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 
mollusea, 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 Anouwra and Uvrodela, 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 
