THE RARE SPIRAL DIATOM AND ITS OCCURRENCE. 185
This leads, however, directly to the much disputed question
as to how the movement of motile diatoms is brought about.
From many observations I have long been convinced that active
movement, and this is the movement usually intended when
speaking of diatom motion, is only possible when the raphe is
in contact with the substratum or some other relatively firm
material and I think the rotating motion of Cylindrotheca, taken
in connection with the spiral course of its canal-raphes, very
much strengthens that view.
Exactly what it is that produces the movement in diatoms
is not yet clear, but, as is now generally admitted, it looks as if
it must be the flowing of some slightly sticky substance, possibly
protoplasm, along the open channel known to exist in the raphe.
Probably this flowing substance does not project very much,
if at all, beyond the level of the raphe or, if it does, it is
so perfectly hyaline that it is invisible. That it exists, however,
seems certain from the way in which extraneous particles
of various kinds can often be seen to be held as it were by
an invisible hand and to travel along the raphe backwards
and forwards, or even twisting round, when the diatom itself
is motionless, or they may actually be seen going along the
raphe in the same direction as the diatom when the latter
is in motion. The same action can be seen in Cylindrotheca,
but in this case the particles take a curved path over the
surface of the frustule and occasionally I have seen them go
completely round following the course of the canal-raphes.
Whether motile diatoms can move when freely suspended in
water is, in my opinion, still a moot point. It may, of course,
be possible that the moving protoplasm, or whatever it is in the
raphe, acts frictionally on the water with which it is in contact
and thus brings about some, but, I should imagine, very slow
motion.
The next point to be considered in connection with Cylin-
drotheca is its extraordinary flexibility. This is due no doubt
to the very feeble silicification of its cell-wall. Diatoms generally
appear to be enclosed in very strong and rigid siliceous cases as
may be inferred from the way in which they withstand the drastic
treatment of "cleaning" and mounting as microscopical objects.
But there are some which have so little silica in their cases that
they disintegrate or even disappear altogether under such treat-
ment and one of these is Cylindrotheca (Fig. 3). One proof
of its flexibility is shown by the way in which it can be bent into
a crescent when rolled about in a live-box (Fig. 5). A more
striking illustration of this character, however, is sometimes to
be seen when the forward slender part of a moving specimen
bends, upon coming in contact with a resisting object, even to