740 Proceedings of Royal Society of Edinburgh. 
yet the cilia themselves, when examined under the microscope, 
would be in active motion. The cilia in themselves are thus not 
the cause of movement ; there has to he co-operation or co-ordination 
of some sort before the ciliary motion can give rise to movement of 
the part bearing the cilia. There are, therefore, two motions con- 
nected with cilia to be distinguished — one, the ordinary so-called 
“ ciliary motion,” which creates currents in the liquid, and keeps up 
a constant stream ; another, which may be called ciliary motive- 
power, which is sufficiently powerful to move the cilia-bearing mass. 
The practical bearing of the distinction is evident in the investiga- 
tion of the action of drugs, &c., upon ciliary movement. It will be 
necessary in future, not only to determine what arrests ciliary 
motion, but also what affects their motive power, apart from their 
own proper movement. 
The absolute and relative rate of movement of the detached parts 
is a subject replete with interest, and the movements of the gill 
of the sea-mussel, slow as they may seem when compared with the 
dashing Infusorian, can actually hold its own; but, as pointed out by 
JSTageli, quoted by Sachs in his Lectures on the Physical Plants — 
c Whether the movements of a body appears to us rapid or slow, 
however, depends also on the relation between its size and the space 
passed over in a definite time. If an elephant and a mouse travel 
an equal distance in the same time, we call the first slow, the second 
quick. A man in walking passes over somewhat more than half 
his length in one second. The most . rapid swarm-cell travels, in 
the same time, a distance which is 2J times as great as its diameter. 
Judged by this standard, the gill of Mytilus only traverses its own 
height in about a minute, and so far is relatively slow. 
In both the Odontophora and the Lamellibranchiata the cilia on 
the velum cause a rotation of the embryo within the egg-capsule, but 
it is a curious fact that the cilia do not always act. Thus on the 
development of Sepia, it is noted that “ the whole embryo now be- 
comes ciliated, though the ciliation does not cause the usual rotation; 
while, in such a closely allied form as Loligo, it does occur. Loligo 
differs mainly from Sepia in the early enclosure of the yolk by the 
blastoderm and in the embryo exhibiting the phenomena of rotation 
within the egg-capsule so characteristic of other Mollusca.* The 
* Balfour’s Embryology , p. 247. 
