THE POLYGASTRIA. 33 



line (one-twelfth of an inch) in dimensions. The largest 

 species; as, for example, the Stentors, are visible to the 

 naked eye ; and these are not more than two-thirds of a 

 line in length. But these living atoms are often clus- 

 tered together in such inconceivable numbers, as to 

 form considerable masses, of various colours. The Poly- 

 gastria occur both in fresh and salt water, in humid 

 earth, peat-bogs, and morasses. Our remarks will be 

 limited to the living species contained in the group to 

 which the reader's attention was first directed. 



VIBRATILE CILIA. As these processes are common 

 to the whole class of Infusoria, and only differ in the 

 several types in number, position, and relative magni- 

 tude, it is desirable, in this place, to give a more par- 

 ticular account of these remarkable instruments, which 

 recent discoveries have shewn to exist also in the in- 

 ternal organs of man, and of the other vertebrated 

 animals, and to be the agents by which many of the 

 most important functions of the animal economy are 

 performed. 



These delicate filiform appendages, appear like very 

 minute hairs rapidly vibrating in the water, and are only 

 discernible by the aid of a good microscope. They are 

 constantly performing a rotatory or circular oscillation; 

 and. as there is a bulb at the base of each cilium, it is 



D 



