CHAPTER III. 



CLASS INFUSOKIA. 



The animals known as "Infusorian Animalcules," are 

 all microscopic in their dimensions, and, tlioi;gh of uni- 

 versal occurrence, they are for this reason only known to 

 scientific observers. They derive their name from the 

 fact that they occur very frequently in what are called 

 "vegetable infusions." By this term is meant the fluid 

 which is obtained by pouring hot water upon any vege- 

 table substance (Latin, in, in ; Jundo, I pour), and then 

 straining off the solid particles. The Infusoria, then, are 

 so called because they are always, or almost always, to be 

 found in fluids of this nature, which have been exposed 

 for a longer or shorter time to the air. They are also of 

 almost universal occurrence in all stationary collections of 

 water, whether fresh or salt. As the type of this class 

 we may select the animalcule known as Parcancecium., a 

 very brief description of which will suffice to indicate the 

 leading points of interest in these animals. 



Parcnncecium (fig. 2) is one of the most common of 

 animalcules in stagnant water, and is only visible under 

 the microscope. Its body is somewhat oval or slipper- 

 shaped, and is almost quite transparent. Externally it is 

 covered by a delicate membrane, which supports a num- 

 ber of little vibrating hair-like processes, which are called 

 "cilia" (Latin, cilmm, an eyelash). These little filaments 

 when in active motion cannot be detected by the eye, 

 owing to their transparency, but they can readily be seen 

 when nearly at rest ; and they are still more easily recog- 

 nised by seeing that aU minute solid particles which may 

 be floating in the water are forcibly driven away when 

 they approach within a certain distance of the animal. 

 By the vibrations of these " cilia," which lash to and fro 

 like so many little whips, currents are set up in the sur- 



