THE INVERTEBRATES 



147 



second. The eggs are deposited by the free adult female in 

 slender strings twisted around the stems of water-plants. 

 The young hairworm on hatching sinks to the bottom of 

 the pond, where it moves about hunting for a host in which 

 to take up its abode. 



The terrible Trichina spiralis, which produces the disease 

 called trichinosis, is another roundworm of which much is 

 heard. This very small 

 worm lives in its adult con- 

 dition in the intestine of man 

 as well as in the pig and other 

 mammals. The young, 

 which are born alive, burrow 

 through the walls of the 

 intestine, and are either car- 

 ried by the blood, or force 

 their way, all over the body, 

 lodging usually in the mus- 

 cles. Here they form for 

 themselves litde cells or 

 cysts in which they lie. 

 The forming of these thou- 

 sands of tiny cysts injures 

 the muscles and causes 

 great pain, sometimes death 

 to the host. Such infested 

 muscle or flesh is said to be "trichinosed," and the flesh 

 of a trichinosed human subject has been estimated to con- 

 tain 100,000,000 encysted worms. To complete the devel- 

 opment of the encysted and sexless Trichinae the infested 

 flesh of the host must be eaten by another animal in which 

 the worm can live, e.g., the flesh of man by a pig or rat, 

 and that of a pig by man. In such a case the cysts being 

 dissolved by the digestive juices, the worms escape, develop 

 reproductive organs and produce young, which then migrate 



Fig. 60. Tapeworm; head, magni- 

 fied, at left; the whole worm 

 may be several yards long. 

 (After Leuckart.) 



