INTRODUCTION. 



b. Those provided with aerial vesicles (Siphonophori). 



c. Those furnished with vibrating hairs, by which they 



move (Ctenophori). 



The class of POLYPS includes three orders. 



a. Fresh-water polyps, and similar marine forms (Hy- 

 dro'ids), fig. 132. 



1). Marine polyps, like the sea-anemone and coral- 

 polyp (Actinoids), figs. 48, 143. 



c. A still lower form, allied to the mollusks by their 

 shell (Rhizopods). 



In addition to these, there are numberless kinds of micro- 

 scopic animalcules, commonly called infusory animals (In- 

 fusoria), from their being found specially abundant in water 

 infused with vegetable matter. Indeed, a great many that 

 were formerly supposed to be animals are now found to be 

 vegetables. Others are ascertained to be crabs, mollusks, 

 worms, &c. in their earliest stages of development. 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, notwithstanding the beautiful 

 researches already published on this subject, and as most 

 of them, are likely to be finally distributed among vegeta- 

 bles and the legitimate classes in the Animal Kingdom, we 

 have not assigned any special place for them. 



