THE KOMANCE OF NATURAL HISTORY. 



species, in which the form is a lengthened oblong, 

 the frustules have the faculty of sliding partially 

 over each other, and thus the chain resembles a 

 series of long steps. 



Sometimes the frustules, perhaps of a graceful 

 wedge-like outline, are attached at the end of long 

 slender threads, which grow from a common 

 point, and radiate in a beautiful 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 spontane- 

 ous motion; and it is very interesting to mark 

 them creeping along in a vagrant, jerking manner 

 over the field of the microscope, making no in- 

 considerable progress. 



There are, then, several circumstances which 

 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 waters 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, con- 



* See supra, p. 101. 

 152 



