THE 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 liquid 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 aérial 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 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 ciate 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 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. disc which supports the 
throat, and assists in deglutition. cand respiration. The bone of the 
arm (humerus) i is single, and long in proportion to the fore arm. In 
the Frogs (Raza), 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 joined together so as to 
form a much-elongated single bone. 
