THE DESMIDS 31 



of small animals and a few quite large ones. Their 

 empty frustules, also, sink to the ocean floors, and form 

 deposits of geological importance. 



There are some Diatoms which move in a gliding or 

 creeping manner. They must necessarily be of the un- 

 attached kind and of suitable shape. The Navicular 

 (little boats) are among such forms familiar to the 

 microscopist. They may be seen moving across the 

 field of the microscope like tiny ships; they frequently 

 halt in their onward progress, reverse engines, as it 

 were, and move backward as easily as forward. They 

 are also to be seen rotating on their axis. Encyonema 

 occurs in colonies in a gelatinous filament, and each cell 

 is able to move backward or forward within the limits 

 of the filament. It is difficult to explain these spon- 

 taneous movements; they may be due to a sheath of 

 protoplasm surrounding the valves or the extrusion of 

 minute threads of the same substance through pores in 

 the frustules. A free-moving form, Pinnularia viridis, 

 is shown, as seen under a medium microscopic magnifica- 

 tion, in Fig. 21, Plate II. 



Exactly where Diatoms are to be placed in the gamut 

 of plant -life is not decided. They are often included 

 among the Algae. They give little, if any, indication of 

 their derivation, nor do they appear to be a step in any 

 particular direction. We may regard them as one of 

 Nature's side-issues ending in a cul-de-sac. That they 

 are of economic importance as well as natural interest 

 has been shown. They are of moment to our present 

 study on account of their display of sex. 



In furtherance of our argument, we turn from the 

 brown Diatoms to the exquisitely green Desmids. These 



