DESMIDS, DIATOMS, AXD FRESH-WATER ALG^E. 95 



and rapidly sliding backward and forward over each 

 other ; free-swimming (Fig. 64). 



This is probably one of the most interesting of the 

 common fresh-water diatoms, on account of its strange 

 movements. When quiet, as it prob- 

 ably will be immediately after being . - r "~ ''-i ; * 

 placed on the slide, the band will 



Fig. 64.-Bacillaria. 



somewhat resemble a row of fence 

 pickets lying closely side by side. Suddenly each 

 picket shoots forward until they are all nearly end to 

 end, the band becoming a long irregular line, and quite 

 as suddenly closing together again. This alternate back- 

 ward and forward gliding is continued until the diatoms 

 become apparently exhausted, or the oxygen in the wa- 

 ter is consumed. What prevents one frustule from 

 slipping off the end of the other is not known ; indeed 

 the cause of the entire performance can only be guessed 

 at. All the species of the genus Bacillaria are said to 

 live in salt water. The form, which I have ventured to 

 identify as a sweet-water variety of B. paradoxa is not 

 uncommon in fresh-water ponds. 



4. FRAGELAIUA CAPUCINA (Figs. 65 and 65o). 

 Frustules very narrow, never wedge- shaped, band 

 long. Fig. 65 shows the band of 

 united frustules ; Fig. 65& the ap- 

 pearance of a single valve more 

 highly magnified. The ends of the 



Figs. 05 and G5a. Fragela- 



valves are somewhat w T edge-shaped. rtacnpndna. 



