CHAPTER II. 



THE SUB-KINGDOM PROTOZOA. 



THE contents of the preceding chapter constitute a brief chronological 

 summary of the more important advances gained in our knowledge of the 

 Infusoria from the date of their first discovery by Leeuwenhoek up to the 

 present time. A comprehensive survey of the organization and affinities of 

 the members of this zoological group, as illumined by the light of recent 

 research, has now to be proceeded with. 



As an initial step in this direction, a short space must, however, be first 

 devoted to a consideration of that larger subdivision of the animal kingdom, 

 of which as a whole the Infusoria are most generally and here definitively 

 accepted as a constituent group or groups. This subdivision, the Protozoa 

 of Von Siebold, or Archezoa of Max Perty, has undergone much modi- 

 fication at the hands of biologists since its first institution in the year 1845. 

 Great diversity of opinion exists, even at the present day, with respect to 

 the delimitations both of its own borders and those of the minor sections 

 and orders into which it may be most conveniently and naturally sub- 

 divided. As here accepted, the sub-kingdom Protozoa may be defined 

 as embracing all those forms of life referable to the lowest grade of the 

 animal kingdom, whose members are for the most part represented by 

 organisms possessing the histologic value only of a single cell, or of a con- 

 geries or colonial aggregation of similar independent unicellular beings. 

 In such cases as Opalina and other multinucleate forms, in which from the 

 compound character of the nuclear or endoplastic element the organism 

 would appear to be composed of several cells, these cells are indistinguish- 

 ably fused with each other, and have not allocated to them separate func- 

 tions or properties as in all more highly organized multicellular animals or 

 Metazoa. 



The essential body-substance of all Protozoa consists of apparently 

 homogeneous, or more or less conspicuously granular, slime-like sarcode or 

 protoplasm, all organs of locomotion or prehension consisting of simple 

 or variously modified prolongations of this element. The food-substances 

 ingested by the Protozoa may be incepted by a single well-defined oral 

 orifice or cytostome, or there may be a plurality of such apertures. Among 

 the Rhizopoda and many Flagellata, on the other hand, such material may 

 be indefinitely received at any point of the periphery, while in yet a fourth 

 series, chiefly endoparasitic such as the Opalinidae there is no oral 



