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 (Tenia) (Fig. 108), 
which lives parasitically in the intes- 
tine of man, is a good example of 
one of these classes. “It has the fa 
form of a narrow ribbon, which may [qU! 
attain the length of several yards, 
attached at one end to the wall of we. 108,—Tape-worm (Tenia 
the intestine, the remainder hanging um). In upper left- 
7 5 a . hand corner of figure the 
freely in the interior.” Its body 18 head tauch:. magnified, 
composed of segments or serially After Leucxanr. 
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 
