PSEUDOPODIA OF DIATOMS. 62X 



The fact that they do not move or quickly retract under 

 stimulus would not necessarily distinguish them altogether 

 from true pseudopodia ; at any rate, if the processes of Arche- 

 rina are to be called by that name these must also, for it is 

 impossible to separate them. It might, however, be an advan- 

 tage to have some distinctive name for such stiff, slowly 

 changing or unchanging pseudopodia. 



It remains to treat briefly of the morphological resemblances 

 between these diatoms and other forms. In doing this I wish 

 simply to point out resemblances or differences, not to draw 

 any large conclusions. 



The first point that strikes one about these pseudopodia is 

 their extreme unlikeness to any plant or part of a plant. 

 Secondly, it is remarkable that when pseudopodia are dis- 

 covered on diatoms these resemble a type which, so far as I 

 know, is absolutely confined to the Heliozoa, forms with which 

 the diatoms are already connected by the presence of a more 

 or less siliceous test. All the Heliozoa are characterised by the 

 relative stiffness of their pseudopodia ; but in Archerina this 

 relative stiffness becomes practically absolute, as in these dia- 

 toms. I do not know of any described form except Archerina 

 which has these non-retractile pseudopodia. 



But the similarity in type of the pseudopodia is not confined 

 to rigidity ; it extends to general form. As far as I know, 

 every single detail of form in the diatoms can be matched 

 amongst the Heliozoa with the exception of the repeated 

 branching of those of Melosira. The resemblance of the diatom's 

 processes to those of Archerina is still closer : nevertheless in 

 99 cases out of 100 it is quite easy to distinguish on a dry slide 

 the pseudopodia of Archerina from those of the diatoms. 



They are distinctly wider and taper regularly. But in the 

 100th case you get an Archerina colony the pseudopodia of 

 which are absolutely undistinguishable from those of the 

 diatoms. Their size and shape are the same as those of 

 Cyclotella, and on a dry cover they show the thickened bases 

 and the denser portions of the filament which I have described 

 as characteristic of the diatoms. 



