arrives that they find themselves organised for 

 another existence, when they leave their watery retreat and betake 

 themselves to dry land. "The tadpole, meanwhile being subject to 

 a series of changes in every system of organs concerned in the 

 daily needs of the coming aerial and terrestrial existence, still passes 

 more or less time in water, and supplements the early attempt at 

 respiration by pullulating loops and looplets of capillaries from the 

 branchial vessels.' (Owen.) 



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 it as to a place of safety. To the Proteus and the 

 amphibious Sirens, where the limbs are confined to the pectoral 

 region, swimming is most natural. 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 

 discharged, 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. Ribs there are none. The sternum is highly 

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

 its mesian 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 disc 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 iliac 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, the leg being joined together so as to 

 form a much-elongated single 



