DESMIDS, DIATOMS, AND FRESH-WATER ALG^E. 67 



mids. And wliile the desmids move stately and slowly 

 in one direction, a diatom may travel quickly half-way 

 across the field of view, and without a moment's hesita- 

 tion, and without turning round, may at once return by 

 its former path or dart off obliquely. A moving diatom 

 always seems to have important business on hand, and 

 to be anxious to accomplish it. An object, therefore, 

 that may be either a desmid or a diatom is not a desinid 

 if it moves rapidly and changes its course suddenly and 

 quickly. The cause of this motion is in either case a 

 mystery. Many theories have been proposed to explain 

 it, but none are satisfactory. If the reader can discover 

 how the desmids and diatoms move themselves his name 

 will be remembered among naturalists to the end of 

 time. 



The surface of a desmid may be smooth, finely stri- 

 ated lengthwise, roughened by minute dots or points, or 

 it may bear several wart-like elevations or spines of dif- 

 ferent shapes ; its edges may be even or notched, pro- 

 longed into teeth, or variously cut and divided. It is 

 these ornaments, in connection with the graceful form 

 and the pure homogeneous green color, that make the 

 desmids so attractive to every student of microscopic 

 aquatic life. Fresh-water diatoms occasionally have tooth- 

 like processes, but they are never spine-bearing ; yet the 

 markings of their surfaces are among the most exquisite 

 of Nature's handiwork, and the most varied. Dots, hem- 

 ispherical bosses, hexagons, transverse and longitudinal 

 lines of astonishing fineness, are among their many 

 4* 



