O’Mrara—Report on the Irish Diatomacee. 237 
the rapid motion by which it is characterised, may have insensibly 
inclined succeeding observers to assign to the Diatomaccee generally a 
place in the Animal Kingdom. This theory is supported by the 
authority of the illustrious Ehrenberg, who regarded the numerous 
globules noticeable in the cells as so many stomachs, and therefore 
gave to a group embracing these and other forms the general designa- 
tion of Polygastrice. But notwithstanding the deference justly due 
to so great an authority, more recent observers are, 1 may say, 
unanimously of opinion that the Diatomacese belong to the Vegetable 
Kingdom—an opinion sustained by the analogy which the forms of 
this. group exhibit as regards their general structure, and more 
especially by the mode of reproduction which they possess in common 
with other organisms generally regarded as vegetable. 
The Motion of the Diatomacee. 
One of the first phenomena which attracts the notice of the students 
of the Diatomacese is the extraordinary power of motion with which 
the frustules are endowed. To account for this motion, various 
theories have been suggested, reducible to two general classes. By 
some it has been supposed that in the process of imbibing water con- 
taining nourishment and expelling what is superfluous, currents are 
produced which haye the effect of propelling the frustules backwards 
and forwards through the water. As concerns this hypothesis, I quite 
concur with the opinion expressed by Ralfs, that it should be regarded 
rather as a figment of the imagination than founded on the observation 
of facts. Others have suggested that the frustules are furnished with 
special organs of locomotion. The occurrence of hair-like processes 
on the frustules has afforded a colourable reason for such a statement ; 
they are, however, only occasional, and have the appearance of 
parasitic growth, rather than of normal organs of the plant. Ehren- 
berg conceived that a pedal organ was extruded from what he regarded 
as an orifice in the centre of the valve: but so far from the exis- 
tence of such an organ haying been satisfactorily sustained, the fact 
that what that eminent observer, as well as others of deservedly high 
reputation, considered to be an opening, is now generally regarded as 
a thickening of the silicious plate, is fatal to the theory. So while 
the motion of the Diatomacez continues to excite attention, it must be 
confessed that the mechanical agency by which the motion is effected 
remains unexplained. 
Structure of the Cell. 
There is one remarkable feature in the structure of the Dia- 
tomacex which distinguishes them from cognate organisms, that is, the 
fact that the cell is invested with a silicious covering, consisting 
of two distinct plates, more or less parallel to one another, and held 
together by a rim or hoop. This silicious covering has been appro- 
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