INFUSORIAL ANIMALCULES. 51 



of Animalcules y and will make a general comparison be- 

 tween the system adopted in that work and the present, 

 he cannot fail to observe that, although the principles of 

 the classification of Miiller and Ehrenberg are widely dif- 

 ferent, yet many of the groups of animalcules occupy, as a 

 whole, similar positions, in the two systems. This analogy 

 I was much struck with in the place of the Vibrio of 

 Miiller and the Bacillaria of Ehrenberg, while the coni' 

 mencing and concluding genera in each system are similar. 



Dr. E. divides the Infusoria into two grand classes ; the 

 animals belonging to the first are called Polygastrica, and 

 are distinguished from the second class, named Rotatoria, 

 by the function of digestion in the former being carried 

 on by numerous globular vesicles, or stomachs, while the 

 creatures belonging to the latter, like most large animals, 

 have only one stomach for digestion. 



PoLYGASTRicA. — The microscopic observer, having pro- 

 cured a number of animalcules, will not fail to observe 

 within the interior of many a number of circular spots ; 

 these are often very large in proportion to the size of the 

 creature, and if the water is clear, they are more trans- 

 parent than the other parts of the animalcule. These 

 vesicles the reader may readily distinguish in many of the 

 drawings contained in the first six plates, and part of the 

 seventh, which represent animals of the class Polygastrica. 

 Like any other division of nature, some of the members 

 composing it exhibit the essential characteristics of the 

 class more prominent than others, and thus the genera 

 Kolpoda and Paramecium contain the largest forms in 

 which these vesicles exist. The reader will do well to 

 refer to the drawings of these genera, which he can readily 



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