CONTROLLABLE DISEASE AGENTS 



333 



certain snails about our pas- 

 tures, the protection of the 

 sheep is a comparatively simple 

 matter. 



249. The tapeworm. The 

 name tapeworm is applied 

 to several species of flat- 

 worms of the genus Tcenia 

 (Fig. 156). It has a com- 

 paratively simple structure, 

 consisting of hardly more 

 than a series of flat sacs 

 containing excretory tubes 

 and reproductive organs, 

 with an anchoring organ, or 

 holdfast, at the end of the 

 series; but the life history 

 is somewhat complex, since 

 it includes adaptations for 

 finding two different hosts 

 in each generation. There 

 are three or four species of 

 tapeworms that inhabit the 

 human intestine. The tape- 

 worm can be dislodged and 

 driven out by medical treat- 

 ment (though sometimes 

 with great difficulty), but 

 its entry into the body can 

 be more easily prevented 

 by making sure that our 

 pork and beef and fish are 

 thoroughly cooked, since in 

 the resting stage in the mus- 

 cles of the secondary host it 

 is easily killed by the heat. 



Fig. 156. The tapeworm 



The adult stage, a, lives in the intestine of 

 a human being and may grow to be many 

 yards long. The segments, b, that make up 

 the worm are hardly more than flat, thin- 

 walled sacs that absorb food from the sur- 

 rounding liquid in the intestine and discharge 

 eggs and sperms into it. The so-called head, 

 c, of the worm is an organ of attachment, 

 made up chiefly of hooks and suckers; im- 

 mediately back of this head new segments 

 are constantly being produced. The seg- 

 ments farthest from the attachment are there- 

 fore the oldest — and largest. The fertilized 

 egg hatches into a larva in a sort of capsule 

 and is passed out of the body of the host 

 with the feces. When taken into the body of 

 another animal — a pig, for example — the 

 larva works its way into the muscles of the 

 new host, e, and settles into a resting stage 

 for an indefinite time. When the flesh of this 

 animal is eaten by a human being, the young 

 worm projects the attaching head from the 

 capsule, d, and fastens itself to the lining of 

 the intestine 



