90 AQUATIC MICROSCOPY FOR BEGINNERS. 



members of the Diatomacex after he has observed and 

 recognized one. More than this he can scarcely hope 

 to do. 



The golden brown coloring matter will often be 

 seen contracted to a narrow strip, or to a spot at each 

 end, while frequently the frustule will be entirely 

 colorless. Diatoms are the favorite food of many 

 microscopic animals, which absorb the cell-contents, 

 often leaving the hard and indigestible valves color- 

 less, but otherwise unchanged. 



Key to Genera of Diatoms. 



1. Growing in bands or ribbons (a). 



2. Growing on colorless stems or in a jelly-tube (c). 



3. Growing with their concave sides attached to other 



plants (e). 



4. Free-swimming (/). 



a. Band curved or coiled. Meridion, 1. 



a. Band zigzag; frustules attached together by the 



corners. Didtoma, 2. 

 a. 'Band uneven, frustules long, narrow, rapidly 



sliding on one another. Bacilldria, 3. 



a. Band straight, or nearly so, edges even, frustules 



motionless (b). 



b. Each frustule six times as long a:s broad. Fragi- 



Idria, 4. 



b. Each frustule twice as long as broad. Himatiti- 



dium, 5. 



c. In a narrow jelly-tube; valves boat-shaped. En- 



cyon^ma, 6. 



c. On the ends of- colorless stems (d). 



d, Valves boat-shaped. Cocconhna, 7. 



d: Valves wedge-shaped. Gomphon^ma, 8; 



