INFUSORIA. 37 



CHAPTER III. 



INFUSORIA. 



The Infusorian Animalcules are minute Protozoa of almost universal 

 distribution, abounding in almost all collections of fresh or salt 

 water in which organic matter may be present. They commonly 

 have the power of becoming " encysted," and of remaining dormant, 

 when deprived of water or of other conditions necessary to their 

 active existence, the external layer of the body forming a kind of 

 case within which the soft central protoplasm is enclosed. Their 

 desiccated bodies, or their similarly desiccated germs, may thus be 

 carried through the atmosphere from place to place, and may at aiiy 

 moment pass into a condition of active life, if they should meet with 

 favourable conditions. Hence, whenever water containing organic 

 matter in solution is freely exposed to the air, it will almost cer- 

 tainly be found to be charged with a larger or smaller number of 

 these minute organisms. It is for this reason that the name of 

 Infusoria was originally applied to this group of the Protozoa ; it 

 having been observed that " organic infusions" — 2.e., water in which 

 an animal or vegetable substance had been boiled — very commonly 

 came to contain Infusorian Animalcules after exposure to the air 

 for a limited period. 



Morphologically, the Infusorian Animalcules are unicellular ani- 

 mals, consisting of a primitive mass of soft protoplasm, provided 

 with an outer membranous wall, and furnished with one or more 

 nuclei. Very usually the outer wall of the body is pierced by a 

 definite oral opening ; and they are sometimes for this reason collec- 

 tively spoken of as the "stomatode," or mouth-bearing. Protozoa. 

 There are, however, certain groups of the Infusoria in which there 

 cannot be said to be any proper mouth. 



There are two principal groups of the Infusoria, known as the 

 Ciliated lufusorians and the Flagellated Infusoiians ; the former 

 (fig. 17, A, B, C) having the external surface covered with vibrating 

 cilia, in part or wholly ; whilst the latter (fig. 1 7, D and E) have one or 

 more long lash-like filaments or " flagella," and are usually destitute 

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