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TEXTBOOK OF ZOOLOGY 



face, size, chemical content of internal organs, etc. (intestinal para- 

 sites of birds are seldom found in mammals, which have lower body 

 temperatures, and parasites adapted to the oxygen-rich interior of 

 a frog's lung can find no suitable habitat in a lungless fish) ; and 

 (3) ability of the parasite to adapt itself to the wide range of en- 

 vironmental conditions found in different hosts; thus D. latum, 

 though it finds its optimum conditions in the humaji intestine, is 

 adaptable enough to survive under the very different chemical con- 

 ditions 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 probably all individuals 

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



Fig. 386. — Giardia lamblia, an intestinal flag-ellate. A, face view; B, semlprofile 

 view; G, cyst. (Reprinted by permission from Introduction to Hmnan Parasitology 

 by Chandler, published by John W^iley and Sons, Inc.) 



the microscopic Protozoa many individuals harbor still smaller 

 protozoans. 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 



