44 NATURE'S SCAVENGERS. 



Let it be remembered, too, that Infusoria, when 

 found in either do not themselves constitute the 

 impurity of fresh or salt water ; they merely act as 

 ' nature's invisible scavengers/ whose duty it is to 

 remove all nuisances that may spring up ; and most 

 unceasingly do these tiny creatures labour in the per- 

 formance of their all-important mission of usefulness. 



