190 
Proceedings of Royal Society of Edinburgh, [march 5, 
margin and the posterior end, which will at least counteract the 
marginal current, so that the turning round of the posterior end will 
not take place. When the posterior end turns round, as it occasionally 
does, we might infer that for some reason or other the intra-lam ellar 
current was not acting. Thus, combined with the translatory there 
will he a rotatory movement, and the anterior end will move 
quickest, making the posterior end relatively the pivot. 
The current produced moves at the rate of 2 inches per minute. 
The problem of motion here is not quite so simple as it may appear 
to some. 
To account for the currents is comparatively easy. If the gill in 
its natural position be conceived of as a boat moored to the shore 
(the body), and the cilia as so many rowers with their oars, then the 
current is simply due to* the successive sweeping of the oars through 
the water, while the boat to which they belong is stationary, and 
when the gill is detached it is simply a case of the boat let loose 
from- its moorings and free to move. We assume here, and we are 
justified in the assumption, that the ordinary motile functions of the 
gill continue after detachment as before. Further, it is easy to 
account for the two kinds of movement — translatory and rotatory — 
depending as they do on the mode of distribution, the number and 
the power of the cilia. 
If the boat were regularly manned, and the rowers equally dis- 
tributed and of equal strength, then the movement would be straight 
forward or translatory, but if unequally distributed, there would be 
a rotatio-n upon the Aveak side — a turning round of the boat. 
So far the matter is simple, but when we consider that the cilia, 
although they perform mechanical work, do not do their work 
mechanically (and in this respect they resemble oarsmen), that they 
can move without causing visible motion, and that they change 
their direction, then the problem becomes rather complicated. How- 
ever the normal direction of the currents is as enumerated above, 
and the normal direction of movement ought to be capable of mathe- 
matical expression. 
The movement of the entire mantle-lobe is rotatory, as cilia are 
found on one side only, and the posterior end remaining nearly 
stationary acts as a pivot, the direction of rotation always being 
towards the attached margin or inwards. 
