I 



Dr. Sharpey o» the Mechanism of Respiration. 335 



are now known to be organs of respiration, and are called the exter- 

 nal gills, to distinguish them from the internal gills, by which the 

 animal respires at a later period. Of these appendages there are 

 three on each side, the tAvo anterior of which are larger than the 

 posterior, and consist each of five or six branches diverging from a 

 common trank, which is attached to the body of the animal. The 

 blood circulates through them in a single stream, which passes out- 

 wards to the extremities, where it is bent back, and returns in a 

 contrary direction. 



Being desirous of ascertaining the dimensions of the globules of 

 the blowl at this period, I happened to cut off one of the external 

 gills as the readiest mode of attaining my object, and laid it on a 

 glass micrometer with a drop of water. On viewing it then with 

 a lens, my attention was attracted by a very singular and unlooked- 

 for appearance ; the globules of blood which had escaped from the 

 cut part of the gill were moved rapidly along its surface towards 

 the points of the branches in a constant and uniform manner. On 

 further inspection I soon satisfied myself that the blood globules 

 were entirely passive in this phenomenon, and that other light par- 

 ticles brought near to the surface of the gill were moved in a simi- 

 lar manner ; their motion being manifestly owing to a current pro- 

 duced in the water along the surface of the gill, in a determinate 

 direction. A conclusive proof of this was afforded by putting the 

 gill which had been cut off, into a watch-glass with a little water. 

 Here, when it happened to be fixed against any obstacle, small 

 bodies in its vicinity were moved along it as before towards the ex- 

 tremity of the branches ; but when unimpeded, the gill itself ad- 

 vanced through the water in a direction contrary to that in which 

 the particles were moved, the trunk being turned forwards ; the 

 tendency to produce a current in one direetion thus causing the 

 gill, now no longer fixed, to move in an opposite one. The current 

 commences at the root of the gill and riins along the branches, at 

 the points of which it does not continue its primitive direction, but 

 is turned off sideways and immediately ceases. 

 ' > I'soon found that the gill was not the only part of the animal 

 which excited motions in the water. Nearly the whole surface of 

 the' body produced the same effect. A general current commenced 

 on the fore part of the head, proceeded along the back and belly 

 and the two sides, to the tail, along which it then continued to the 

 extreitiity. It was not so strong as that on the gills, but agreed 

 with it in other respects. 



The power of giving rise to the currents, whatever may be its 

 nature, is confined entirely to the external surface of the animal ; 

 portions of the skin being removed and put into water in which a 

 powder Avas diffused, the particles of powder were moved along the 

 external surface only. Parts detached from the animal continue 

 to excite currents for several hours after their separation, and the 

 smallest portion produces this effect. In these cases the current 



