589.61 406 



V 



The Movement of Diatoms 



THIS paper was suggested by a perusal of Robert Lauterbom's 

 " Uutersucliungen tiber Bau, Kernteilung unci Beweguug des 

 Diatomeen." It would be impossible to do justice to a work in 

 which the letterpress occupies 165 quarto pages, within the limits 

 of a single article. The present communication, therefore, will deal 

 mainly with the movement of Diatoms, omitting, for the present, 

 any reference to the interesting chapters on the protoplasm and its 

 inclusions, and on nuclear and cell division — a portion of the work 

 well worth separate treatment. For ready comprehension, however, 

 the subject must be introduced by an account of the structure of 

 the frustule in Pinnularia and Surrirella. 



Structure of the Frustule in Pinnularia major. — The general 

 appearance of Pinnularia is sufficiently well known to render any 

 special description superfluous. The chief features visible in a sur- 

 face view of the frustule, as shown in Plate III., Fig. 7, are (1) the 

 presence of a median longitudinal undulating line, the ' raphe,' with 

 terminal and central nodes, and (2) a double row of transverse 

 markings, the so-called ' striae ' or ' costae.' The true significance 

 of these structures is brought out clearly by Lauterborn. 



The raphe had previously been interpreted by the majority of 

 investigators as an open cleft placing the interior of the cell in 

 direct communication with the surrounding medium, and Pfitzer 

 also sought to justify this view on various grounds. Flogel, however, 

 contended that the cleft was closed on its inner side by a thin 

 membrane. In order to settle this point Lauterborn carefully 

 examined empty frustules of Pinnularia major, and also transverse 

 sections (2-3 fxl) of examples fixed with chromosmium and stained 

 with haematoxylin. One of these sections, from the region between 

 the central mass of protoplasm and the extremity of the cell, is 

 shown iii Fig. 1. The cell- wall exhibits a marked thickening in 

 the neighbourhood of the raphe, which runs as a narrow angular 

 cleft from exterior to interior without any distinguishable trace of 

 an inner limiting membrane. The appearance of this cleft varies 

 considerably according to the position of the section in the series 

 (see Figs. 2 and 3), being at some points simply oblique, at others 

 variously bent, and this irregularity in shape explains why the aspect 

 of the raphe alters with a change of focus. In some sections (Fig. 3) 



