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The Flatworms — Platyhelminthes 



When the average person hears the word "worm" he may think of 

 fish bait or slimy, crawling creatures that are quite repulsive to many 

 people, those of the feminine sex particularly, and he may use the word 

 to express his contempt for some spineless, repulsive person by calling 

 him a worm. To the zoologists, however, the word has a much broader 

 meaning; three of the great phyla of the animal kingdom are given over 

 to worms and they are known to be extremely abundant with hundreds 

 of important species and of very great economic importance. 



This first phylum of worms is called the flatworms because the body 

 is flattened in contrast to the next phylum of worms which has a rounded 

 body. 



Symmetry of Flat Worms 



An examination of the worms of this group reveals that their bodies 

 consist of two sides, right and left, each of which is practically a duplica- 

 tion of the other except that the placement of parts is exactly reversed. 

 We might say that one half is like a mirror image of the other half. Such 

 an animal can be cut in only one body plane to give two halves approxi- 

 mately alike, since the two ends of such animals are different also. We 

 call such an arrangement bilateral symmetry and we will find this body 

 plan in nearly all the animals yet to be studied. 



Animals with this type of symmetry have six body directions or sur- 

 faces. If you think of a dog as a typical bilaterally symmetrical animal 

 we can easily describe these. The head end, or front, of the dog is ante- 

 rior, while the tail end, or rear, is posterior; the back, or the surface 

 which is up, is dorsal, while the abdomen, or the surface which is down, 

 is ventral; and, finally, the sides are right lateral and left lateral. In 

 order for these directions to apply to man he would have to get down on 

 his hands and knees, but the plan of symmetry is the same. Whenever 

 we have only one structure, such as a nose, it is placed in the median 

 plane of the body ; if we have two, such as eyes, they are placed in the 

 same spots on opposite sides of the body. There is some force at work 

 which correlates the two sides of the body and they will be balanced. 



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