31 GENERAL HISTOET OF 



which, as a rule, are attached ; for example, Stentor and VorticeUa, 

 as also many of the doubtfal Infusoria — the Bacillaria, which, 

 becoming free, enjoy a slow swimming movement. Perhaps the 

 moat interesting example of movement by contractility, is that seen 

 in the stalk or pedicle of the genus VorticeUa, in which its activity 

 is so great, and its character so peculiar, that it leaves no doubt of 

 the presence of muscular fibre and muscular irritability. 



The circular arrangement of very distinct cilia about the upper 

 end — head and mouth, in VorticeUa, Stentm', &c., and the whirling, 

 wheel -like motion of those ciUa, would seem, as Prof. Owen remarks, 

 " to indicate the passage to the higher or rotiferous group." 



Sectioi^' XIII. On the Digestive System of the Pohjgastrica. — The 

 microscopic observer, having procured a number of animalcules, will 

 not fail to observe within the interior of many a number of cii'cular 

 spots ; these are often very large in proportion to the size of the 

 creature, and if the water is clear, they appear more transparent 

 than the other parts of the animalcule. These vesicles the reader 

 may readily distinguish iu many of the drawings contained in the 

 first six plates, and part of the seventh, which represent animals of 

 the class Pohjgastrica. Like any other division of natural bodies, 

 some of the members composing it, exhibit the essential characteristics 

 of the class more prominently than others, — thus the genera 

 Kolpoda and Paramecium contain the largest forms in which these 

 vesicles exist. The reader will do well to refer to the di-awings of 

 these genera, which he can readily do by means of the List of 

 figures at the end of the volume. 



The older naturalists considered these vesicles as the ova ; and 

 Baron Gleichen made many experiments to endeavour to see their 

 expulsion, but without success. This idea of the Baron's, respecting^ 

 the nature of these bodies is the more remarkable, as it is to him we 

 owe the original expeiiments of feeding animalcules with coloured 

 food ; and the fact of these parts becoming immediately coloured, 

 while the surrounding portions remain transjjarent, could scai'cely 

 have escaped his notice. 



From the observations of Dr. E., these globidar vesicles appear 

 to be distinct stomachs, of which a single animalcule belonging to 

 this class possesses many, as noticed in another place. When one of 



