One-Celled Animals — Phylum Protozoa 



The phylum Protozoa includes the one-celled animals. With only one 

 cell for each body, it is evident that these animals must be quite small. Be- 

 cause of their small size they are not well known to the average person, since 

 they can be seen in detail only by the comparatively few people who have 

 access to a good microscope. With the aid of this valuable instrument, 

 however, we find that these one-celled forms are quite numerous. A small 

 pond of stagnant water may appear to be lifeless and uninteresting, but a 

 drop of water from such a pond, when viewed under the microscope, be- 

 comes a miniature aquarium teeming with life of great variety. The be- 

 ginning students of zoology never fail to marvel at the activities of this new 

 world of life which the microscope reveals to them. Animals of many 

 shapes and sizes dart back and forth ; others move sluggishly across the 

 field of vision ; some smaller forms may be ensnared and devoured by 

 larger forms ; life in all its complicated reactions and relationships goes on 

 here as it does among the larger animals. 



The protozoa are sometimes spoken of as the simplest forms of animal 

 life, and certainly they are simple in body organization when compared with 

 the complex body of a larger animal with its many different kinds of cells. 

 On the other hand, it must be remembered that all forms of animals must 

 carry out certain vital life processes, and all of these processes must be ac- 

 complished by a single cell in the case of the protozoa. In the multicellular 

 forms of animal life there can be a distribution of tasks among the different 

 cells — some cells can function in digestion, some in excretion, some in repro- 

 duction, etc. The one cell of a protozoan, however, must function in all of 

 these activities. It must act as a nerve cell when it receives stimuli — it must 

 function as a muscle cell when it reacts to the stimuli — it must produce all 

 of the enzymes necessary for digestion — it must take care of its own inges- 

 tion, egestion, excretion, etc. In this sense then the protozoa must be con- 

 sidered as very complex organisms. A protozoan cell will be more complex 

 physiologically than any single cell taken from a large animal such as man. 



At first thought one might think that the protozoa could be of little eco- 

 nomic importance — they are so small — how could their activities be of any 

 great concern to man? They are, however, of considerable economic im- 

 portance. Many of them serve as food for larger water animals which, in 



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