ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 268 



contracts at will ; and performs as is well known many very active 

 movements. Take another example from amongst the simpler forms of 

 life — cryptogamic plants, whose spores, possessing contractile fila- 

 ments, have considerable powers of extension and contraction, and are 

 otherwise employed as motile agents. We may then without further 

 argument assume that diatoms are furnished with a somewhat similar 

 agency, whereby their movements are effected. . . . 



Somewhat conclusive evidence of the prehensile contractile fila- 

 mentous theory is, I believe, furnished by the addition of a minute 

 quantity of colouring matter to the contents of the cell in which the 

 diatoms are confined. When a particle or two of colouring matter 

 comes within reach of a filament, it is seized upon, and follows the 

 subsequent movements of the diatom. Occasionally a coloured par- 

 ticle will be seen to be affected in the following manner. At a point 

 equal to the length of the frustule, it is grasped en passant ; or it may 

 be seized at some intermediary distance of the extreme limit of the 

 prehensile filament, when it is instantly drawn towards the frustule 

 with a jerky motion, and secured. On more than one occasion, a cell 

 of Palmogloea was seized in the way described, and seen to travel 

 along the longitudinal sutural aspect of the valve, and in a contrary 

 direction to that of progression, the progress of the frustule being at 

 the moment perceptibly slower and somewhat more jerky. When not 

 so engaged it appeared to be occupied in securing points d'appui on 

 the slide and cover-glass. Any and every movement, however, must 

 be performed at some disadvantage in so confined a space as that 

 afforded by a very shallow cell : a succession of normal movements is 

 scarcely possible under the circumstances. . . . 



In once again entering upon the investigations of the movements 

 of diatoms, I have been anxious to divest myself of all preconceived 

 opinions, of either cilia or other organs, and having obtained during 

 the past summer a bountiful supply of lively specimens, I at once 

 made them the subject of careful and prolonged study. Portions 

 taken from the bulk, together with cells of Palmoglcea, were trans- 

 ferred to a growing-slide, the last named being arranged in groups of 

 from two to a dozen or more primary cells. By expansion of the 

 transparent outer membrane these cells soon displayed the phenomenon 

 of self-division; but neither in the compound state nor as single 

 cells did any of them exhibit any other kind of motivity. ^ The 

 diatoms, on the contrary, were incessantly on the move, passing to 

 and fro, over the field of the Microscope. Now and again they would 

 seize upon a Palmogloea and carry it off. Any retraction of the con- 

 tractile filament of the diatom produced a sudden jerky movement of 

 the cell ; but as soon as it relinquished its hold it returned to a state 

 of perfect rest. Occasionally a fresh attack was made and the cell 

 was seen to follow in the track of the diatom. The movements were 

 so remarkable, that no one observing them could, I venture to think, 

 refer them to other motile organs than that of prehensile filaments or 

 other voluntary contractile bodies under the perfect control of the 

 frustule. 



At another time a minute organism belonging to a different genus 

 would dart across the path of a diatom or come into contact with it, 



