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F. RANSOM—DIATOMS: 
One of the most interesting phenomena connected with the 
natural history of diatoms is the power that many species possess 
of independent movement, a fact which has already been alluded to 
as a source of error to the early observers who placed them in the 
Animal Kingdom. Various theories have been suggested to explain 
the cause of this motion. Nearly all come under one of the three 
following heads—firstly, existence of endosmotic and exosmotic 
currents; secondly, existence of cilia on some part of the frustule; 
and thirdly, existence of some exterior or protruding protoplasm. 
The hypothesis that the movements are caused by osmotic currents, 
originated by Nageli in 1849, was for some time the most popular 
theory, and probably still numbers many adherents. That such 
currents do exist is to be expected, but that they are sufficiently 
powerful and varied to account for these propulsions, is scarcely 
probable. The motion is described by the Kev. William Smith, 
who inclined towards the osmotic theory, in his 1 Synopsis of the 
British Diatomaceee,’ as follows:—“This motion is of a peculiar 
hind, being generally a series of jerks, producing a rectilinear 
movement in one direction, and a return, upon nearly the same 
path, after a few moments’ pause, by another series of isochronal 
impulsions. The movement is evidently of a mechanical nature, 
produced by the operation of a force not depending upon the volition 
of the living organisms. An obstacle in the path is not avoided, but 
pushed aside; or, if it be sufficient to avert the onward course of 
the frustule, the latter is detained for a time equal to that which it 
would have occupied in its forward progression, and then retires 
from the impediment, as if it had accomplished its full course.” 
This is an accurate description of the more regular of these move¬ 
ments, but it is frequently noticeable that considerable digressions 
occur from the rectilinear course without any apparent obstacle 
being encountered. It is also observed that the direction is to 
some extent mechanically regulated by the shape of the frustule, 
as in the crescent-shaped Cymbella , whose course usually describes 
an arc of a circle. Dr. Carpenter in his sixth edition of ‘ The 
Microscope’ still favours the osmotic theory, and endeavours to 
dispel the objection that such motion is not observable in other 
unicellular organisms by stating that there are no other such 
organisms which are so nearly surrounded by an impermeable 
envelope and consequently in which osmose is limited to so small 
a portion of the cell-wall. The fact, however, that no current can 
be detected in the water in proximity to either extremity of the 
moving frustule, indicates that the passage of liquid through the 
cell can exert but very little force. Another objection to both this 
and the ciliary theory is that diatoms appear to be only capable of 
movement when in contact with some solid body. If either of these 
hypotheses were correct, it might be supposed that such contact 
would be an obstruction rather than an assistance. 
The ciliary theory has now been generally abandoned, although it 
has occasional revivals which create some sensation at the time of 
their appearance. The idea was first propounded by Ehrenberg 
