Each of these had a
body shaped like a pear, the small end being slightly incurved and
produced into a long curved filament, or _cilium_, of extreme tenuity.
Behind this, from the concave side of the incurvation, proceeded another
long cilium, so delicate as to be discernible only by the use of the
highest powers and careful management of the light. In the centre of the
pear-shaped body a clear round space could occasionally be discerned, but
not always; and careful watching showed that this clear vacuity appeared
gradually, and then shut up and disappeared suddenly, at regular
intervals. Such a structure is of common occurrence among the lowest
plants and animals, and is known as a _contractile vacuole_.

The little creature thus described sometimes propelled itself with great
activity, with a curious rolling motion, by the lashing of the front
cilium, while the second cilium trailed behind; sometimes it anchored
itself by the hinder cilium and was spun round by the working of the
other, its motions resembling those of an anchor buoy in a heavy sea.
Sometimes, when two were in full career towards one another, each would
appear dexterously to get out of the other's way; sometimes a crowd would
assemble and jostle one another, with as much semblance of individual
effort as a spectator on the Grands Mulets might observe with a telescope
among the specks representing men in the valley of Chamounix.

The spectacle, though always surprising, was not new to me. So my reply
to the question put to me was, that these organisms were what biologists
call _Monads_, and though they might be animals, it was also possible
that they might, like the _Bacteria_, be plants. My friend received my
verdict with an expression which showed a sad want of respect for
authority. He would as soon believe that a sheep was a plant. Naturally
piqued by this want of faith, I have thought a good deal over the matter;
and, as I still rest in the lame conclusion I originally expressed, and
must even now confess that I cannot certainly say whether this creature
is an animal or a plant, I think it may be well to state the grounds of
my hesitation at length. But, in the first place, in order that I may
conveniently distinguish this "Monad" from the multitude of other things
which go by the same designation, I must give it a name of its own. I
think (though, for reasons which need not be stated at present, I am not
quite sure) that it is identical with the s