CHAPTER VIII 



THE MICROSCOPIC ANIMALS— PROTOZOA 



Types of Protozoa 



In samples of water taken from a pool or pond and examined 

 under the microscope there are quantities of actively moving 

 forms of life which quickly catch the eye. Many of these, in- 

 deed nearly all of the brightly colored ones, are the unicellular 

 plants which were considered in the preceding chapter. Others 

 are protozoa, one-celled animals, and these represent forms re- 

 lated to what was probably the vanguard of the long procession 

 of animal types culminating in man. Some of them have no 

 motile organs, but creep about on the bottom, and afford a fasci- 

 nating picture of protoplasmic moving, e.g., Ameba. Others 

 move with microscopic rapidity, turning on their axes or pro- 

 gressing in a zigzag course through the water. The motile organs 

 of these forms, called flagella, are, owing to their fineness and 

 to their active movements, difficult to see, even under very power- 

 ful lenses, but are revealed by adding a drop of iodine to the 

 preparation. They extend from the forward (anterior) end of 

 the cell into the water as one, two, or more exquisitely fine, hair- 

 like processes. In life they undulate from base to tip and at the 

 same time describe a cone the apex of which is the point of inser- 

 tion in the body. In still another type of protozoa the movement 

 is more regular, but is also a combination of rotation and for- 

 ward progression. In these the surface of the cell may be seen 

 to be covered with a coating of beating cilia or with a conspicuous 

 zone of cilia at the anterior end. These, like flagella, are readily 

 demonstrated by the use of iodine. 



These forms, ordinarily found in stagnant water, represent 

 three of the four great groups or types of organization of the 

 protozoa — Mastigophora (whip-bearing), Sarcodina (from 

 sarcode, an early term for protoplasm), and Infusoria (living in 

 infusions). The fourth type, Sporozoa, are not found in water, 



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