ALGJE. 



195 



siliceous wall (Figs. 36, 37). That the latter theory 

 is true in some species has, we think, been clearly 

 demonstrated in some of the large marine forms, but 

 it is not so evident in the more delicate species. 

 The movements of the diatoms excited the wonder 



ta? 



E^ 



Fig. T,6.—Pinnulana major. 



Fig. yj.—Siauroneis phenicenteron- 



and curiosity of the earliest observers, and was a 

 great argument in favour of their animal nature. 

 The means by which these movements are produced 

 has not yet been explained. By some it was sup- 

 posed to be caused by the extension and retraction 

 of a foot or feet, and by others the expulsion of 

 water from the ends of the cell. It is only among 

 the elongated, or staff-shaped forms, that these move- 

 ments are conspicuous, but that all non-parasitic 



