132 INVERTEBRATE ZOOLOGY 



from penetrating and injuring them. There are evidences that 

 eggs of Ascaris may remain ahve in the soil for a period of 5 to 6 

 years. This adds very greatly to the difficulty of eradicating 

 Ascaris. 



Extensive migrations are also carried on in the body of the host 

 by the hookworms (Ancylostoma of the Old World and Necator 

 of the New World). The adult worms, which cause very serious 

 loss of blood and affect the entire body of the host, occur in the 

 intestine of man where the thin-shelled eggs are produced and 

 eliminated along with fecal matter. In the soil, the young 

 worms hatch and feed for a while on the fecal matter. A new 

 skin forms beneath the old one, which is finally shed, and the 

 larva enters upon the second period of its life. After another 

 molt, the larva is ready to infest a new host individual. This it 

 does either by entering the body along with contaminated water 

 or food or by active penetration of the skin. Hands and feet 

 are the chief inroad for the larvae, which attack the skin exposed 

 to the soil and follow along hair follicles or between epidermal 

 scales to the lymph spaces. Once in the lymph stream they are 

 carried passively to the subclavian, thence to the heart, and on 

 with the blood stream to the lungs. Here the larvae leave the 

 capillaries, enter the air sacs, and wander through the bronchi, up 

 the trachea into the esophagus, and down through the stomach 

 to the intestine where they become attached to the wall by 

 means of their specially adapted sucking mouth. About 7 

 to 10 weeks from the time the larva enters the skin, eggs begin 

 to appear in the feces, indicating that the worms have reached 

 maturity and the life cycle is thus completed. 



Still a different condition is found in the life cycle of Trichinella 

 spiralis. There are two distinct stages in the parasitism by this 

 worm: the sexually mature worm which occurs in the digestive 

 tract and the encysted larvae in the muscles. Both sexes occur 

 in the intestine of man and of other mammals of which the pig 

 and rats are the most important hosts. The gravid female 

 pierces the wall of the intestine with her posterior extremity. 

 With the genital pore inserted in a lacteal vessel, the young are 

 liberated and carried by the lymph and blood streams into the 

 tissues of the host's body. It is not known to what extent this 

 migration is active or passive. Upon reaching muscular tissue, 

 the larvae become encysted and all further development is 

 contingent upon the muscles containing the encysted larvae 



