CHAPTER II 



THE SUBKINGDOM PROTOZOA 



The Protozoa are sundered from the rest of the Animal Kingdom by 

 a perfectly sharp distinction. The distinction consists in this : that in 

 the body of a protozoon, whether there be one nucleus, or a few, or 

 many, no nucleus ever has charge solely of a specialized part of the 

 cytoplasm; whereas in other animals there are always many nuclei, 

 each in charge of a portion of cytoplasm which is specialized for a 

 particular function, such as contraction, or conduction, or secretion. 



Stated thus, the definition of the Protozoa is quite unambiguous. 

 Unfortunately, ambiguity is usually imparted to this subject by the 

 introduction of a concept, that of the "cell", which has a different 

 extension for different authorities. If that concept, primarily of use 

 in other connections, is to be introduced here, we must frame our 

 definition in one of two ways, according to the meaning which we 

 attach to the word "cell". If we apply this term to every nucleus 

 together with its cytoplasm, we must define the Protozoa as " animals 

 which consist of one cell or of several cells which are all alike, save 

 sometimes the reproductive cells ". If, on the other hand, we give the 

 term "cell" its earlier extension, applying it only to the specialized 

 units of nucleus and cytoplasm which together compose the bodies 

 of the higher animals and plants, we shall define the Metazoa as 

 "cellular animals" and the Protozoa as "non-cellular". It will then 

 be convenient to employ the term "energid" for application to any 

 nucleus with its cytoplasm, whether they together constitute the body 

 of a protozoon or a cell of a metazoon. 



In any case the facts remain the same, and they provide one of the 

 main sources of the interest which the study of the Protozoa offers, 

 namely the carrying out of the processes of life, and often of a complex 

 life, by an organization which, though it may visibly be of correspond- 

 ing complexity, is purely cytoplasmic. Considered in this light the 

 structure of, for instance, the more complicated ciliates and flagellates 

 is exceedingly instructive. In three other respects the Protozoa are 

 peculiarly interesting. In their bodies dead " formed " material, how- 

 ever plentiful it be as a covering or scaffold for the body, never as- 

 sumes the importance which it has as ground substance or skeleton in 

 the Metazoa, where the size of the body is such that the protoplasm 

 cannot maintain its organization without support against forces that 

 tend to deform it. Consequently, in observing the physiology and 

 behaviour of a protozoon, we are seeing in the actual protoplasm of an 



