22 



THE YOUNG NATUEALIST. 



ing the skeletons of a sparrow and a lizard ; such an examination will shew 

 that the bones of the sparrow's wing are the same, bone for bone, as those of 

 the lizard's fore-leg, but drawn backwards and upwards, so as to work in the air. 

 The knee of the sparrow's leg seems to be bent the opposite way to that of the 

 lizard's hind-leg ; but this is only apparent, as what seems at first sight the 

 knee of the sparrow is really its ankle, for the bones of its foot have grown 

 long and leg-like, and it always stands upon its toes, the rest of its foot 

 forming a firm support to hold its body up in the air. The true knee, is, in 

 birds, nearer the body ; and strange as it may seem, some birds occasionally 

 rest the whole of their foot on the ground, presenting a most curious appear- 

 ance. 



Though perhaps the Archegosaurus and some other doubtful fossil forms 

 may present closer links between batrachians and reptiles than any existing 

 forms, yet we are not without links in the existing Chelonians, which of all 

 reptiles are the lowest and the most closely allied to the batrachians. 



The most interesting point to be observed, is the fact that some Chelonians 

 possess delicate internal gills in the pharynx, homologous to the inner gills 

 of tadpoles. Agassiz pointed out the existence of these pharyngeal processes, 

 and careful experiments have borne out the inferences he drew therefrom. 

 A soft-shelled turtle f Aspidonectes spinifer) was confined in a glass aquarium, 

 and completely immersed in the water, and it was noticed that the throat and 

 the floor of the mouth became alternately swollen and collapsed, much like 

 the respiratory motions of a frog in the air. As no air escaped from the 

 turtle, the bulging of the throat and mouth must be caused by rilling the 

 mouth and pharynx with water and expelling it again, while the air must be 

 forced from the lungs to the mouth and back again after being partially 

 aerated. The gradual transformation of gill-breathing batrachians to true 

 air-breathing reptiles, of which the Chelonians seem to give us some idea, is 

 more definately illustrated in the case of the Alpine black salamander [Sal. 

 atra.) This animal, originally a batrachian, and as such laying its eggs in 

 water, has been forced by altered circumstances (gradual elevation of land) to 

 retain its young in its body till they become air-breathers ; yet, if the young 

 be removed prematurely from the body of the mother and put into water they 

 go through all their changes like the common newts. 



Having traced the connection between gill and lung breathers, the next 

 step will be to consider the respiratory and circulatory organs of reptiles and 

 birds. The heart and lungs, in different orders of Sauropsidans present wide 

 differences. In the Chelonians, as has been mentioned, the circulation and 

 respiration are almost batrachian in character. Most reptiles possess a three- 

 chambered heart, and in some, as Ophidia, one lung is merely rudimentary, 



