174 TEXTBOOK OF ZOOLOGY 



Economic Relations of the Phylum 



The planarians and other free-living flatworms are of practically 

 no economic importance, but the phylum includes a large number 

 of forms, principally Trematodes and Cestodes, which are parasitic 

 in higher vertebrate animals, including man. Such groups as the 

 intestinal flukes, liver flukes, lung flukes, blood flukes, pork tape- 

 worm, beef tapeworm, margined tapeworm of dog, gid worm, hy- 

 datid worm, common tapeworm of dog, chicken tapeworm, dwarf 

 tapeworm, sheep tapeworm, tapeworm of horse, and fish tapeworm 

 are all serious parasites. They cost many thousands of dollars and 

 much debility each year. A more complete discussion of this topic 

 will be found in the chapter on Animal Parasitism. 



Phylogenetic Advances of Platyhelminthes 



(1) Anteroposterior principal axis, (2) bilateral symmetry, (3) a 

 distinct third germ layer, the mesoderm, (4) an excretory system 

 of flame cells, (5) central nervous system extending with the axis 

 of the body, (6) specialized gastrovascular cavity, and (7) perma- 

 nent sexual reproductive organs. 



