334 ESSENTIALS OF ZOOLOGY 



piece will replace the missing parts about as the process is carried 

 out in fission. A piece from the middle of the animal will regener- 

 ate a head portion at the anterior margin and a tail portion at the 

 posterior margin. 



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. 



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. 



References 



Hegner, R. W. : Invertebrate Zoology, New York, 1933, The Macmillan Company. 

 Hyman, L. H.: Studies on the Morphology, Taxonomy, and Distribution of 



North American Triclad Turbellaria, 1928-31, Tr. Am. Micro. Soc, "Vols. 



47 and 50. 

 Ward, H. B., and Whipple, George C. : Fresh-AVater Biology, New York, 1918, 



John Wiley & Sons, Inc. 



