THE WORMS 49 
their internal organization, so closely resembling that of 
the free-living forms as to need no further description, in- 
dicates that they are 
descendants of the lat- 
ter. In the greater 
number the body is 
flat, and a few species 
still retain their outer 
coat of cilia ; but since 
these are no longer of 
service as locomotor 
organs they have gen- 
erally disappeared, and 
in their place numer- 
ous adhesive organs, 
such as spines, hooks, 
and suckers (Fig. 30), 
have arisen, which en- 
able the animals to 
hold on with great te- 
nacity. Thus attached { \ 
to its host, and using 
Fie. 30.— A parasitic flatworm (Epidella). m 
it as a convenient and mouth ; 0, opening of reproductive system ; 
comparatively safe 8, sucker and spines for attachment. The di- 
, gestive system is stippled; nervous system 
means of locomotion, black. Enlarged 8 times, from Nature. 
the parasite may still ‘ 
continue to capture small animals for food or may derive 
its nourishment from the tissues of the host. In addition 
there are numbers of internal parasites, living almost ex- 
clusively in the bodies of vertebrate animals, scarcely a sin- 
gle one escaping their ravages. 
48. Life history.—In the external parasites the young 
hatch out and with comparative ease make their way to 
another host; but the young of an internal parasite, inhab- 
iting the alimentary canal, have a very slight chance in- 
deed of ever reaching a similar location in another host. 
