Platyhelminthes. 



349 



mesoderm, and endoderm. Many of them are parasites, 

 and some wholly lack a digestive tube. When present 

 the digestive tube has but one opening, the mouth. They 

 show a tendency to reproduction by self-division, and most 

 of them when cut in two develop two individuals. 



The Tapeworm. Probably the most widely known of 

 the flatworms are the tapeworms. These are parasites in 

 the digestive tube of various verte- 

 brates, including man. As the name 

 indicates, the body is ribbon-shaped, 

 sometimes attaining a length of thirty 

 feet. The body consists of segments, 

 or proglottids, a tapeworm ten feet 

 long having about eight hundred 

 segments. There is no mouth nor 

 digestive tube ; none is needed, as the 

 worm lives surrounded by material 

 digested by another animal, and the 

 parasite simply absorbs nourishment 

 through its skin. There is a distinct 

 head, whose chief work is that of at- 

 taching the worm to the lining of the 

 intestine ; this is secured by a circle 

 of hooks at the end of the head and 

 four sucking disks on the sides. For 

 a short distance from the head the body is unsegmented ; 

 then segments are formed by constrictions at intervals ; 

 farther on, the segments grow larger. 



Development of the Tapeworm. The hinder segments 

 of the tapeworm contain embryos. These segments drop 

 off and the embryos are set free, passing out with the 

 excrement. They are eaten by another animal, the hog, 

 for instance; in the intestine the embryo bores through the 



FIG. 196. TAPEWORM 

 ( Tcenia solium) . 



In upper left hand corner of 

 figure the head much mag- 

 nified. After Leuckart. 

 From Jordan and Kel- 

 logg's Animal Life. 



