CHAPTER V 

 PLATYHELMINTHES. FLAT-WORMS 



In older texts the group name "Vermes" was used to include all of 

 the worms. In modern books the worms are usually placed in three or 

 more phyla the Platyhelminthes or flat-worms, the Nemathelminthes 

 or round- worms, the Annulata or segmented- worms; and sometimes 

 the curious little worm-like rotifers are placed in a phylum of their 

 own, though they and some other worms are of uncertain affinities. 

 The first three of these groups will be taken up in order, as each has 

 considerable economic importance. 



The flat-worms, though a lowly group, are more highly specialized 

 than the ccelenterates. They have three .germ layers, the mesoderm 



forming important systems of 

 organs muscles, urogenital organs, 

 etc. They are bilaterally instead 

 of radially symmetrical and, as the 



FIG. i 9 .-Planaria. A common name indicates are of a distinctly 



fresh- water flat-worm. X5- J 



flattened form. Their internal 



structures are difficult to make out in the laboratory, and vary con- 

 siderably in different species, being highly degenerate in some parasitic 

 forms. 



The flat- worms are usually divided into the three following classes : 



I. Turbellaria, Fig. 19, free living forms with ciliated ectoderm; found 

 in fresh and salt water, rarely on land. Represented by the familiar 

 genus Planaria of our ponds and pools. 



II. Cestoda, Fig. 20, long, much flattened forms divided into 

 numerous (sometimes many hundred) more or less independent 

 segments, no alimentary canal present; parasitic in habits. Repre- 

 sented by the common tapeworms. 



III. Trematoda, Fig. 21, usually short and broadly flattened, 

 with ventral suckers and much branched digestive and reproductive 

 organs. Parasitic in habits. Represented by the liver and other 



flukes. 



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