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TEXTBOOK OF ZOOLOGY 



Onciiocerca volvulus, transmitted by certain biting flies, is a com- 

 mon cause of blindness in some parts of Mexico. Besides the 

 human nematodes there are thousands of others parasitizing lower 

 animals, both vertebrates and invertebrates. The other two classes 

 of the phylum Nemathelminthes are entirely parasitic; the Acan- 

 thocephala, or thorny-headed worms, are common intestinal para- 

 sites of many vertebrates, including the hog and occasionally man; 



Fig. 390. — Elephantiasis, some extreme cases. A, of legs and feet; B, of 

 scrotum ; C, varicose groin gland ; D, of scrotum and legs ; E, of mammary glands. 

 (Reprinted by permission from Introduction to Human Parasitology by Chandler, 

 published by John Wiley and .Sons, Inc. A and B sketched from photograps from 

 Castellani and Chalmers; C, D, and E from Manson.) 



the Gordiacea or horsehair worms (Fig. 88) are parasites of insects 

 until nearly mature; they crawl out of their insect hosts when the 

 latter fall into water, become sexually mature, and lay their eggs. 



Arthropoda. — All of the classes in this phylum are predominantly 

 free-living, but several classes also include parasitic species. The 



