that attends the efforts of a chess player when he 

 adjusts the pieces on his board. 



Diatoms, of course, are not peculiar to the sea 

 alone : they occur also on moist rocks and soil, in 

 fresh-water streams and ponds, and on the surface 

 of perennial mud. Marine forms are sometimes 

 attached to seaweeds, and many forms are often 

 found in the stomachs of clams and sea-squirts and 

 other animals that siphon their food from the 

 water; but by far the greatest number are those 

 that float independently, free. Each individual, or 

 frustule, is composed of two valves, the silicious 

 covering, or skeletal case, structurally not unlike 

 an ordinary pill-box. That is to say, one half of 

 the case is slightly larger than the other and fits 

 over it like a cover. One peculiarity of the living 

 diatom, which is likely to strike the observer's first 

 notice, is its extraordinary power of motion. Some 

 species move slowly to and fro in a straight line; 

 others are relatively rapid, leaping or darting 

 around erratically or sometimes following a zigzag 

 course in the water. The cause of this motion has 

 never definitely been determined; but speculation 

 has not been wanting to account for it. The most 

 probable theory is that which supposes the diatom 



[119] 



