no Aquatic Organisms 



valves are connected by a membraneous portion of the 

 cell wall known as the girdle. A diatom may appear 

 very different viewed from the face of the valve, or from 

 the girdle (see fig. 35a and b, or j and k). They are 

 circular-like pill-boxes in one great group, and more or 

 less elongate and bilateral in the others. 



Diatoms are rarely green in color. The chlorophyll 

 in them is suffused by a peculiar yellowish pigment 

 known as diatomin, and their masses present tints of 

 ami >er, of ochre, or of brown ; sometimes in masses they 

 appear almost black. The shells are colorless; and, 

 bring composed of nearly pure silica, they are well 

 nigh indestructible. They are found abundantly in 

 guano, having passed successively through the stomachs 

 of marine invertebrates that have been eaten by fishes, 

 that have been eaten by the birds responsible for the 

 guano deposits, and having repeatedly resisted diges- 

 tion and all the weathering and other corroding effects 

 of time. They abound as fossils. Vast deposits of 

 ^ them compose the diatomaceous earths. A well-known 

 bed at Richmond, Va., is thirty feet in thickness and of 

 vast extent. Certain more recently discovered beds 

 in the Rocky Mountains attain a depth of 300 feet. 

 Ehrenberg estimated that such a deposit at Biln in 

 Bohemia contained 40,000,000 diatom shells per cubic 

 inch. 



Singly they are insignificant, but collectively they are 

 very important, by reason of their rapid rate of increase, 

 and their ability to grow in all waters and at all ordinary 

 temperatures. Among the primary food gatherers 

 of the water world there is no group of greater import- 

 ance. 



In figure 35 we present more or less diagrammatically 

 a few of the commoner forms. The boat -shaped, freely 

 moving cells of Navicula (a, b, c) are found in every 

 pool. One can scarcely mount a tuft of algae, a leaf 



