ANIMAL PARASITISM 745 



are the human intestinal flagellate, Giardia lamhlia (Fig. 386), and 

 the blood-inhabiting trypanosome, Trypanosoma rhodesiense, caus- 

 ative agent of African sleeping sickness which is carried by the 

 Tsetse fly, Glossina (Fig. 388). Examples of parasitic Infusoria are 

 the human intestinal ciliate, Balantidium coli (Fig. 389), the various 

 species of Opalina, and related genera found in the excretory bladder 

 or cloaca of frogs and toads. Of the thousands of species of Sporozoa, 

 all of which are parasitic, probably the best known are the three 

 species of the genus Plasmodium (Fig. 393), which cause human 

 malaria, and Babesia higemina, which produces Texas tick fever of 

 cattle. 



Platyhelminthes. — This phylum also contains four classes, two of 

 which, Trematoda (flukes) and Cestoda (tapeworms), are all para- 

 sitic, while the other two, Turbellaria and Nemertinea, are mainly 

 freeliving but contain some species which are parasitic on aquatic 

 invertebrates. Among the best known examples of Trematodes are 

 Fasciola hepatica (Figs. 398 and 399), the sheep liver fluke; Clon- 

 orchis sinensis (Fig. 397), the Chinese human liver fluke; and Schis- 

 tosoma Jiaematohium, one of the three species of human blood flukes. 

 Probably the best known tapeworms are Taenia saginata (Fig. 402), 

 the beef tapeworm. Taenia solium, the pork tapeworm and Diphyllo- 

 hothrium latum, the broad fish tapeworm, all three common parasites 

 of the human intestine, and EcJmiococcus granidosus, 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 (Figs. 394 

 and 395), the Old World hookworm; Ascaris lumhricoides (Fig. 90), 

 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 

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

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

 tropical mosquitos and causes elephantiasis, a disease in which the 

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



