DIATOIIACILE. 157 



like outline, are attached at the end of long slender threads, 

 which grow from a common point, and radiate in a beauti- 

 ful fan-like manner ; at other times, the frustule is of an 

 irregular trapezoidal form, and is connected with its 

 fellows by a short intervening band. Perhaps the most 

 common form of all is that of an italic / without the 

 terminal dots, each frustule being unconnected with 

 others. These have the power of spontaneous motion ; 

 and it is very interesting to mark them creeping along in 

 a vagrant, jerking manner over the field of the microscope, 

 making no inconsiderable progress. 



There are, then, several circumstances wliich combine 

 to make the economy of these creatures full of interest, 

 and give them a strong hold on our imagination. 



1. Their inconceivable multitudes, and their universal 

 distribution, especially in the wat€rs of our globe, from 

 the equator to the poles, or at least as near to them as 

 man has been able to investigate, the everlasting glaciers 

 of the icy seas being conspicuously stained with them. 



2. The vast part assigned to them in the economy of 

 creation, since, as we have seen, they not only enter largely 

 into the composition of the solid crust of the globe, but 

 sustain (mediately) the life of its very hugest creatures.* 



3. The very great variety of forms assumed by the 

 different kinds. 



4. Their marvellous elegance and beauty, consisting iu 

 their material, their shapes, and their sculpturing. 



5. Their sponta:neous movements, and the mystery 



• See tupra, p lOfl. 



