64 AQUATIC MICROSCOPY FOR BEGINNERS. 



off obliquely on a new one. Amoving diatom always 

 seems to have impdrtant business on hand, and to be 

 anxious to accomplish it. An object, therefore, that 

 may be either a d'esmid or a diatom is not a desmid if. 

 it moves rapidly and changes its course suddenly and 

 quickly. 



The cause of this motion is in each case a mystery. 

 Many theories have been proposed to explain it, but 

 none is satisfactory. If the reader can discover how 

 the desmids and the 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 even 

 spines of different shapes ; its edges may be even or 

 notched, prolonged into teeth, or variously cut and 

 divided. It is these ornaments, in connection with 

 the graceful form and the pure usually 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 Na- 

 ture's handiwork, and among the most varied. Dots, 

 hemispherical bosses, hexagons, transverse and longi- 

 tudinal lines of astonishing nunlber- and fineness, are 

 among their many surface sculpturings, the delicacy 

 and the Closeness of which defy description by any 

 but the mathematician. So numerous and close to- 

 gether are the surface lines of some that they are used 

 to test the good qualities of the best and highest- 

 power objectives. There are nO perfectly smooth dia- 



