PARASITISM AND DEGENERATION 



183 



protozoan is as simple as an animal's body can be, being 

 composed of but a single cell, degeneration can not occur 

 in the cases of these parasites. There are, besides Grega- 

 rina, numerous other parasitic one-celled animals, several 

 kinds living inside the cells of their host's body. One 

 kind lives in the blood-corpuscles of the frog, and another 

 in the cells of the liver of the rabbit. 



97. The tape-worm and other flat-worms. In the great 

 group of flat-worms (Platyhelminthes), that group of ani- 

 mals which of all the principal animal groups is widest 

 in its distribution, perhaps a major- 

 ity of the species are parasites. In- 

 stead of being the exception, the 

 parasitic life is the rule among these 

 worms. Of the three classes into 

 which the flat -worms are divided 

 almost all of the members of two of 

 the classes are parasites. The com- 

 mon tape-worm (Tcenia) (Fig. 108), 

 which lives parasitically in the intes- 

 tine of man, is a good example of 

 one of these classes. " It has the 

 form of a narrow ribbon, which may 

 attain the length of several yards, 

 attached at one end to the wall of 

 the intestine, the remainder hanging 

 freely in the interior." Its body is 

 composed of segments or serially 

 arranged parts, of which there are 



about eight hundred and fifty altogether. It has no mouth 

 nor alimentary canal. It feeds simply by absorbing into 

 its body, through the surface, the nutritious, already di- 

 gested liquid food in the intestine. There are no eyes 

 nor other special sense organs, nor any organs of locomo- 

 tion. The body is very degenerate. The life history of 

 the tape-worm is interesting, because of the necessity of 



FIG. 108. Tape-worm (Tcenia 

 solium). In upper left- 

 hand corner of figure the 

 head much magnified. 

 After LETJCKABT. 



