12 Laboratory Guide in Zoology 



THE CLASSIFICATION OF THE PROTOZOA 



The lowest branch of the animal kingdom is the Pro- 

 tozoa. The members of this branch are nearly all aquatic, 

 microscopic animals of very simple structure. They con- 

 sist of a single cell, or in some cases of an association of 

 cells, which are not, however, distinctly differentiated into 

 tissues. The animals of this branch consist essentially of 

 bits of protoplasm, each capable of absorbing food and 

 reproducing its kind. Food is taken into the body ; its 

 digestible portions are assimilated, and its indigestible 

 parts are rejected. The food so utilized may be either 

 animal or vegetable in its nature. 



The Protozoa may be reproduced by fission, a process of 

 simple division ; by budding, a process in which a small 

 portion of the animal is divided off, separating from the 

 parent cell, and finally developing into a new organism ; 

 and by conjugation, a sexual process by which in certain 

 forms two or more individuals fuse together, become en- 

 cysted, and later split up into a large number of spores, 

 each of which develops into an adult organism. There 

 are various modifications of each of these processes. 



The Protozoa are now divided by leading authorities 

 into five classes, namely : — 



I. Rhizopoda. 

 II. Mycetozoa. 



III. Mastigophora. 



IV. Sporozoa. 

 V. Infusoria. 



The members of the lowest class of Protozoa, the 

 Rhizopoda, are usually characterized by the presence of 

 an outer layer of protoplasm which frequently sends out 



