9 



cilia are broader in one direction than in the other, and by giving to 

 each of them a motion similar to that of the feathering of an oar, 

 which has been before alluded to, and which I have found to be really 

 the case. 



To see this feathering movement in the most satisfactory manner, 

 the cilia must have become nearly quiescent ; and if that portion 

 which is attached to the gill-ray, and which, for the sake of distinc- 

 tion, may be termed the root, be carefully examined, it will be found 

 to make a circular movement whilst the filament itself is being bent up- 

 wards or downwards, the cilia also become alternately light and dark, 

 in consequence of their being of a flattened figure, and the circular 

 movement causes them to present at one time the broad surface, and at 

 another the edge. The root of each cilium is of a globular figure, and 

 Ehrenberg supposes that minute muscles are attached to this part, 

 which give to it all its motions. In a preceding portion of the paper 

 it was stated that there were two rows of cilia on each of the bars, 

 the one much larger than the other, and differing in their motions, the 

 one up and down in a vertical plane, the other quick and undulatory ; 

 and it would appear probable that the large cilia, by their motions, 

 may cause the currents to and from the shell, whilst the smaller ones 

 may be for the purpose of keeping in motion all the fluid which had 

 entered between the laminae of the gills. 



In conclusion I would beg leave to state that at all times it requires 

 some little care to get a satisfactory view of the motions above de- 

 scribed ; an hour nearly should elapse from the time the portion of 

 gill is cut off until it be examined, and then the observer can hardly 

 fail in making them out. 



Having examined all the books I could think of which relate to the 

 subject of ciliary movement, I have found that no notice has been 

 taken of it, and deeming the subject of sufficient interest, I was indu- 

 ced to lay the same before the Society this evening. 



