O'Meara — Report on the Irish Biatomacecv. 237 



the rapid motion by which it is characterised, may have insensibly 

 inclined succeeding observers to assign to the Diatomacefe generally a 

 place in the Animal Kingdom. This theory is supported by the 

 authority of the illustrious Ehrenbcrg, 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 Polygastricae. But notwithstanding the deference justly due 

 to so great an authority, more recent observers arc, I may say, 

 unanimously of opinion that the Diatomaceae belong to the Vegetable 

 Kingdom — an opinion sustained by the analogy which the fonns 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 tlie Biatomacece. 



One of the first phenomena which attracts the notice of the students 

 of the Diatomaceae 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 arc 

 produced which have 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 ha\'ing been satisfactorily sustaiued, 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 Diatomaceae 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- 

 tomaceae 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- 



2 K2 



