PARASITISM AND DEGENERATION 



355 



lias no mouth nor alimentary canal. It feeds simply by ab- 

 sorbing into its body, through the surface, the nutritious, 

 already digested liquid food in the intestine. There are no 

 eyes nor other special sense organs, nor any organs of locomo- 

 tion. The body is very degenerate. The life history of the 

 tapeworm is interesting, because of the necessity of two hosts 

 for its completion. The eggs of the tapeworm pass from the 

 intestine with the excreta, and must be taken into the body 

 of some other animal in order to de- 

 velop. In the case of one of the several 

 species of tapeworms that infest man, 

 this other host must be the pig. In 

 the alimentary canal of the pig the 

 young tapeworm develops and later 

 bores its way through the walls of the 

 canal and becomes imbedded in the 

 muscles. There it lies, until it finds its 

 way into the alimentary canal of man 

 by his eating the flesh of the pig. In 

 the intestine of man the tapeworm con- 

 tinues to develop until it becomes full 

 grown. 



In a lake in Yellowstone Park the 

 suckers are infested by one of the flat- 

 worms (Ligula) that attains a size of 

 nearly one fourth the size of the fish in 

 whose intestines it lives. If the tape- 

 worm of man attained such a compara- 

 tive size, a man of two hundred pounds' weight would be in- 

 fested by a parasite of fifty pounds' weight. 



Another group of animals, many of whose members are 

 parasites, are the round worms or threadworms (Nemathel- 

 minthes). The free-living roundworms are active, well- 

 organized animals, but the parasitic kinds all show a greater 

 or less degree of degeneration. One of the most terrible para- 

 sites of man is a round worm called Trichina spiralis (Fig. 213). 

 It is a minute worm, from one to three millimeters long, which 

 in its adult condition lives in the intestine of man or of the pig 

 or other mammals. The young are born alive and bore through 

 the walls of the intestine. They migrate to the voluntary 

 muscles of the hosts, especially those of the limbs and back, 



FIG. 212. Tapeworm, 

 Tcenia solium. In the 

 upper left-hand cor- 

 ner is the much en- 

 larged head. (After 

 Leuckart.) 



