18S4-85.] Edinburgh Naturalists' Field Club. 247 



njion as indicative of an animal nature. Ehrenberg has ascribed 

 the motion to the existence of a snail-like foot projecting from the 

 central pore or umbilicus, but no such structure can be detected by 

 the eye. Niigeli, in 1853, denied the existence of special locomo- 

 tive organs, and declared that the motion was due to the attraction 

 and emission of fluids which were necessary for the maintenance of 

 tlie nutritive processes. The fluids in question were supposed to 

 be unequally distributed over the surface, and to be suflicieutly 

 powerful to overcome the resistance of the water. By further sup- 

 posing that one-half of the cell admitted while the other emitted 

 currents, and that a regular alternation of these processes took place 

 at the two halves, the alternate forward and backward movements 

 were explained. Hogg, in 1855, attributed the motions to cilia, 

 while Smith ascribed them not to any external organs, but to 

 exosmotic and endosmotic processes occurring simultaneously, and 

 at the extremities of the frustules. This view has been accepted by 

 Rabenhorst ; but another probable cause had already been pointed 

 out by Wenham, and accepted by Siebold, namely, the undulations 

 of an external membrane, which, however, may not have been 

 clearly recognised by any observer. An ingenious suggestion has 

 recently been made by Engelmann,^ namely, that the unseen gase- 

 ous molecules escaping from the Diatom cell cause the movement, 

 he having found in Bacteria a means of demonstrating under the 

 microscope the evolution of oxygen by the living frustules. O'Hara, 

 in 1882, again accepted the general explanation previously ad- 

 vanced by Wenham ; while still later, 1883, Hogg attributed the 

 movements to coiatractile jjrehensile filaments. In the same year, 

 van Ermengem^ ascribed them to thermo-dynamical, and, perhaps, 

 electro-capillary forces ; while Adams^ has even speci;lated on the 

 occurrence of cilia lining the surface of the enclosed vegetable 

 matter, which might bring about the resiilts observed. This view, 

 however, can hardly be looked upon as tenable, the hypotheses 

 ascribing the movements to cilia, to osmotic or other physical pro- 

 cesses, to the undulations of an external membrane, or to the 

 escape of gaseous molecules, being much more probable. 



The velocity of the movements of Diatoms varies very greatty. 

 Some advance at the rate of about one-third of an inch per minute, 

 while others do not pass over more than one fifty -fifth of an inch in 

 the same space of time. Between these numbers many other 

 speeds have been recorded. 



The methods of multiplication hitherto observed among Diatoms 

 are (1) fissiparous division or temnogensis ; (2) conjugation ; and 



1 'Bot. Zeit.,' 1881, p. Ml e< seq. 



^ 'Bull. Soc. Belg. Micr.,' vol. ix. pp. 41-43. 



' ' Amer. Month. Micr. Jour.,' vol. iv. p. 59. 



