.324 THE VITAL FUNCTIONS. 



leave no other trace of their former existence 

 than the original division of the arterial trunks, 

 which had supplied 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- 

 nus, the Siren lacertina, and the Menohranchus 

 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 by nature at an early 

 stage, so that their adult state corresponds to the 

 larva condition of the frog-t 



In all warm blooded animals respiration be- 

 comes a function of much greater importance, 



* See Fig. 357, p. 274. 



t Geoffroy St. Hilaire 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 

 these peritoneal canals, and the supposed function of the swim- 

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

 piration. 



