86 AQUATIC MICROSCOPY FOR BEGINNERS. 



peculiar motion was one great reason for classing them 

 among the atiimals, although some undoubted plants 

 have even a more rapid movement. 



No class of microscopic objects, except, perhaps, 

 the Infusoria, is so abundant. No ditch or pond is 

 without them. No pool is too small to harbor them ; 

 even a depression made by a cow's hoof in a wet 

 meadow soon becomes a home for them. They will 

 probably form some of the first things to attract the 

 attention of the novice in the use of the microscope. 



Their shape is as varied as their number is great, 

 and their hard and glass-like surface is most beauti- 

 fully lined and dotted, and sculptured in .delicate 

 tracery. Most plants are comparatively soft, but the 

 diatoms are noteworthy for the hard case enclosing 

 the semi-fluid, yellowish-brown contents, a case that 

 is indestructible. It may be heated to redness, it may 

 be boiled in strong acids and in alkalies, and at the 

 end be as it was before, as gracefully formed and as 

 beautifully marked. Indeed, properly to study the 

 surface markings, the diatoms should be treated 

 by some method to destroy the coloring matter often 

 obscuring these geometrical designs, which, for many 

 purposes, make them so highly valued. For the be- 

 ginner, however, who desires only to recognize a dia- 

 tom when he meets with- one in the field of his micro- 

 scope, and to learn its name, if possible, such prepa- 

 ration is unnecessary. 



These plants are also peculiar in their structure. 

 In this they have often been compared to a pill-box. 

 The diatom is formed of two parts called valves, one 

 of which may be likened to the pill-box proper, and 

 the other to the lid, since it slips over the upright 



