68 FLATWOEMS, ROUNDWORMS, AND ROTIFERS 



of mature eggs have been given off from the body. The 



eggs are carried away into ditches and waste places, where 



they may be eaten by pigs. The embryos 



soon hatch from the eggs in the intestines 



of the pigs, and immediately bore their 



way through the tissues to the muscles of 



the animal. In this situation the embryo 



changes into a flask-shaped form, known 



as the larva (Fig. 35). The presence of 



these larval tapeworms in the muscles of 



the pig give to the pork an appearance 



that causes it to be known as "measly 



pork." When such pork is eaten raw or 



insufficiently cooked, the larvee soon de- 



FiG. 35.— Larva velop into mature tapeworms in the di- 



of tapeworm. gestive tract of man. 



It is best to cook pork thoroughly before eating it, 



although tapeworms are not usually fatal to life. This rule 



of prevention also apphes to the beef tapeworm. 



Roundworms 



On the whole the members of this group are more worm- 

 like than the flatworms. Their bodies are, in general, 

 cyUndrical but unsegmented. Many of the roundworms 

 are parasitic. Perhaps the small worms so often seen in 

 strong vinegar and known as "vinegar eels" are the most 

 famihar examples of the roundworms. The nematode 

 worms that live in soil and attack the roots of certain plants, 

 causing galls to form on the roots, are also more or less 

 famihar. To this group also belong the so-called "hair 

 worms," or "thread worms" (Fig. 36). These long slender 

 worms are often found in drinking troughs and so much 



