CLASSIFICATION AND GENERAL CHARACTERISTICS 473 



mals of every class. Generally they are harmless, but some varieties 

 may produce pathological changes and even fatal diseases severely 

 epidemic. 



As their name indicates, they are all characterized by reproduction 

 through spore formation, but they exhibit the utmost diversity of struc- 

 tural and developmental characteristics. As a rule, each 'species is 

 parasitic on one kind of tissue of a particular species of host. They 

 are generally taken into the system in the spore stage either (1) with 

 the food of the host, (2) by the bites of insects, or (3) by inhalation. 

 The spore membranes are dissolved by the fluids of the host, and thus 

 one or more germs or sporozoites are set free to bore into the special cells 

 of the host. Here they grow, some remaining intracellular permanently, 



FIG. 142 



Diagram of variations in life cycle of flagellates : 1, a young flagellate ; 2, adult flagellate ; 3, lon- 

 gitudinal division of adult free form ; 4, daughter flagellate ; 5, encystation ; 6-8, division into 

 isogametes ; x and z, division into macrogametes and microgametes, characteristic for some forms ; 9, 

 conjugation of the isogametes ; y, conjugation of the macrogametes and microgametes ; 10, resting- 

 stage zygote ; 11-12, division into young. (After Doflein.) 



others only in the young stages. The latter either pass different phases 

 of their more or less complicated life history in different parts of the body 

 of one and the same host, or they pass some phases of their life cycle in 

 the cells of an intermediate host. 



The sporozoa vary widely in size as well as in other characteristics. 

 From the smallest, several of which can be contained in a single blood 

 cell, there are all gradations in size up to those that may be seen by 

 the naked eye (Prospora gigantea, 16 mm.). 



Besides being characterized by the power to produce more or less 

 resisting spores, the sporozoa are also characterized by the fact that as 

 a class they possess none of the special organs found in other protozoa 

 for ingesting or digesting solids. Many develop flagella or show amoeboid 



