MOTION OF DIATOMS. : 185 
Turning to another part of the slide, I found another free moving 
specimen “with a similar gelatinous mass in contact with it. The 
diatom was moving freely, and towing the mass along with it, at- 
tached to its hinder end. Soon the mass began to move forward 
on the shell, the motion of the diatom ceased and was presently 
reversed, the order of sequence being distinctly as stated. In 
several instances the motion of the gelatinous mass from the rear 
end of the diatom forward, plainly preceded the change in the 
direction of the frustule, as is the change of ciliary motion (assum- 
ing that to be the motive power, for the sake of illustration,) did 
not instantly stay the diatom, but required an appreciable moment 
of time to overcome the momentum. My observation of this shell 
continued for a full hour, the changes of direction being frequent, 
and all the accidental modifications and phases of the phenomena 
were strikingly confirmatory of the existence of some force applied 
along the line of the raphe, acting sometimes in one direction and 
sometimes in another, in such a way as would be fully explained 
by supposing ciliary action along that line, but which do not seem 
to be so easily accounted for by osmotic action, certainly not by 
osmotic action at the ends of the frustule. 
On one or two occasions the acting force did not appear to be 
reversed at the same instant at the two ends of the diatom. Twice 
the foreign matter moved against the current of general motion, 
slowly, it is true, but really in such a way as to indicate that the 
force acting upon it was not in the same line of direction as was 
that exerted on the other half of the frustule. But when the motion 
controlling the gelatinous mass became vigorous, it either became 
dominant or was indicative of harmonious action at both ends of 
the shell, so that the motion of the diatom through the water be- 
came very pronounced and strong. 
I looked for similar phenomena among the other kinds of dia- 
toms in the gathering, but saw nothing of the sort except in the 
instances described. The Navicule were very lively, but I saw no 
examples of action upon foreign matter that came in their way. 
Neither could I detect any current, even along the Nitzschias ; the 
motion of the gelatinous substance occurring only when it came in 
contact with the shell and apparently sticking to it. 
‘My study of the diatom-shell has led me to accept the opinion 
that the raphe is a real fissure in the shell, but in many species it 
is not a simple and vertical linear-opening of the shell. It is more 
like the joint formed by the overlapping of the edges of curved 
tiling on a roof: a thickened line of silica borders one lateral half 
of the shell, while the other half dips under it with a thin film. It 
is true that an osmotic force may be conceived as working along 
the raphe, as well as that a line of cilia should do so; but the diffi- 
culty is to account for such action upon an extraneous mass as that 
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