INTRODUCTORY OBSERVATIONS. 79 



surface in every modification should be liumid ; for as soon as 

 it becomes dry, its function ceases, and tlie animal speedily 

 dies. But as the Frog, for instance, is frequently exposed 

 to a dry atmosphere, it is essential that there should be some 

 provision made for a constant supply of moisture to the 

 skin, which has just been shown to be a respiratory surface. 

 This is effected precisely as in other surfaces which perform 

 this function, — namely, by a secretion of fluid from the sur- 

 face itself. The extent of the skin is, however, so great that 

 the whole internal moisture of the animal would speedily be 

 exhausted, unless a reservoir were provided for an extraor- 

 dinary demand ; and I now proceed to show what this reser- 

 voir is, and by what means it is replenished. Every one 

 knows that when a Frog is hastily seized, or even quickly pur- 

 sued, it voids a considerable quantity of water, which is gene- 

 rally, but erroneously, supposed to be the urine. This water 

 is limpid and pure, containing no traces of the usual compo- 

 nent elements of the urinar}'' secretion. It is contained in a 

 sac, which has also been mistakenly believed to be the urinary 

 bladder. This is the reservoir to which I have alluded 

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

 mosphere, 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 purpose of respi- 

 ration, when it is again taken up, and restored to the sur- 

 face 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 ex- 

 ceedingly 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 metamorphosis 



