Dr. G. C. Wallich on the Diatomacece. 17 



generality of authorities, respecting the movements of tlie Diato- 

 maceffi, may be best stated in the paragraphs from the ' Synopsis ' 

 (vol. i. p. xxiii) : — 



" Of the cause of these movements I fear I can give but a very 

 imperfect account. It appears to me that they do not arise from 

 any external organs of motion. The more accurate instruments 

 now in the hands of the observer have enabled him confidently 

 to affirm that all statements resting upon the revelations of more 

 imperfect object-glasses, which have assigned motile cilia or feet 

 to the Diatomaceous frustulc, have been founded upon illusion 

 and mistake. Amongst the hundreds of species which I have 

 examined in every stage of growth and phase of movement, 

 aided by glasses which have never been surpassed for clearness 

 and definition, I have never been able to detect any semblance 

 of a motile organ. 



" I am constrained to believe that the movements of the Dia- 

 tomaceae are owing to forces operating within the frustule, and 

 are probably connected with the endosmotie and exosmotic ac- 

 tion of the cell. The fluids w-hich are concerned in these actions 

 must enter and be emitted through the minute foramina at the 

 extremities of the silicious valves ; and it may be easy to con- 

 ceive that an exceedingly small quantity of water expelled through 

 these minute apertures would be sufhcient to produce movement 

 in bodies of so little specific gravity." 



Had it only been necessary to explain the ordinary and sepa- 

 rate movements of the Diatomaceous frustule, the theory of 

 endosmotie and exosmotic action might perhaps have been 

 deemed satisfactory. But the moment we come to consider the 

 behaviour of the frustule with reference to minute objects in its 

 vicinity, and duly interpret the phenomena I have endeavoured to 

 describe, and which are inseparable as cause and efiect, it becomes 

 evident that no such action can, by any possibility, account for 

 them. To explain such phfenomena consistently, we are irre- 

 sistibly, as I conceive, led to one inference, namely, the existence 

 of elongated prehensile filaments, capable of alternate extension 

 and retraction, of extreme tenuity, yet of extraordinary strength 

 and elasticity, — in virtue of which both the ordinary to-and-fro 

 motions and the secondary motions affecting surrounding bodies 

 are performed. 



All "free" Diatomaccaj may be held to possess these organs. 

 But where they emerge, whether they arise in one, or two, 

 or several pairs from each valvular extremity, and whether 

 they are to be considered as processes sent out from the pri- 

 mordial utricle, it would be foolish, at present, to hazard an 

 opinion. 



If it has not been deemed rash to assume the existence of 

 Ann. i^ Mat/. N. Hint. Ser. 3. Vol. v. 2 



