398 
POPULAR SCIENCE REVIEW. 
some of its “ droppings ” under the microscope, is certain to 
observe some forms of Navicula, Nitzscliia, or Synedra, ex- 
hibiting these movements. The motion is of a peculiar kind, 
a slow, regular advance in a straight line ; a little hesitation ; 
then another slow, rectilinear movement, followed perhaps by 
another; then a few moments* pause, and a return upon nearly 
the same path by similar slow, rectilinear, recurrent movement. 
It is particularly noticeable that an obstacle in the path does 
not affect the Diatom, which does not shrink from, or avoid, 
foreign bodies in any way. There is not the slightest evidence 
that the contact with a foreign body affects the nature of the 
movement, as it nearly invariably does in the lowest Protozoa. 
Either the obstacle is pushed on one side, by the motor power 
of the Diatom, or, should it be too large for this, the Diatom 
remains in apposition with it for as long a space of time as 
would have been occupied in its forward movement, had that 
been unopposed. The rate of these movements in the Diatoms 
is slow when compared with some of the rapid, darting evo- 
lutions of Infusoria ; but, as compared with the rate of 
crawling movements in animals, it is exceedingly rapid. Some 
of the most rapid Diatoms are estimated by the Rev. W. Smith 
to move about four hundred times their own length in three 
minutes ; that is to say, a Diatom of an inch in length 
moves more than the yto of an inch in a second. That this is 
a very rapid movement, if considered as a crawling movement, is 
apparent from the fact, that a snail moving at the same propor- 
tionate rate would accomplish the distance of a mile in two hours. 
No organ capable of producing these movements of the 
siliceous frustule are apparent to the observer on his first 
inspection, and many varied hypotheses have been advanced 
as to their cause, based frequently upon the most careful 
observations, though more often on erroneous ones. 
The various explanations offered come all under one of three 
heads, viz., 1st, the existence of endosmotic and exosmotic 
currents; 2nd, the existence of cilia on some part of the 
frustul%; 3rd, the existence of a snail-like foot, external to 
the frustule. 
Nageli, in 1849, was the originator of the first of these 
hypotheses, and his explanation was adopted by Yon Siebold 
and our chief English authority, the Rev. William Smith. 
Nageli says : — 
The cells have no special organs for these movements. But as in con- 
sequence of their nutritive processes they both take in and give out fluid 
matters, the cells necessarily move when the attraction and the emission of 
the fluids is unequally distributed on parts of the surface, and is so active as 
to overcome the resistance of the water. 
Yon Siebold demonstrated the existence of currents on the 
