PROTOZOA 



71 



for these are obligatory parasites* living in the fluids and cavities 

 of all kinds of animals. We will discuss the first three types in 

 this chapter, but the Sporozoa will be considered in a later chap- 

 ter (see page 105). Altogether there are upward of 15,000 

 known species of protozoa, varying in size from half a micron to 

 half an inch (Porospora, a parasite of the Norwegian lobster) 

 for single individuals with one nucleus, and up to 2\ to 3 inches 

 for individuals with many nuclei (Nummulites) . 



Animal Flagellates (Mastigophora) 



Characteristics 



The typical form of an animal flagellate is ellipsoidal, or 

 elongated in the direction of movement (Fig. 37). Its proto- 

 plasm is usually somewhat denser on the outside, thus forming 

 an indefinite cortical zone rather than a definite membrane. 



Fig. 37— COLLODICTYON 



A highly voracious animal flagellate 

 with four flagella 



From a photomicrograph by the author 

 Magnification, 720 



Closely attached to it, however, on the outside is a delicate, usu- 

 ally invisible, lifeless covering known as the periplast or pellicle. 

 This, indeed, is present in all protozoa. In a strict sense, there 

 are no naked forms of protozoa. 



The smallest and the simplest of the flagellates are the most 

 important in relation to man. Here the derived organization 

 (see page 38), typical of the Mastigophora, is seen in its sim- 



* Obligatory parasites cannot live under any but parasitic conditions. 



