24 INTRODUCTION. 



scopic animalcules, commonly united under the name of 

 infusory animals, (Infusoria,) from their being found specially 

 abundant in water infused with vegetable matter. These 

 minute beings do not, however, constitute a natural group in 

 the Animal Kingdom. Indeed, a great many that were for- 

 merly supposed to be animals are now found to be vegetables. 

 Others are ascertained to be crustaceans, mollusks, worms 

 of microscopic size, or the earliest stages of development of 

 larger species. In general, however, they are exceedingly 

 minute, and exhibit the simplest forms of animal life, and 

 are now grouped together, under the title of Protozoa. But, 

 as they are still very imperfectly understood, notwithstand- 

 ing the beautiful researches already published on this sub- 

 ject, and as many of them are likely to be finally distributed 

 among vegetables, and the legitimate classes in the Animal 

 Kingdom to which they belong, we have not assigned any 

 special place for them. 



