SECTION II 

 PHYLUM PROTOZOA 



In the preceding section we learnt the essential structure of an 

 animal cell, and it was pointed out that in the lowest organisms 

 the entire individual consists of a single cell. All such unicellular 

 animals are placed in the lowest primary subdivision of the animal 

 kingdom — the phylum Protozoa. 



We have also learnt that cells vary considerably in character. 

 They may be ainceboid or capable of protruding temporary processes 

 of protoplasm called pseudopods ; fiagellate, or produced into one 

 or more — always a small number — of threads having an intermit- 

 tent lashing movement ; ciliated, or produced into numerous 

 rhythmically moving threads of protoplasm ; or encysted, the proto- 

 plasm being enclosed in a cell- wall. Moreover, under certain 

 circumstances, amoeboid cells may fuse with one another to form 



These well-marked phases in the life of the cell allow us to 

 divide the Protozoa into subdivisions called Classes. The same 

 organism may be amoeboid, flagellate, encysted, and plasmodial 

 at various stages of its existence, but nevertheless we find 

 certain forms in which the dominant phase in the life-history is 

 amceboid, others which are characteristically flagellate or ciliated, 

 others again in which the tendency to form plasmodia is a 

 distinctive feature. In this way five well-marked groups of 

 unicellular organisms may be distinguished. 



Class 1. Rhizopoda. — Protozoa in which the amceboid form is 

 predominant, the animal always forming pseudopods. Flagella 

 are often present in the young, and occasionally in the adult. 

 Encystation frequently occurs. 



Class 2. Mycetozoa. — Terrestrial Protozoa in which the plas- 

 modial phase is specially characteristic, as also is the formation 

 of large and often complex cysts. 



Class 3. Mastigophora. — Protozoa in which the flagellate form 



