32 GENERAL HISTOEY OF 



are, as I have satisfied myself by direct investigation, germs of fresh 

 water worms, some of which I have seen hatched from eggs of 

 Planaria laid under my eyes." Mr. Girard coincides with Agassiz 

 in these statements, and adds that the Kolpoda cucullus is one of the 

 embryonic stages of a species of freshwater Planaria. (Proceedings 

 of American Association, 1848, p. 402). 



jN'one of the Pohjgastrica exceed in dimensions the 12th of an 

 inch, and some of the smaller species, as those of Monas, Bodo, Bac- 

 terium, and Vibrio, even when full grown, are bnt the 1 -2000th part 

 of that measiu'e ; indeed, so minnte must be many of the yoimg of 

 these Inftisoria, that they cannot be recognized by our microscopes. 

 The genera Stentor and Spirostotmmi on the other hand, contain 

 species as large as the gi'eater wheel animalcules (Rotatoria), and are 

 easily detected by the naked eye. Again, others, individually so 

 small as to be almost in^-isible, fonn, 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 Pol3'pi. 



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 also exist in fluids produced by animal secretions ; 

 moist earth, too, is another situation in which some are to be foimd. 

 As an instance of the later habitat, there has been recently found 

 some earth near jS'ewcastle almost entirely composed of living 

 species of the genus Bacillaria, and other loricated Infusoria. 

 Various kinds reside in the vapoiu- of the atmosphere, in which, 

 fi'om then- light weight, they rise in coimtless multitudes, and are 

 blown about by the wind in invisible cloud-like masses. 



The degree of motion possessed by the several genera of Pohjgas- 

 trica, varies greatly, whilst the sort of movement is as various. In 

 soA'cral instances these peculiarities in locomotion are useful in 

 characterizing genera ; such will be found particularly indicated in 

 the third or systematic part of this work. Some genera are attached, 

 and still enjoy some extent of relative movement, such are Vorticella, 

 Zooihamnium, and Syncdra. Most of those miattachcd arc, more or 



