ANIMAL PARASITISM 



435 



finds its optimum conditions in the human intestine, is adaptable 

 enough to survive under the very different chemical conditions found 

 in the dog, while Taenia saginata is usually unable to survive under 

 these conditions. 



PARASITES AND THE GROUPS IN THE 

 ANIMAL KINGDOM 



What kinds of animals are infested by parasites? Not only all 

 phyla and classes, but all known species and probablj^ all individuals 

 of higher forms serve as hosts for some kind of parasite. Even in 



Fig. 180. — Giardia lamblia, an intestinal flagellate. A^ face view ; B, semiproflle 

 view ; C, cyst. (Reprinted by permission trom Introduction to Human Parasitology 

 by Chandler, John Wiley and Sons, Inc.) 



the microscopic Protozoa many individuals harbor still smaller proto- 

 zoans. For example, several species of parasitic Protozoa are found 

 in Amoeba proteus. 



To which of the main groups of animals do parasites belong? All 

 animal phyla, except Echinodermata, include some species which 

 lives as parasites, but the great majority of parasites belong to one 

 of these four phyla: Protozoa, Platyhelminthes, Nemathelminthes, 

 and Arthropoda. 



Protozoa. — Of the four classes in this phylum, one, Sporozoa, is 

 entirely parasitic; the other three (Sarcodina, Mastigophora, In- 



