18 Dr. G. C. Wallich on the Diatomacea. 



cndosmotic and exosmotic action in solving the problem before 

 us, surely it cannot be so to assume that of prehensile organs 

 of the kind shadowed forth. In the one case the alternating 

 action in opposite directions has no parallel. On the contrary, 

 looking at the unicellular nature of the Diatomaceous frustule, 

 we are at once met with strong negative testimony. Not so, 

 however, with regard to the motile filaments ; for we are pre- 

 sented with analogous phsenomena in the spores of certain Acro- 

 genous plants, which move from place to place by the alternate 

 expansion and contraction of the hairs with which they are 

 furnished. 



Or, if we are inclined to admit an analogy between the pre- 

 hensile and motile organs now spoken of and the pseudopodia of 

 the Rhizopods or, to go yet a step further, if we look at the 

 contractile pedicels of certain Infusoria, we are provided with 

 examples from the animal kingdom of the occurrence of similar, 

 or very nearly similar organs. 



From the character of the primary movements of the Dia- 

 tom, and those secondary movements which are produced by 

 its instrumentality on objects in its neighbourhood, it is highly 

 probable that the prehensile and active portion of the organs is 

 chiefly confined to their extremities, their stem being somewhat 

 rigid, and thereby unfitted to create currents amongst minute 

 foreign particles at its point of exit. Of filaments of this nature 

 we have examples, although on an enlarged scale, in certain 

 Monadina (Monas attenuata, for instance), and certain Euglena?, 

 such as Peranema globulosa. By such organs we might naturally 

 expect to see particles in the vicinity of a Diatom grasped and 

 swayed about, precisely as occurs in reality, at the same time 

 that the ordinary motions are produced. 



Professor Smith has stated that, on colouring the water with 

 " carmine or indigo, he had never been able to detect in the 

 coloured particles surrounding the Diatom those rotatory cur- 

 rents which indicate, in the true Infusorial animalcules, the 

 presence of cilia." But, surely, had endosmotic and exosmotic 

 action been the real source of motion, similar currents might 

 to have- attended the expulsion from the apertures of even so 

 small a quantity of water forcibly enough to create those 

 movements. 



The same writer has also stated (Synops. vol. i. p. 23) that 

 " motion may at times be detected in other forms than the free 

 species ; as those of Gomphonema, when forcibly separated from 

 their stipes, occasionally exhibit an evident tendency to change 

 their position /' but that such motions are devoid of the isochro- 

 nous quality so notable in the others. 



This is precisely what might be looked for, as I conceive the 



