336 Dr. Sharpey on the Mecha,nism of Hespiration 



moves always in the same direotion with respect to the surface of 

 the detached parts, as it had done previously to their separation. 



I continued for some time to observe this phenomenon in the 

 larva of the frog, in order to find out whether it underwent any 

 alteration in the progress of the development of that animal. It is 

 known that after a time the external gills become covered by a fold 

 of the skin, and inclosed in the same cavity with the internal gills, 

 where they gradually shrink and ultimately disappear. When this 

 change is effected, the animal respires by the internal gills, the wa-y 

 ter entering the branchial cavity at the nostrils, and escaping from 

 it by an opening on the left side. On examining it while this change 

 was taking place, and for some time after, I found that the power 

 of exciting motions in the water underwent little or no alteration. 

 The external gUls, after their inclosure, still retained their pecu-. 

 liar property, and continued to do so as long as any trace of them 

 was visible ; the current on the body remained the same ; on the 

 tail it had acquired a twofold direction, diverging from its middle 

 part, or the continuation of the vertebral column, obliquely up-, 

 wards and downwards towards the upper and lower edge. As the 

 animal advanced in growth, however, the currents gradually disap-^ 

 peared over the greater part of the surface, continuing longest at 

 the posterior and lateral part of the body ; at length, when the pos- 

 terior extremities were so far advanced that the thigh and leg, and, 

 the division of the foot into toes, could be discerned with a magni- 

 fying-glass, which is the latest period I have made any observation, 

 the current existed only at the commencement of the tail, and on 

 a small part of the surface of the body adjacent to the hind leg. 



The internal gills, though tried in various stages of development, 

 never produced any current. 



Whatever might be the cause which more immediately gave rise 

 to these currents, it appeared to me that the purpose for which 

 they were intended, was to effect a renewal of the water on the 

 respiratory surfaces ; respiration being performed in this animal 

 not only by means of the gills, but very probably also by the gene- 

 ral surface of the body. It was, therefore, not unreasonable to 

 presume, that similar phenomena might be exhibited by the larv8& 

 of other Batrachia, by the Proteus, Siren, &c. and perhaps other: 

 tribes of aquatic animals, particularly those in which the respira-. 

 tory organ was unprovided with a muscular apparatus, capable of 

 causing a renewal of the water on its surface. I have, according-, 

 ly, found currents in the larva of the newt, in the Mollusca and 

 other invertebrate animals, and to a greater extent, indeed, than I 

 at first anticipated. 



I first examined the larva of the newt or water salamander, a 

 few days after its exclusion from the egg. The external gills at 

 that time consist of three appendages on each side, the posterior 

 and middle of which have each two short lateral processes, the 

 commencement of branches. The anterior extremities appear as a. 



