r>AR.\SITISM AND DEGEXEllATIOX 



35.' 



o 



has no mouth nor uhmcntary canaL It fccils sinii)ly Ijy ab- 

 sorbing into its body, through tlie surface, tlio nutritious, 

 ah-eady digested hc^uid food in the intestine'. '^J'here are no 

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

 tion. The l:)ody is very degenerate. The hfe history of tlie 

 tapeworm is interesting, because of tlie necessity of two hosts 

 for its completion. The eggs of the ta])eworm ])ass from the 

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

 of some other animal iiu order to de- 

 velop. In tiie case of one of the several 

 species of tapeworms that infest man, 

 this other host must be the ])ig. In 

 the ahmentary canal of the j)ig 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. 



Fig. 212. — T.npeworm, 

 TivniasoUutn. In the 

 m)per Icft-haiul cor- 

 ner is the much en- 

 larged licail. (After 

 Leuckart.) 



In a lake in Yellowstone Park the 

 suckers are infested by one of the flat- 

 worms {Ligida) 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 lie in- 

 fested by a parasite of fifty ])ounds' weight. 



Another group of animals, many of whose members are 

 parasites, are the roundworms or threadworms (Xemathel- 

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

 organized animals, but the parasitic kinds all sliow a greater 

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

 sites of man is a roundworm called TricJiiun spiralis (Fig. 2Ki). 

 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, 



