DIATOMAVE^. 227 



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

 found in the latter also. 



The principal genera are Oosmanum (Fig. 153), Euastrum and 

 Micrasterias, •wbicli are constricted in tlie middle ; aud Gktsterium, in 

 whicli the individuals are cylindrical or fusiform.* 



304.— Order Diatomaceae.t The Diatoms are micro- 

 scopic unicellular algse, resembling ini manj jDarticulars 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 cts to stand at right angles to the remainder 

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

 bos cover. The two valves are generally slightly different 

 in size, so that one slips within the other {A, Pig. 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 valves, or as the side view (C, Pig. 

 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 Eev. Francis Wolle's "Desmids of 

 ihe United States," 1884, for an account of our species. 



f Most of our species are figured and described in Henri Van 

 lleurck's " Synopsis des Diatomees de Belgique," 1880-5. 



