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 

 it as a convenient and 

 comparatively safe 

 means of locomotion, 

 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. 



FIG. 30. A parasitic flatwonn (Epidella). m 

 month ; o, opening of reproductive system ; 

 s, sucker and spines for attachment. The di- 

 gestive system is stippled ; nervous system 

 black. Enlarged 8 times, from Nature. 



