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CHAPTER III. 



NATURE AND ORGANIZATION OF THE INFUSORIA. 



PROCEEDING to the more immediate consideration of the special group or 

 groups of organisms that form the subject-matter of this volume, it is in the 

 first place scarcely requisite to observe that the title of the Infusoria as 

 employed from the date of its earliest introduction up to the present time 

 has carried with it a most wide and indeterminate meaning. Formerly 

 utilized for the distinction of almost every microscopically minute aquatic 

 organism, whether belonging to the animal or vegetable series, it is found 

 to embrace for the most part in its more modern application several highly 

 differentiated classes or sections of the sub-kingdom Protozoa, and in some 

 cases, even yet, organisms whose true position should undoubtedly be among 

 the representatives of the vegetable world. In accordance with the views 

 held by the present author, the Infusoria as a group, even when restricted to 

 forms exhibiting a decided animal organization only, scarcely possesses an 

 intrinsic or coherent status, embracing as it does, though incompletely, 

 representatives of all four of the primary natural sections of the Protozoa 

 that have been previously enumerated. Adapted, however, as closely as 

 possible to meet existing exigency, this same group or legion, as it may 

 be conveniently denominated, corresponds as here embodied most closely 

 with those three classes of the Protozoa included in the preceding tabular 

 view of this sub-kingdom under the titles of the Flagellata, Ciliata, and 

 Tentaculifera. In other words, it comprehends, with the exception of 

 the typical Rhizopoda and two subordinate Flagellate orders of the 

 Spongida and Mycetozoa, the whole of the representatives of the Protozoa. 

 But for the limits of space at command, the first, if not the second, of these 

 two last-named orders would likewise have been admitted and described 

 in extenso on equal terms with their associated groups ; its individual repre- 

 sentatives, as explained at length in Chapter V., conforming in all essential 

 structural and developmental details with those of that special order here 

 distinguished by the name of the Choano-Flagellata. From the evidence 

 already submitted, it is clearly apparent that the Infusoria, from whichever 

 point of view selected, can be regarded as irregularly gathered excerpts 

 only from that primary subdivision of the animal kingdom known as the 

 Protozoa, and that no correct estimate of the affinities nor definition of the 

 characters of its multitudinous representatives can be accomplished apart 

 from their consideration as constituent integers of this one harmonious 



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