10 INTRODUCTION TO ZOOLOGY. 



Existing as they do, everywhere in countless multitudes, 

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

 are weU 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 

 Bculpture {Fig. 4). 



^i,nia^i\Hiiuiiiwiii!«!iiM!iffliiP'i;i !;n!!KiVfif;!fflif{frm;t!n^^ • e ^^''S^ '^s^^S** 

 ^ ajii.'Hiiiiiiiiiiijjiiiiiiiiii/Hiiiiiiiiiimiiiiiiiiisiiiiiiiiiiiiuiinmiimmnj i? cion 01 them m aitterenu 



parts of the world is 

 perhaps the most sur- 

 prising circumstance in 

 their history. Ehren- 

 berg found that a hill 



Fig.4.— Shells OF Infusoria. in Bohemia, composed 



chiefly of the polishing 

 substance known in the arts as " 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 

 the shores of a lake near Urania, in Sweden, is found a de- 

 posit of a similar kind, called by the peasants " mountain- 

 meal," and which they use mixed up with flour as an article 

 of food. Deposits of fossil infusoria are not confined to 

 foreign countries. A few years since, the Bann Kcservoir 

 Company were deepening a small lake a few miles from New- 

 castle, in the county of Down, and the workmen found a 



