CHAPTER XVII 



THE INFUSORIA 



THE term Infusoria had originally a much wider application than 

 at present, being used to denote the various microscopic animalcules 

 which make their appearance in infusions exposed to the air. Hence 

 the Infusoria included any Protozoa, and even organisms distinct 

 from them, such as Rotifers. Just as the word " insect " has been 

 restricted in its zoological application to a single class the Insecta 

 Hexapoda so the term Infusoria has become narrowed down to 

 denote the Infusoria Ciliata and Suctoria, which constitute, taken 

 together, one of the most definite and sharply-marked classes of the 

 Protozoa, characterized by two principal structural features : first, 

 the possession of cilia during the whole or a part of their active life ; 

 secondly, the differentiation of the nuclear apparatus into a vegeta- 

 tive macronucleus and a generative micronucleus (p. 153). 



The Infusoria fall naturally into two subclasses : the Ciliata 

 proper, in which the cilia are retained throughout life ; and the 

 Acinetaria or Suctoria, in which cilia are present only during 

 the early or larval phases of the life-history, and are lost in the 

 adult organism, which is of sedentary habit, and in which food- 

 capture is effected by special organs suctorial tentacles. 



SUBCLASS I. CILIATA. 



The Ciliata, the most abundant and familiar of microscopic forms 

 of life, may be considered in a sense the highest of the Protozoa, 

 since in no other class does the cell-body attain to so great a com- 

 plication of parts and organs or to so high a degree of structural 

 differentiation. Not even in the Metazoa are single cells to be 

 found of such visibly complicated structure, since in the Metazoa 

 the cell is specialized usually for one particular function of a living 

 body, while in the Ciliata the single cell performs all the functions 

 of life. Moreover, the differentiation of the nuclear apparatus into 

 generative and vegetative portions may be considered analogous 

 with, and parallel to, the differentiation of gennen and so ma in the 

 Metazoa ; and Lewin (172) regards the micronucleus as living inde- 



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