8^ AMPHIBIA. 



moved by absorption, and the developement of the little ru- 

 dimentary air sac, so exactly analogiis to the air-bag of fishes, 

 into the most beautiful cellular lungs, are changes which, but 

 for the ocular demonstration to which they are yearly sub- 

 mitted, could scarcely be believed. Into the anatomical 

 details of the steps by which these changes take place, it is 

 not my intention to enter ; but the general developement of 

 the new organs, both of these functions and of that of pro- 

 gression, Avill be given in the account of each different form. 



We find then that these typical forms of the Amphibia 

 become essentially altered during the progress of their growth 

 in all their principal systems of organization ; in the nervous, 

 the circulating, the respiratory, the digestive, and the repro- 

 ductive organs : nor does reproduction ever take place in 

 these animals until the other changes have been perfected. 

 But in the perennibrancliiate forms, as the Siren, the Pro- 

 teus, &:c. it seems as if the metamorphosis were stopped sud- 

 denly at that period when the lungs begin to be developed, 

 before the branchiae have at all diminished in size or in ac- 

 tivity of function. The reproductive organs, however, go on 

 to their full developement, and the animals never undergo 

 any further change of form or habit, but continue throughout 

 life to breathe both the atmosphere by their air cells, and 

 water by their branchiae, as well as either medium indifferent- 

 ly by means of their skin. In a word, the pulmonary, the 

 branchial, and the cutaneous modes of respiration are in these 

 curious animals going on simultaneously, although there can 

 be no doubt that the branchial is the most essential to their 

 well being, and the pulmonary the least so. 



Enough has probably been said on the general physiology 

 of these animals , for it would not consist with the object of 

 this work to enter more minutely into the details of this part 

 of the subject ; but to the physiological enquirer, few classes 

 of animals present a more extensive or interesting field of 



