84 AMPHIBIA. 



derable quantity of water, which is generally, but errone- 

 ously, supposed to be the urine. This water is limpid and 

 pure, containing no traces of the usual component elements 

 of the urinary secretion. It is contained in a sac, which 

 has also been mistakenly believed to be the urinary blad- 

 der. This is the reservoir to which I have alluded. 

 When, therefore, the Frog is happily placed in a damp 

 atmosphere, or in water, the skin absorbs a quantity of 

 water, which there is every reason to believe is secreted 

 into the bladder just mentioned, where it is kept in store 

 until the dryness of the skin requires a supply for the pur- 

 pose of respiration, when it is again taken up, and restored 

 to the surface by which it had been first absorbed. 



There can be no doubt that this cutaneous respiration is 

 of much more frequent occurrence in animals having naked 

 and moist skin than has hitherto been ascertained ; and 

 it is exceedingly probable that it will be found to take 

 place, not only in all the aquatic amphibia, but in the 

 mollusca, and many others, having integuments of a similar 

 character. 



Allusion having already been made to the metamorpho- 

 sis of the typical Amphibia, I shall here offer a few general 

 remarks upon this most remarkable phenomenon. In the 

 whole of the tribes of Frogs, Toads, and Newts, or, in 

 other words, in the two orders Anoura and Urodela, the 

 young animal, as it emerges from the egg, and for a con- 

 siderable time afterwards, exhibits a form and structure 

 essentially similar to those of fishes. The organs of respi- 

 ration and of circulation differ in no important point from 

 those which exist in that class, and their mode of aquatic 

 progression is absolutely identical. At a certain period 

 of their growth a gradual change takes place in the struc- 

 ture and function of each of these systems, during which 



