DIATOMACE^: 



619 



any department of natural history, the more does it prove that the 

 range of variation is far greater than had been previously imagined ; 

 and this is especially likely to be the case with such humble 

 organisms as those we have been considering, since they are obviously 

 more influenced than those of higher types by the conditions under 

 which they are developed ; whilst, from the very wide geographical 

 range through which the same forms are diffused, they are subject 

 to very great diversities of such conditions. 



The general habits of this most interesting group cannot be 

 better stated than in the words of Mr. W. Smith : ' The 



Fid 4G5. 



FIG. 46G. 



FIG. 465. Mastogloia Smitlrii : A, entire stipe ; B, frustule in its gelatinous 

 envelope; C-F, different forms of frustule as seen in side view; G, front view 

 H, frustule undergoing subdivision. 



FIG. 466. Mastogloia lanceolata. 



Diatomacece inhabit the sea or fresh water ; but the species peculiar 

 to the one are never found in a living state in the other locality ; 

 though there are some which prefer a medium of a mixed nature, and 

 are only to be met with in water more or less brackish. The latter are 

 often found in great abundance and variety in districts occasionally 

 subject to marine influences, such as marshes in the neighbourhood 

 of the sea, or the deltas of rivers, where, on the occurrence of high 

 tides, the freshness of the water is affected by percolation from the 

 'adjoining stream, or more directly by the occasional overflow^ of its 

 banks. Other favourite habitats of the Diatomacece are stones of 

 mountain streams or waterfalls, and the shallow pools left by the 



