EFFECTS OF PARASITE 203 



and is carried to a position in some organ. Here it re- 

 mains until the rabbit is eaten by a dog and the liber- 

 ated worm attaches itself to the intestinal coat, grows, 

 lays eggs and thus completes the cycle ; in the hookworm, 

 a small threadworm inhabiting the intestine of man, the 

 eggs escape from the host, develop to a considerable 

 degree in the ground, and the young worms then bore 

 through the skin on the soles of the feet of barefooted 

 laborers. Entering the blood, they are transported to 

 the lungs, where they bore their way through the wall, 

 make their way to the throat, and are swallowed. In 

 the intestine they adhere by hooks, and complete the 

 cycle of development. 



Effects of Parasite on Host. — In the case of 

 an animal whose skin is pierced by the proboscis of a 

 mosquito the injury and loss of body substance is unim- 

 portant. Even when a parasite, such as a tapeworm, is 

 a permanent resident, the injury due to its attachment 

 and absorption of nutritive materials is not of any great 

 importance. It may happen, however, that the slight 

 break in the skin or wall of the digestive tract or in the 

 outer surface of plants may serve as a point of entry for 

 other parasites, such as bacteria, molds, and various one- 

 celled animals^ whose effects may be of a most serious 

 character. In some cases the secondary parasites may be 

 carried in the proboscis of a fly, flea, or mosquito and, 

 passing through the skin when this is pierced, cause 

 plague, sleeping sickness, malaria, and many other terri- 

 ble scourges. Also there are many known cases of para- 

 sites occurring in such great numbers that the blood 

 vessels of the host have become clogged to a serious and 

 even fatal extent. Finally, there are many parasites, 

 both plant and animal, whose wastes, liberated in the 

 body of the host, cause some of the most serious diseases 

 afflicting man and the domesticated animals. 



