MANUAL OF NATUEAL HISTORY. 367 



INFUSORIAL-ANIMALCULES. 



THESE microscopic beings are usually termed "In- 

 fusory-Animalcules," which term formerly embraced 

 the Rotifers, and several other kinds of animals, en- 

 tirely different in their organization. It is now, 

 however, restricted to the present class, characterized 

 by a number of internal sacs or stomachs, which 

 simple feature will at once distinguish them from 

 other minute forms. The Polygastrica are all aqua- 

 tic, some inhabiting the waters of the ocean, and 

 some being found in fresh water. The discoverer of 

 most of them, and the great authority on their his- 

 tory, is Ehrenberg, whose works display extraordi- 

 nary patience and profound research. The name of 

 "Monad" is often employed to express the lowest 

 grade of organized beings, and with some degree of 

 truth, for their structure is reduced to very simple 

 elements, and they are so minute that a single drop 

 of water may contain five hundred millions of indi- 

 viduals. Some of the Polygastrica, as the Proteus, 

 change the forms of their bodies in a very surprising 

 manner, appearing under the microscope sometimes 

 as a round atom of jelly, then becoming slender and 

 worm-like, and even throwing out different parts of 

 their body, and assuming shapes enough fully to 

 justify the name bestowed upon them. 



These animals appear to have been equally as 

 numerous during the earlier periods of the world's 

 history as they are now : vast deposits of chalk hav- 



