646 - Heredity and Evolution 



and reach the muscles, where they form a 

 multitude of new cysts. In man, the rare cases 

 of heavy infection (overt trichinosis) involve 

 fever and a very painful swelling of the 

 muscles, with slow recovery. Moreover, if the 

 site of infection includes the diaphragm 

 muscle, breathing may be impaired, even to 

 the point of fatality. 



With elephantiasis, on the other hand, the 

 larval filarians are introduced into the blood 

 stream by mosquitoes of the genus Citlex. 

 Reaching the lymph glands, the larvae ma- 

 ture into adult females about 8 cm long, and 

 males about half that length. In the lymph 

 nodes, the sexually produced larvae may be- 

 come very numerous. Such an accumulation 



Mouth 



Fig. 32-22. Fossilized shells of an extinct brachiopod, 



Sptrifera vanuzem/. Extinct species tor outnumber all 

 modern surviving brachiopods. (Courtesy of the Ameri- 

 can Museum of Natural History, New York.) 



coagulant present in the mouth secretions of 

 the worm. Consequently grave anemia may 

 result in heavily infected persons. 



Trichinosis, caused by the trichina worm 

 (Trichinella spiralis), and elephantiasis, 

 caused by a filarial worm (Wuchereria ban- 

 crofti) are diseases of much rarer occurrence, 

 especially in this country. However, mild in- 

 fections of trichina, in which scarcely any 

 symptoms are noticed, may occur in some 

 15 percent oi the United States population. 



Human infections of trichina may result 

 from eating insufficiently cooked (pink) pork. 

 Cysts (Fig. 32-24) containing dormant trichina 

 larvae may be present in the skeletal muscles 

 of the pig (also of rats, dogs, cats, and certain Fig. 32-23. A nemotode, or "roundworm," diagram 

 bears). Liberated by the digestive enzymes of sh ° win 9 internal structures. A rich variety of semi- 



., i i , ■ . „ i, u microscopic nematodes live in the soil. Most nematodes 



the new host, each larva grows into an adult , ,. . , , , 



1 are tree-living harmless creatures, but a few are para- 



that lives for a while in the host's intestine, sites of considerab | e viru | ence (see text) . (From Gen . 



producing a new crop of larvae. Then the eral Zoo/ogy, by Miller and Haub. Holt, Rinehart and 



active larvae penetrate into the blood stream Winston, Inc.) 



