292 THE MICROSCOPE. 



remove the animal matter, a slight increase in transparency 

 is the only perceptible difference of appearance in these 

 two opposite conditions. 



INFUSORIA. 



The class Infusoria, described by Ehrenberg in his work 

 Infusionsthierehen, published in 1838, he divided into two 

 great groups, the Polygastrica, or many-stomached; and the 

 Rotifera, or rotating, wheel-animalcules: the latter are 

 now classed with animals of a higher type of organisation. 

 The classification of the Infusoria presents considerable 

 difficulties, partly arising from their excessive minuteness, 

 which renders the assistance of our best microscopes 

 necessary to enable us even to see many of them, and 

 partly from the impossibility of avoiding confusion from 

 the intermixture of genera of more highly organised 

 animals, and some plants, as the Volvocinece, the Desmi- 

 diaceat, Bacillarice, <fec., in their various stages of develop- 

 ment. 



The term Infusoria l is applied to them because they 

 were first discovered in water where vegetable matter was 

 decomposing, and therefore, the infusion was considered 

 necessary for their production. Now, however, it is an 

 established fact, that they are in a higher state of organi- 

 sation when taken from pure streams and clear ponds than 

 from putrid and stagnant waters. A little bundle of hay, 

 or sage-leaves, left for about ten days in a mug containing 

 some pure rain-water, caught before entering a butt, 

 produces the common wheel-animalcules, which are found 

 adhering to the sides of the mug near to the surface of the 

 water. The only use of the vegetables seems to be to 

 facilitate the development of the latent life of the atoms 

 of organic matter, and perhaps as the first sources of 

 their food. It is an indispensable condition that air be 

 admitted to the infusion : and this circumstance, dis- 

 covered by Leeuwenhoek in 1676, has always been regarded 

 as one of the principal evidences in favour of the doctrine 

 of spontaneous generation, a doctrine which at one time 

 had many supporters. 



(1) Infusoria, from infusor, a pourer-in. 



