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growths. Independently of the correctness of Ehrenberg's 
views, ciliary action has been assigned as the cause of move- 
ment in Diatoms. Mr. Jabez Hogg, in the Journal of Micro- 
scopical Scien-ce, 1855, p. 285, tells ns that he has repeatedly 
satisfied himself that the motive power is derived from cilia 
arranged around openings at either end — in some also around 
central openings, — which, with the terminal cilia, act as 
paddles or propellers. I have copied Mr. Hogg's figure in the 
plate as an interesting and instructive warning to microscopical 
observers, for Mr. Wenham (whose knowledge of and famili- 
arity with the optical principles involved in the use of the 
microscope is unrivalled) has clearly shown that Mr. Hogg's 
“ cilia " were optical illusions. 
There is yet one more advocate for ce cilia" to be enu- 
merated. The writer in the last edition of Pritchard's 
“ Infusoria" considered that, at the time when he wrote, it was 
not satisfactorily ascertained that minute cilia do not exist 
along the ec raphe," causing the currents of indigo particles 
described by Yon Siebold. 
Lastly, we have to consider the hypothesis of some form of snail- 
like foot. It is no doubt the slow, crawling movement of the free 
Diatoms, together with the very considerable power exhibited 
by them, that first suggested this explanation, and not any 
observation of such an organ. Ehrenberg asserted that a 
snail-like foot was protruded from the central thickened por- 
tion of the frustule, known as the “ umbilicus," which he 
believed to be a perforation. M. Eocke believed in the 
existence of many such protusable feet, of a temporary cha- 
racter, while Mr. Wenham advances the following speculations 
on the cause of the movements, which, from their close 
approximation to the true state of the case as ascertained by 
Professor Schultze, deserve especial attention. He says (■ Journal 
of Microscopical Science , 1856, p. 159) - 
If caused by the action of cilia, such extremely rapid impulses would be 
required to propel the comparatively large body through the water, that 
surrounding bodies would be jerked away far and wide. A similar effect 
would be observed if the propulsion were caused by the reaction of a jet of 
water, which, according to known laws of hydrodynamics must necessarily be 
ejected with a rapidity sufficient to indicate the existence of the current a 
long distance astern. I consider there is no ground for assuming the motions of 
the Diatomacese to be due to either of these causes. They are urged forward 
through a mass of sediment without displacing any other particles than those 
they immediately come in contact with, and quietly thrust aside heavy obstacles 
directly in their way with a slow but decided mechanical power, apparently 
only to be obtained from an abutment against a solid body. In studying the 
motions of the Diatomacese, I have frequently seen one get into a position such 
