THE LIVER-FLUKES, TAPE- WORMS, ETC. 73 



and this in turn still another embryo so that three generations 

 of embryos are present one within the other. 



Tape-worms. The tape-worms are the most common and 

 the best known of the flat worms. There are many species, 

 the adults of all of which live in vertebrate animals. But there is 

 almost always an alternation of hosts during the life of the 

 parasite, the larval tape-worm living in one animal and the 

 adult in another. In the larval stage the tape-worms may 

 occur in various parts of the body of the intermediate host, 

 but the adult or fully developed worm always occurs in the 

 alimentary canal of the final host. Many of the domestic 

 animals suffer from these parasites. At least ten different 

 species of tape-worms have been found in the dog, the inter- 

 mediate hosts including rabbits, sheep, and other animals 

 that the dog may feed on. Many of the domestic fowls are 

 infected by tape-worms, whose intermediate hosts are insects 

 or small aquatic crustaceans, like Cyclops. 



Several kinds of tape-worms infest man. Tania solium, 

 whose intermediate host is the pig, may serve as an example 

 of the group. The adult worm is attached to the inner wall of 

 the intestine of man by a number of fine hooks with which the 

 small head is provided. The long, ribbon-like, symmetrical 

 body lies free in the alimentary canal, where it absorbs the 

 liquid food directly through its thin body-wall. The parasite 

 has neither mouth nor alimentary canal. The body may 

 reach a length of many feet and be composed of as many as 

 850 segments, or proglottids. Each proglottid produces both 

 sperm cells and egg cells, and as these become mature the 

 posterior proglottids drop off one by one and pass out of the 

 alimentary canal with the excreta. If some of these escaped 

 proglottids are eaten by a pig the embryos issue from the eggs, 

 bore through the walls of the alimentary canal of the host, and 

 make their way to the muscles, where they increase greatly in 

 size and develop into a rounded sac filled with liquid. In this 

 stage they are large enough to be readily seen, and the infected 

 spotted meat is called "measly pork." If such infected pork 

 is eaten by man without being cooked sufficiently to kill the 

 parasites, the young tape-worms will attach themselves to the 



