52 GENERAL HISTORY OF THE INFUSORIA. 



sarily move when the attraction and the emission of the fluids is unequally 

 distributed on parts of the surface, and is so active as to overcome the 

 resistance of the water. This motion, consequently, is observed more particu- 

 larly in those cells which, in consequence of their taper form, easily pass 

 through the water ; these cells, moreover, move only in the direction of theii' 

 long axis. If one half of a spindle-shaped or ellipsoidal cell chiefly or en- 

 tirely admits material, the other half, on the contrary, giving it out, the cell 

 moves towards the side where the admission takes place. But as in these 

 cells both halves are physiologically and morphologically exactly alike, so it 

 is that it is first the one and then the other half which admits or emits, and 

 consequently the cell moves sometimes in one, at other times in the opposite 

 direction." 



In our apprehension, this mechanical interpretation of the phenomenon is 

 not sufficient ; the alternate reception and discharge of fluid matters by each 

 opposite half requires an effort of imagination, to conceive, unwarranted by 

 analogy. We shall, however, presently see that Prof. Smith gives the pre- 

 ference to this supposition, amid the many conflicting fancies of authors and 

 the obscurity of the question. 



Encouraged by apparent success in discovering cilia on the fronds of Des- 

 midieae, Mr. Jabez Hogg searched for them on Diatomeae, and tells us {J. M. S. 



1855, p. 235) that he has repeatedly satisfied himseK that their motive power 

 is derived from ciha arranged around openings at either end, — ^in some also 

 around the central openings, which, mth those ciha at the ends, act as 

 paddles or propellers. He, moreover, states his impression that the frustules 

 have a degree of volition sufficient '' to move along and to steer their coiu'se ; 

 for intervals of rest and motion are most clearly to be distinguished." To 

 this behef in cilia on the frustules of Diatoms, Mr. Wenham is as determined 

 an opponent as he is to the Hke hy[Dothesis respecting the Desmidieae (J. M. S. 



1856, p. 159), and he offers the folloTving speculations on the cause of the 

 movements : — " If caused by the action of ciha, such extremely rapid impulses 

 would be required to propel the comparatively large body through the water, 

 that surrounding particles would be jerked away far and Tvdde ; a similar 

 effect would be observed if the propulsion were caused by the reaction of a jet 

 of water, which, according to knovni laws of hydrodynamics, must neces- 

 sarily be ejected with a rapidity sufficient to indicate the existence of the 

 current a long distance astern. I consider that there is no ground for 

 assuming the motions of the Diatomaceae to be due to either of these causes. 

 They are ui'ged forward through a mass of sediment without displacing any 

 other particles than those they immediately come in contact with, and quietly 

 thnist aside heavy obstacles directly in their way, with a slow but decided 

 mechanical power, apparently only to be obtained from an abutment against 

 a sohd body. In studying the motions of the Diatomeae, I have frequently 

 seen one get into a position such as to become either supported or jammed 

 endways between two obstacles. In this case, particles in contact with the 

 sides are carried up and down from the extreme ends with a jerking move- 

 ment and a strange tendency to adherence, the Diatom seeming unwilling 

 to part with the captured particle. Under these circumstances I have dis- 

 tinctly perceived the undulating movement of an exterior membrane ; whether 

 this envelopes the whole sui^face of the silicious valves I am not able to 

 determine, nor do I know if the existence of such a membrane has yet been 

 recognized. The movement that I refer to occupied the place at the junction 

 of the two valves, and is caused by the imdulation of what is known as the 

 * connecting membrane.' This will account for the progressive motion of 

 the Diatomeae, which is performed in a manner analogous to that of the 



