432 The Animal Kingdom 



in the intestine of the host. The young larva is some 200 to 300 microns 

 in length. It does not remain in the intestine for long, and soon bur- 

 rows its way through the intestinal mucosa and into the small capil- 

 laries of the intestinal wall. For about ten days the larva remains 

 in the circulatory system, possibly spending most of this time in the 

 portal system where the supply of food is particularly rich. Finally, 

 the larva makes its way through the liver and heart and arrives at 

 the lungs. By the time it has reached the lungs, it has grown to 

 ten times its original length. Here the larva burrows through the lung 

 tissue and makes its way into the throat and trachea. Then it is 

 swallowed by the host and finally once again arrives at the intestine. 

 The young worm is some 2 to 3 mm. in length when it arrives in the in- 

 testine the second time, and after some two or three months, attains its 

 full size. Its life span is about one year, during which period the 

 female is constantly producing eggs. 



The greatest damage done to the host by these ascarids is during 

 the period when the larvae pass from the lungs. The breaking through 

 of the delicate lung tissue causes hemorrhaging and a pneumonia-like 

 condition if there is a heavy infection. Heavy experimental infections 

 may be fatal, but ordinarily the infection is not that severe. In pigs, 

 the symptoms which occur during this passage of the larvae from the 

 lungs are known as "thumps." In the intestine, the worms may do 

 some damage by injuring the mucosa. Heavy infections may also be 

 sufficient to interfere with the passage of food down the intestine. 



Children are often particularly susceptible to ascarid infections 

 as they are very careless in their defecation and eating habits. Where 

 proper conditions prevail, ascarids are present in all regions of the 

 world. Infections are particularly abundant when people are care- 

 less in their habits and the soil is warm and moist. 



Other species of Ascaris are parasitic in horses, cats, dogs, and 

 poultry. The life histories are similar to the above in most respects. 



The Hookworms. — Another nematode infection of human beings 

 which is of much greater economic importance than Ascaris is that 

 caused by the hookworms. Two human species are known : Ancylos- 

 toma duodenale which is prevalent in Europe, North Africa, Western 

 Asia, and northern China; and Necator aniericanus (Fig. 141,6") which 

 is found throughout the tropical and subtropical regions of the world. 

 Both species are much alike in their habits and appearance, with N. 



