ON THE MOVEMENTS OF THE DIATOMACE. 825 
Several facts which came under my own observation last summer 
while observing the motions of the Pinnularia nobilis, one of the 
largest of our fresh-water diatoms, have convinced me that Professor 
Smith is mistaken in the cause he assigns for these movements. If he 
is correct in his supposition that they are owing to the imbibing and 
ejection of fluid alternately at either end of the valve, then their mo- 
tion must invariably be the same, never varying, advancing and re- 
treating motion; but this is not so, I have repeatedly seen a diatom, 
when met by an obstacle in its path, suddenly change its course by 
a quick lateral motion, and go off in a direction quite different to that 
it was formerly pursuing, and frequently have I seen this done when 
there was nothing apparently to cause it, but as if from mere caprice 
the course had been changed. 
On one occasion, I was fortunate enough to get a large, beautiful 
Pinularia in the centre of the field of view, and just beside it was a 
small piece of decayed vegetable matter. As the diatom moved 
along, this substance, instead of remaining stationary, or being 
carried along with the frustule in its forward motion, as would be the 
case were Professor Smith’s theory correct, was propelled in the 
opposite direction, in a manner precisely similar to what it would 
have been, had it stood beside a ciliated infusorial Animalcule instead 
of a diatom. Its motion, however, was not regular, at least not as 
regular as that of the diatom, but somewhat intermittent, as if the 
repelling force to which it was subject was stronger in some places 
than at others, which fact seems to confirm the idea entertained by 
Mr. Hogg, that the cilia are not placed all along the valve, but at 
intervals. When this substance reached the end of the diatom, the 
rapidity of its motion increased, as if the force applied to it had sud- 
denly become greater, or was more directly applied, and at a short 
distance from the valve, all its motion ceased. On the return journey 
of the diatom, the same process was repeated, the small body begin- 
ning to move before it came into contact with the diatom, and 
continuing its course as before, only in the contrary direction. 
On another occasion, while a frustule of the Pinnularia acuta was 
traversing the field of view, it came in contact with a valve of Cocco- 
nema, lying directly across its path. Striking it fair in the centre, 
it passed partly over the obstruction for about one-third of its own 
length, as represented in the diagram, and then stopped as if it had 
got stuck between the diatom it was thus attempting to pass over, 
