DZATQXAQR&. 



clear water to that which is stagnant, although they are to be 

 found in the latter also. 



The principal genera are Cosmanum (Fig. 153), Eaaxtrum and 

 Micrasterias, which are constricted in the middle ; and Closterium, in 

 which the individuals are cylindrical or fusiform.* 



304. Order Diatomacese.t The Diatoms are micro- 

 scopic unicellular algne, resembling in man) particulars the 

 Desmids, but differing from them in having walls which are 

 silicified, and in the chlorophyll being hidden by the pres- 

 ence of phycoxanthine. The endochrome, as the colored 

 contents are called, is always symmetrically arranged. Each 

 cell (technically called a frustule) is usually composed of two 

 similar and approximately parallel portions, called the valves. 

 Each valve may be described as a disc whose edge is turned 

 down all around, so as to stand at right angles to the remainder 

 of the surface, making the valve have the general plan of a pill- 

 box cover. The two valves are generally slightly different 

 in size, so that one slips within the other (A, Fig. 154), thus 

 forming a box with double sides. In other cases as, for ex- 

 ample, in Diatoma and Fragilaria the valves are simply 

 opposed, and do not overlap. In figures and descriptions of 

 Diatoms, the parts corresponding to the top and bottom of a 

 box are referred to as the valve?, or as the side view (C, Fig. 

 154), and that which in the box would be called the side, is 

 in the Diatom called the front. 



305. The individuals may exist singly, or in loose fami- 

 lies ; they are free, or attached to other objects by little 

 stipes, and they are frequently imbedded in a mucous secre- 

 tion. The free forms are locomotive, and may be seen in 

 constant motion under the microscope. As in the Desmids, 

 the mechanism of this movement is not certainly known ; 



* The student is referred to Dr. H. C. Wood's " Contribution to the 

 History of the Fresh-water Algae of North America," 1872, for an ac- 

 count of our species. 



f Pfitzer and others maintain that the name Bacittariicece should be 

 applied to this order. 



