CILIA. 



629 



ternal gills exhibited the same phenomena, but 

 he could discover nothing of the kind on the 

 internal gills. 



Gruithuisen* observed in the tadpole of the 

 Green Frog that so soon as the circulation of the 

 blood began in any part of the gills, small ob- 

 jects were attracted and repelled from that spot, 

 and that the same took place a few days later 

 on the tail wherever vessels had been formed. 

 lie conceived that the motion of the water was 

 for the purpose of exposing the blood to its in- 

 fluence, and compared it to the current pro- 

 duced by Infusoria by means of cilia. He does 

 not say, however, that he had seen cilia in the 

 tadpole. 



Iluschkef observed that the water in the 

 vicinity of the gills of the young Salamander 

 was thrown into a boiling-like motion, while it 

 flowed steadily at other parts of the body. 



Without being aware of these previous disco- 

 veries, I was led in 1830, by an accidental ob- 

 servation of my own, to go over nearly the same 

 ground.} I had cut off one of the external 

 gills of the tadpole of the Frog, and placed it 

 with a drop of water under the microscope, 

 with the view of measuring the size of the glo- 

 bules of blood that might flow from it, and was 

 astonished to perceive that the globules, on 

 escaping from the cut part of the gill, moved 

 rapidly along its surface towards the points of 

 the branches in a constant and uniform manner. 

 On further inspection it soon became evident 

 that the blood-globules were entirely passive in 

 their motion, and that other light particles 

 brought near the gills were moved in a similar 

 manner ; their motion being manifestly owing 

 to a current produced in the water along the 

 surface of the gill in a determinate direction. 

 A conclusive proof of this was afforded by put- 

 ting the gill which had been cut off, into a 

 watch-glass with a larger quantity of water. It 

 was then seen that when the gill happened to 

 be fixed by any obstacle, small bodies in its 

 vicinity were moved along it as before towards 

 the points of the branches, but when unim- 

 peded the gill itself advanced through the 

 water in a direction contrary to that in which 

 the particles were moved, the trunk being 

 turned forward ; the tendency to produce a 

 current in one direction, thus causing the gill, 

 now no longer fixed, to move in the opposite 

 one. The current began at the root of the gill, 

 and ran along the branches, at the points of 

 which it did not continue its primitive direc- 

 tion, but turned off sideways, and immediately 

 ceased. (See Jig. 309, C). 



I soon found that the gill was not the only 

 part of the animal which excited motion 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, pro- 

 ceeded along the back and belly and the two 



* Salzburg. Medicinisch-Chirurgische Zeitung, 

 1819, ii. p. 447. 



t Isis, 1826, p. 625, (cited in Burdach's Physio- 

 logic, from which I quote, not having seen the ori- 

 ginal.) 



$ Edinb. Med. and Surg. Journal, xxxiv. 



sides, to the tail, along which it continued to its 

 extremity. It was not so strong as that on the 

 gills, but agreed with it in other respects. 



I continued for some time to observe the 

 phenomenon in the larva of the Fro;?, in order 

 to find out whether it underwent any alteration 

 in the progress of the developement of that 

 animal. It is known that after a time the ex- 

 ternal gills become covered by a fold of the 

 skin, and inclosed in the same cavity with the 

 internal gills, when they gradually shrink and 

 at last disappear. On examining the animal 

 while this change was taking place, and for 

 some time after, it appeared that the external 

 gills after their inclosure still retained their 

 peculiar property, and continued to do so as 

 long as any portion of them remained; the 

 current on the body remained the same ; on the 

 tail it acquired a twofold direction diverging 

 from the middle part or continuation of the 

 vertebral column, obliquely upwards and down- 

 wards towards the upper and lower edge. As 

 the animal advanced in growth, the currents 

 gradually disappeared over the greater part of 

 the surface, continuing longest at the posterior 

 part of the body; at length, when the pos- 

 terior extremities were so far advanced in 

 growth that the thigh, leg, and toes could be 

 discerned with a magnifying glass, which was 

 the latest period of observation, the current 

 existed only at the commencement of the tail, 

 and on a small part of the body near the hind 

 leg. The internal gills, though tried in various 

 stages of development, did not exhibit the 

 phenomenon. 



I next sought for the same appearances in 

 the larva of the Newt or Water Salamander, 

 which was first examined a few days after its 

 exclusion from the egg when its gills are very 

 simple. At this period the surface of the 

 animal produces currents agreeing in almost 

 every circumstance with those which take 

 place in the larva of the frog at a correspond- 

 ing stage of its development. Particles of 

 powder diffused in the water are carried along 

 the surface of the body from before back- 

 wards; on the gills they are conveyed along 

 each of the trunks from the root to the ex- 

 tremity. The gills also, when cut off, move 

 through the water with the cut extremity for- 

 wards, in a direction contrary to the currents. 

 I have since found nearly the same phenomena 

 in the gills at a much later period. 



It was evident that the purpose of these 

 currents was to effect a renewal of the water 

 on the respiratory surfaces; respiration in these 

 animals probably being performed not only by 

 means of the gills, but also by the general sur- 

 face of the body. 



It appeared that the power of impelling the 

 water was wholly confined to the external sur- 

 face of the animal; a portion of the skin being 

 raised and detached, floating bodies were 

 moved along its external surface only. Parts 

 cut off from the animal continued to excite 

 currents for several hours after their separation, 

 and the smallest portion produced that effect. 

 In these cases the current always moved in 

 the same direction relatively to the surface of 



