PSEUDOPODIA OF DIATOMS. 617 



a typical specimen I counted about forty-six of these projections, 

 while a roasted cover of the same gathering gave about forty- 

 six as the number of radiating ribs on the valve of the Cyclo- 

 tella. Hence there would appear to be a close connection be- 

 tween the number of the pseudopodia and the structure of the 

 diatom — a point of very great importance. Sometimes pseudo- 

 podia spring from the surface of the cingulum as well as from 

 the valves. 



The pseudopodia are generally fairly straight ; occasionally 

 they branch at some distance from the valve. This was 

 especially the case in the earliest gathering (figs. 1 and 2), 

 where they branched repeatedly. 



On dried cover-glasses it is common to find two or three 

 pseudopodia springing from a short thickened base. In water 

 unstained these bases are extremely hard to see. 



The number of the pseudopodia is, on the whole, strikingly 

 regular. On dried covers seventeen to twenty is the ordinary 

 number seen round a valve in side view (fig. 6) ; sometimes 

 twice that number (fig. 7). 



The pseudopodia vary a good deal in thickness in different 

 gatherings. On dried unstained cover-glasses they often show 

 short portions of their length more opaque and solid-looking 

 than the rest, sometimes getting a beaded look. In this they 

 agree with the pseudopodia of Archerina (figs. 5, 6). 



Rarely one comes across quite thick pseudopodia. Fig. 5 

 represents a striking form, with a few very stout ones. 

 It is noteworthy here that these four spring from the coarser 

 markings on the valve. Occasionally one of the pseudopodia 

 is thickened in the centre, as in fig. 6. Lastly, the pseudo- 

 podia of two diatoms seem to be able to fuse into each other, 

 and increase greatly in width, so as to form a broad band con- 

 necting the two diatoms. Whole chains are thus formed, 

 though two frustules only are more common (figs. 2 and 8). 



Next as to the use of these pseudopodia, and the question 

 why other diatoms do not have them. The chief point to be 

 remembered is that these little Melosiras and Cyclotellas occur 

 mainly as isolated frustules, and are without the power of loco- 



