CHAPTER XIII 

 PARASITIC FLATWORMS 



By henry B. ward 



Professor of Zoology in the University of Illinois 



The parasitic worms do not all belong to a single systematic 

 division. Coming in many cases from widely separated groups, 

 they often show much closer relationship to certain free-living 

 forms than to each other. But because of a likeness in manner 

 of life these forms were grouped together by early students of ani- 

 mal life as the Helminthes and in fact were long regarded as related 

 by reason of similarities in appearance and habit. There are five 

 such groups, usually ranked as classes; they are Trematoda or 

 flukes, Cestoda or tapeworms, Nematoda or roundworms, Acan- 

 thocephala or proboscis-worms, and Gordiacea or hair-worms. 



In any given host only a few parasitic species may be found or 

 again the number of individuals and species of parasitic worms in 

 a single host may be very large. I have taken 5000 flukes from a 

 single fish {Amid), and even larger figures are recorded. At a 

 given time the variety of species may be Hmited; yet as the kinds 

 of parasites change with the food, the season, and the region, the 

 total number found in a certain host may be very large; thus over 

 one hundred species of parasitic worms are reported from man 

 and thirty or forty from some well-known and widely-studied fish 

 or aquatic birds. Some parasites are found in more than a single 

 host species, a few infest a wide range of animals, and others occur 

 in one host only; all in all, parasites are far more numerous than 

 free-living animals both in number of individuals and of species. 



The abundance of parasites varies greatly under different con- 

 ditions of existence. Desert animals are not without them, but 

 they are much more numerous and more varied in water-living 

 animals than in hosts from any other habitat. 



Representatives of some or all groups of parasites occur in the 

 various aquatic vertebrates and invertebrates, and while in a 



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