DESMIDS, DIATOMS, AND FRESH-WATER AIXLE. 89 



II. DIATOMS. 



For a long time there was much discussion as to the 

 animal or vegetable nature of the diatoms, but that they 

 are plants is now the general belief. Their peculiar 

 motion was one great reason for classing them among 

 the animals, 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 with- 

 out 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 beginner 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 beautifully 

 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 alkalies, and at the end be as it was before, as 

 gracefully formed and as beautifully marked. Indeed, 

 to properly study the diatoms they should be treated 

 by some method to destroy the coloring matter often 

 obscuring the surface markings for which they are 

 chiefly valued. For the beginner, however, who desires 

 only to recognize a diatom when he meets with one 



