NEREIS 



195 



sckr 



Another flatworm is such an abject parasite that it has lost 

 most of the organs usuail}- possessed by worms. This is the 

 tapeworm (Fig. 191). When the eggs of the tapeworm are 

 taken into the body of an herbivorous animal, the embryos 

 develop there for a while and then stop. 

 When flesh containing these embryos is 

 eaten by a carnivorous animal, the em- 

 bryos attach themselves to the food canal 

 of their host by means of hooks and 

 sometimes also by means of four suckers. 

 The head thus constituted (called scolex) 

 produces, by rapid growth, a long chain 

 of segments, each of which is full of germs. 

 The chain, or " tape," absorbs fluid food, 

 which soaks through its body-wall. As 

 the segments at the older end of the ani- 

 mal mature, they are set free and pass out 

 of the alimentary tract to be ])icked up, 

 perchance, in the food of an herbivorous animal or else to 

 perish. 



The tapeworms are remarkable because they show to what 

 extremes of degeneration a parasite may go. They have 

 lost so fundamental an organ as the food canal. All com- 

 plex sense-organs and organs of locomotion are lacking, and 

 the worms have become mere sacs of reproductive cells. 

 This is the usual tendency of internal parasites, — to multi- 

 ply enormously their means of reproduction so as to increase 

 the chance of infecting another host. On the other hand, 

 the sense-organs, brain and nervous system, and muscles 

 tend to degenerate because the parasite has little use for 

 them. 



62: cr 



Fig. 189. — Distomum, 

 the liver-fluke. Nat. 

 size. Excr, e.xcretory 

 pore ; mo, mouth ; 

 repr, reproductive ap- 

 erture ; sckr, posterior 

 sueker. l^rom Parker 

 and Haswell. 



