CHAPTER XXV 

 THE PROTOZOA 



WHAT we learned in Chapter IX regarding the minute animals 

 that cause disease such as malarial fever should be enough to 

 teach us the importance of the one-celled animals, the Protozoa, 

 in the affairs of men. Such species as that which causes malaria 

 are, however, not suitable for laboratory study since they can 

 hardly be seen even with a compound microscope and are not 

 available in a living condition. On the other hand, there are 

 many animals consisting of a single cell which can be procured 

 in abundance by simply bringing pondweeds or dry hay into the 

 laboratory, placing it in a shallow dish, covering it with water, 

 and then allowing it to decay for a few days. The scum that 

 collects on the surface of such a " culture " or " infusion " will 

 be found to contain many kinds of Protozoa, and the surround- 

 ing water will swarm with other species. 



Paramecium. The best species to begin with is the slipper 

 animalcule, Paramecium caudatum. Paramecia are large 

 enough to be seen with the naked eye if a proper background is 

 provided. They are cigar-shaped animals with a depression 

 called the oral groove (Fig. 132, o.g) extending from the forward 

 end obliquely backward. The mouth (m) is situated near the 

 end of this oral groove. 



The motile organs are thin, threadlike cilia which beat back 

 and forth and propel the animal forward or backward, and draw 

 food particles into the mouth. 



Just beneath the surface is a layer of spindle-shaped cavities 

 filled with a semifluid substance. These are called trichocysts 



218 



