62 GENERAL HISTORY OF 



but the l-2000th part of that measure; indeed, so minute 

 must be many of the young of these Infusoria, that they 

 cannot be recognised by our microscopes. The genera 

 Stentor and Spirostonum, on the other hand, contain spe- 

 cies as large as the greater wheel animalcules (Rotatoria), 

 and are easily to be distinguished by the naked eye. 

 Again, others, individually so small as to be almost invi- 

 sible, form, when aggregated, green, red, yellow, blue, 

 brown, and black- coloured masses of great extent. Thus, 

 the clusters of some species in the families Vorticella and 

 Bacillaria increase to such an extent that they attain a 

 size of several inches, resembling Polypi. The Micromega 

 forms cartilaginous arborescent masses, which have been 

 looked upon by some as Fuci, Algse, &c. ; Gallionella and 

 Setrizonema, as also Epystilis grandis, often form masses 

 several feet in length. 



The greater number of animalcules belonging to this 

 class are found in fresh water; numbers inhabit the salt 

 water of the ocean ; and some live in astringent solutions, 

 even those containing much tannin. They are found in 

 fluids produced by animal secretions ; moist earth, too, is 

 another situation in which some members of this class are 

 to be found. As an instance of the later habitat, there 

 has been recently found some earth near Newcastle 

 almost entirely composed of living species of the genus 

 Bacillaria, and other loricated Infusoria. It is highly pro- 

 bable that some kinds reside in the vapour of the atmo- 

 sphere, in which, from their light weight, they may be 

 raised in countless multitudes, and blown about by the 

 wind in invisible cloud-like masses. 



It is remarkable, that one -half the families belonging to 



