10 INTRODUCTION TO ZOOLOGY. 



Existing as they do, everywhere in countless multitudes, 

 and endowed with appetites so voracious, it is clear that they 

 are well adapted to be the unseen scavengers of nature, and 

 that one of their uses in creation is to remove those decaying 

 matters which would become offensive to our senses and 

 dangerous to human life. Having removed those dead and 

 decaying substances, and made them a part of their own 

 organization, they in their turn become food for other animal- 

 cules, which again serve as nourishment for fishes. They 

 form, therefore, one of the means by which the salubrity of 

 our atmosphere is preserved, and putrefaction and decay 

 rendered conducive, through their instrumentality, to the 

 support of higher animals, and thus to the sustenance of 

 man himself. 



Some species of the polygastric animalcules, notwith- 

 standing their minuteness, are furnished with shells of various 

 forms and sizes. These are generally formed of silex; and 

 though not displaying the rich colours of the shells of the 

 mollusca, are no less beautiful, for the place of colour is 

 supplied by the most varied and exquisite patterns of natural 

 sculpture (Fig* 4), 



Fig. 4. SHELLS OP INFUSORIA. 



The large aggregation of them in different parts of tho 

 world is perhaps the most surprising circumstance in their 

 history; especially when we call to mind the fact that each 

 of these minute and scarcely visible shells is the production 

 of an animal which has passed away, and left behind but 

 this tiny memorial of its existence. 



Ehrenberg found that a hill in Bohemia, composed chiefly 

 of the polishing substance known in the arts as u tripoli," was 

 one mass of the siliceous fossil shells of these creatures ; and 

 that in a stratum fourteen feet in thickness, a cubic inch con- 

 tained the remains of 41,000,000,000 of individuals.* On 



* Taylor's Scientific Memoirs, vol. i. p. 407. 



