324 THE VITAL FUNCTIONS. 



trace of their former existence than the original 

 division of the arterial trunks, which had sup- 

 plied them with blood directly from the heart, 

 but which, now uniting in the back, form the 

 descending aorta.* 



There is a small family, called the Perenni- 

 branchia, belonging to this class, which, instead 

 of undergoing all the changes I have been des- 

 cribing, present, during their whole lives, a great 

 similitude to the first stage of the tadpole. This 

 is the case with the Axolotl, the Proteus angui- 

 7ms, the Siren lacertina, and the Menohraiichus 

 lateralis, which permanently retain their external 

 gills, while at the same time they possess imper- 

 fectly developed lungs. It would therefore seem 

 as if, in these animals, the progress of develope- 

 ment had been arrested at an early stage, so that 

 their adult state corresponds to the larva condi- 

 tion of the frog-t 



In all warm blooded animals respiration be- 

 comes a function of much greater importance, 



* See Fig. 357, p. 274. 



f GeofFroy St. Hildire thinks there is ground for believing that 

 Crocodiles and Turtles possess, in addition to the ordinary pul- 

 monary respiration, a partial aquatic abdominal respiration, 

 effected by means of the two channels of communication which 

 have been found to exist between the cavity of the abdomen and 

 the external surface of the body : and also that some analogy 

 may be traced between this aquatic respiration in reptiles, by 

 \\\Q%Q peritoneal canals, and the supposed function of the swim- 

 ming bladder of fishes, in subserviency to a species of aerial res- 

 piration. 



