68 AQUATIC MICROSCOPY FOR BEGINNERS. 



5 calls for a green net often visible to the naked eye, 

 whicfi describes the specimen, giving the name of its 

 genus, Hydrodktyon, and referring the student to the 

 Algse, Section III. of this chapter. After using this 

 preliminary Key for a few times, he will be able to de- 

 cide at a glance through the microscope to which sec- 

 tion his specimen belongs. 



Key to the Desmids, Diatoms, and Fresh-water Algee. 



1. Plants formed of a single, crescent-shaped, spher- 



ical, barrel-shaped, oblong and constricted, or 

 circular and flattened, cell, sometimes arranged 

 side by side. in long ribbons, but seldom end to 

 end; color green or brown (a). 



2. Plants formed of many cells arranged end to end in 



long threads; coloring matter usually green, often 

 in spiral bands or other patterns on the cell-wall 

 {a). 



3. Plants formed of several green cells grouped in the 



shape of a flat disk with from six to many short 

 blunt star-like points; floating free. Pedidstrufii 

 {Alga, III.). 



4. Plants formed of froni two to eight narrowly-oval 



green cells placed side by side, each terminal cell 

 with two curved colorless bristles; floating free. 

 Scened^smus (Algm, III.). 



5. Plants forming a green net visible to the naked eye. 



Hydrodictyon (Alga, III. ) . 

 a. Color green, the plant a floating hollow sphere. 



Volvox {Alga, III), 

 a. Color green, the plant not a hollow sphere {6). 

 a. Color golden-brown {c). 



