INTRODUCTORY OBSERVATIONS. 87 



anatomical details of the steps by which these changes take 

 place, it is not my intention to enter ; but the general de- 

 velopment of the new organs, both of these functions and 

 of that of progression, will 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 reproductive organs : nor does reproduction ever 

 take place in these animals until the other changes have 

 been perfected. But in the perennibranchiate forms, as the 

 Siren, the Proteus, &c. it seems as if the metamorphosis 

 were stopped suddenly at that period when the lungs begin 

 to be developed, before the branchiae have at all diminished 

 in size or in activity of function. The reproductive organs, 

 however, go on to their full development, 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 indifferently 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 physio- 

 logy 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 in- 

 quirer, few classes of animals present a more extensive or 

 interesting field of investigation. After all that has been 

 done, much remains vet to be ascertained, in the functions 



