OF THE DIATOME^. 51 



moTement in one direction, and a return upon nearly the same path, after a 

 few moments' pause, by another series of isochronal impulsions. The move- 

 ment is evidently of a mechanical natui'e, produced by the operation of a force 

 not depending upon the volition of the living organisms : an obstacle in the 

 path is not avoided, but pushed aside ; or, if it be sufficient to avert the 

 onward course of the fi'ustule, the latter is detained for a time equal to that 

 which it would have occupied in its forward progression, and then retires 

 from the impediment as if it had accomplished its fuH course. There is cer- 

 tainly no character of animality in the movement ; and the observer, familiar 

 "vvith the phenomena of life in the earlier stages of vegetable existence, is 

 constrained to see a coimterpart in the involuntary motions of the filaments 

 of the Oscillatorieae, or of the gemmiparous spores of the Fuci and Con- 

 fervae." 



This same view was taken by Morren in 1839 {op. cit.), who says — '' The 

 movement of the BaciUaria, however free it may be, is by no means so free 

 and active as that of the spores of Algae, which are plants, or, at least, parts 

 of plants ; and the motion is no positive ground for the belief in their ani- 

 mality." 



The cause of the motion of the Diatomese has hitherto not been satisfacto- 

 rily determined. To the hypothesis of a snail-like expanding foot projecting 

 fi'om the central pore or umbilicus, advanced by Ehrenberg, we have already 

 alluded (p. 41); and since no one original observer, in spite of the best-dii^ected 

 efforts, has been able to detect the remotest evidence of such an organ, and 

 as all evidence goes to show that no actual perforations exist at the point 

 indicated for its extrusion, it would be useless to raise any argument upon it. 

 This distinguished natui^alist subsequently satisfied himself of the presence 

 of other locomotive organs in a Navicida (Surirelhi gemma, XII. 3, 4), which 

 he has thus described : — " Instead of a snail-like expanding foot, long delicate 

 threads projected, where the ribs or transverse markings of the sheU joined 

 the ribless lateral portions, and which the creatui'e voluntaiily drew in or 

 extended. An animalcule ^th of a line long had 24 for every two plates, 

 or ninety- six in all ; and anteriorly, at its broad frontal portion, foui^ were 

 visible. Whether these organs were supernumerary, and existed along T\dth 

 cirrhi, &c., and with the flat snail-hke foot which the rest of the Naviculce 

 possess, could not be determined. Longitudinal clefts at the broad side of the 

 shell were not present ; but as many as 96 lateral openings for the exit of the 

 ciiThi were perfectly distinct." These ciliary processes were farther stated 

 to be actively vibratile, and to be retracted or extended at short periods. 



Prof. Smith has remarked on this appearance, that the presence of haii^s 

 apparently on all parts of the frustule may often be detected, and that he has 

 noticed them on nearly every occasion on which he has gathered this species, 

 but in no case has he been able to perceive any motion in such hairs ; and he 

 therefore concludes that they are merely a parasitic growth, such as the 

 mycelium of some Algae. He has also seen similar appendages to other Dia- 

 tomeae, but in every case devoid of motion. 



The notion of exsertile and retractile feet has been renewed by M. Focke 

 {Comptes Remlm, 1855, p. 167), who attributes the movements of Navicida to 

 such organs of a temporary kind, which he says pass through openiags he 

 has detected on the sides of the lorica. 



Nageli offered the following, and, to Siebold's mind, satisfactory, explana- 

 tion of the forward and backward movement, as well of many Desmidie* 

 as of Diatomeae {J. M. S. 1853, p. 195). " The cells," he writes, " have no 

 special organs for these movements. But as in consequence of their nutri- 

 tive processes they both take in and give out fluid matters, the cells neces- 



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