808 



ANIMALS AND THEIR ENVIRONMENT 



m^,,/m///////m/////////////////////////////////////////////////^^^^^ 



Figure 39.9. Hookworm. A, Longitudinal section through head of adult showing 

 mouthful of intestinal wall being sucked. Eggs («) pass out in the host feces, hatch 

 in the soil (C) and grow to the infective stage (D). These penetrate the host skin 

 and migrate by way of the blood, lungs, and throat to the small intestine. {A after 

 Ash and Spitz; others after Chandler.) 



host and undermining his health year after year. Two species are com- 

 mon in the small intestine of man, Ancylostoyna duodenale and Necator 

 americanus (Fig. 39.9). The adult gathers a bit of intestinal lining in 

 its mouth and sucks blood from the capillaries. These are one-host 

 parasites with a free-living larva. Eggs pass out in the feces and hatch 

 in the soil, where the larvae develop to the infective stage. Once on 

 the host they bore through the skin into the blood, are swept through 

 the circulatory system to the lungs, where they burrow into the air 

 cavities, crawl up the bronchial tubes, and are swallowed. In warm, 

 moist climates where people are often barefoot, hookworms are common 

 and contribute greatly to the lethargy, indifterence and poverty of man. 

 In recent years the prevalence of hookworm in southeastern United 

 States has been greatly decreased through improved health habits and 

 economic status. 



364. Parasites in Body Tissues 



Parasites that live within the tissues of the host may enter through 

 the skin or from the digestive tract. Some of these feed upon the tissues; 

 others lie among the cells and are saprozoic. The two largest and most 

 important groups are the trypanosomes (class Flagellata) and the blood 



