CHAPTER X 



THE CILIATA— PARAMECIUM AND VOETICELLA 



Though the Protozoa which we have studied present many 

 differences of form and habit, none of them show any great 

 complexity of structure. The Volvocinse, indeed, seem to form 

 an exception to this statement, but even in them the unit 

 of organisation, the flagellate cell, is simple enough. We have 

 now to consider a group of which the individual members, 

 whilst retaining the characters of a single cell, attain to a very 

 considerable degree of complication, and may be spoken of as 

 being highly differentiated. But this elaboration of structure, 

 be it clearly understood, is the outcome of specialisation of the 

 protoplasm composing the cell-unit, and as such is different 

 from that other mode of elaboration which, as we have seen in 

 the case of the frog, is the outcome of the aggregation of many 

 cells differing in kind and interwoven so as to produce tissues 

 and organs of great complexity. In other words, the cell-body 

 of the ciliate Infusorian is a microcosm ; parts of it are 

 differentiated for the performance of the several vital functions, 

 whereas, in the higher multi-cellular animals, each component 

 cell of a tissue is specialised for the performance of only one of 

 those functions. Thus we may say that in the Ciliata the cell 

 attains its greatest complexity of visible structure. 



The species known as Paramecium caudatum and Parame- 

 cium aurelia are most conveniently selected for the study of 

 ciliate structure and reproduction. Both are equallycommon and 

 may nearly always be found in stagnant pools containing dead 

 leaves and other decaying organic matter. Usually they are 

 not very abundant in pools, but large quantities of them may 

 easily be reared by collecting confervse and water-weeds in 

 summer weather, placing them in a jar of rain-water covered 

 over by a gla^s plate and leaving them to rot. For the first two 

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