438 



ESSENTIALS OF ZOOLOGY 



COCCUS granulosus, a dog and wolf tapeworm whose larval stage is 

 the cause of a horrible human disease. 



Nemathelminthes. — The single class Nematoda includes at least 

 95 per cent of the species in this phylum ; most of them are free- 

 living, but there are also thousands of parasitic species. Examples 

 of parasitic species are the human hookworms, Necator americanus, 

 the American hookworm, and Ancylostoma duodenale, the Old World 

 hookworm ; Ascaris lumhricoides, the large intestinal roundworm of 

 hog and man ; Dracunculus medinensis, the Guinea worm, often over 

 a yard long, which crawls around under the human skin (believed 

 by some to be the ''fiery serpent'' mentioned in Exodus) ; Trichinella 

 spiralis, which causes the often fatal human disease, trichinosis, when 



Fig. 183. — Balantidium coli, an infusorian parasite of the intestine. Active form 

 from intestine. c.v., anterior contractile vacuole; cyt., cytostome ; f.v., food 

 vacuole; n, nucleus. (Reprinted by permission from Introduction to Human Para- 

 sitology by Chandler, John Wiley and Sons, Inc.) 



its larvae, encysted in pork, are eaten by man; and Wuchereria han- 

 crofti, the filaria which is injected into the human blood by certain 

 tropical mosquitoes and causes elephantiasis, a disease in which the 

 infected limbs may become larger than the body of the victim. 

 Onchocerca 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 Acanthocephala, 

 or thorny-headed worms, are common intestinal parasites of many 

 vertebrates, including the hog and occasionally man ; the Gordiacea 



