INFUSORIAL ANIMALCULES. 545 



homogeneous gelatinous matter, and the surface is destitute of 

 cilia, the movements being effected by the extension and con- 

 tractions of processes or filaments of the general substance, and 

 the food taken by the same means. 



III. POUIFERA, or Sponges, compound animals consisting of a 

 horny or silicious framework, covered by a gelatinous fluid, 

 which contains a multitude of minute creatures resembling the 

 Rhizopoda in their characters. They form masses of various 

 sizes and forms, permeated by larger or smaller canals, which 

 Mlow the water to pass to all parts of them. 



CLASS I. INFUSORIA. 



1201. Whenever any decaying organised matter exists in a 

 fluid state, and is exposed to air and warmth, it will speedily 

 be found peopled with minute inhabitants of the most varied 

 forms and diversified movements, possessed of considerable 

 activity, and evidently endowed with an energetic system of 

 nutrition. The cause of the sudden appearance of these Ani- 

 malcules where no germs were previously suspected to exist, 

 and where it was not easy to suppose that they had been con- 

 veyed, has been a matter of much speculation. Many have had 

 recourse to the supposition that the germs formed part, in a 

 latent state, of the living tissues of the animal and vegetable 

 structures, from the decomposition of which they were evolved ; 

 and others have even supposed them to have arisen from acci- 

 dental combinations of inorganic elements. But these Animal- 

 cules are not confined to infusions of organised matter ; they are 

 found in the stagnant waters around our cities ; in the waters 

 of rivers, harbours, and lakes ; and even, it is believed, in every 

 fluid drop of the ocean. From their minute size and almost 

 universal distribution, therefore, we cannot doubt that they are 

 by far the most numerous living beings which exist on the face 

 of the globe. Indeed, from calculations it would appear that a 

 single cup of water may easily hold a number of the smallest 



