8 



in others to some part of the internal organs only. In the animal in 

 question, the common sea-mussel {Mytilus edulis), they principally 

 occupy the gills or branchiae, which are four in number, two on each 

 side, and are easily known by their being brought into view when the 

 shell is opened in the ordinary way, they being situated between the 

 two lobes of the membrane lining the shell, which is termed the man- 

 tle. Each gill is composed of a series of bars placed parallel to each 

 other, like the teeth of an ordinary comb, but with some connecting 

 bars in the shape of small protuberances between them ; besides this, 

 each gill consists of two layers of such bars, which are united together 

 at their edges, but have one layer free at the base, a few small bars 

 placed rather obliquely serve to connect the two layers of each toge- 

 ther. The edges where the union of the two layers takes place, are of 

 a rounded figure, and on these the cilia occur in the greatest abun- 

 dance, they being literally crowded with them on every part, but on 

 the bars there are two rows on each side, one of the rows being much 

 larger than the other, the large ones are more superficial than the 

 others, and their motion is not so rapid as that of the small ones, in 

 the former the beating or lashing motion is very perceptible, whilst 

 in the latter it occurs so rapidly as to present the appearance of a con- 

 tinued series of undulations. It is to certain phenomena connected 

 with the movement of the large cilia that I would direct the attentioi 

 of the Society this evening. 



If one of the rays be placed under the microscope with the larje 

 cilia uppermost, when the movement is nearly stopped, each ciliwi 

 . • will be found (when viewed with a power of about 500 linear) to 



present, besides the usual curved motion in a vertical plane, aro- 

 ther although slight yet important movement on itself, in a directbn 

 nearly at right angles to the preceding, which movement occupies 

 just one quarter of a circle, and is precisely analogous to that of ihe 

 quills in the wings in birds during their flight, or to what we are m#re 

 familiar with, viz., the feathering of the oar in rowing. When we con- 

 sider the fact that the fluid acted on by the movement of the cilit is 

 always propelled in one direction, we are at a loss to imagine how sich 

 could be the case ; if the cilia had only an upward and downward no- 

 tion, why should not the current of water proceed first in one direc- 

 tion and then in the other ? To get over this difficulty, and to accoint 

 for the fact according to this view of the subject, the downward strole, 

 or that which impels the water, must be performed in a much less space 

 of time and with a greater degree of force than that in the backwaid 

 direction, but the problem can be easily solved by allowing that the 



