PARASITIC FLATWORMS 367 



have a well-developed and functional digestive system; in Mermis 

 the system is active during early life and becomes inert and de- 

 generate in the adult stage. Finally in the Acanthocephala there 

 is no trace of an alimentary system at any stage in the life-history. 

 The gains are no less marked. Hold-fast organs, like suckers and 

 hooks, enable the parasite to maintain its position against the con- 

 stant and vigorous movements of the host. Such organs of simi- 

 lar structure appear in widely separated groups, e.g., suckers in 

 flukes and threadworms. 



While these structural likenesses between parasitic worms of 

 different groups are striking and important, they are in a real 

 sense superficial and do not serve to conceal more than tempo- 

 rarily the fundamental differences in structure between the various 

 groups. 



The flatworms (Plathelminthes) are soft-bodied, usually elon- 

 gate and somewhat flattened forms. In the phylum are included 

 the free-living Turbellaria (Ch. XII) and Nemertina (Ch. XIV), 

 as well as two classes of parasitic worms : the Trematoda or flukes, 

 and the Cestoda or tapeworms. The other three classes of para- 

 sitic worms named previously are grouped together under the 

 phylum Nemathelminthes or roundworms, which forms the topic 

 of a separate chapter. The structure of each group will be dis- 

 cussed separately, but certain biological features are general enough 

 to deserve brief mention first. 



Aquatic animals possess some external parasites; among them 

 the species of ectoparasitic flatworms, rare in fresh water, belong 

 to a single subdivision of the flukes or Trematoda; all other flukes 

 and the Cestoda which are all parasitic live as endoparasites in 

 some part of the host organism where they find better protection 

 than on the surface. The most common place of residence is the 

 alimentary canal or its adnexa, air-bladder, lungs, liver, etc. 

 Parasites occur regularly in the body cavity and other serous 

 spaces, in the kidney and bladder, in the sex organs, in the heart 

 and blood vessels, encysted in the skin, connective tissue and 

 muscle, and finally in the nervous system, even entering the eye 

 or brain and its cavities. 



Parasites may be collected by opening an animal in a dissecting 



