CHAPTER THIRTY-ONE 



PLATYHELMINTHES 



I. IT was formerly customary to include under the The Vennes 

 name Vermes^ a great variety of different organisms, UJ^JJ? 8 

 roughly classed as worms. These did not. of course, writers 

 include the so-called worms which are the larvae of in- 

 sects, but they did include such creatures as rotifers, 

 which would not usually be thought of as worms at all. 

 It could be said of the Vermes that they were bilaterally 

 symmetrical, usually greatly elongated or "vermiform," 

 with a distinct body cavity between the intestine and 

 outer wall. They could thus be excluded from the 

 ccelenterates and echinoderms, while the absence of 

 distinct jointed appendages served to indicate that they 

 were not Arthropods. In recent works these Vermes or 

 Vermidea have been divided, so that today we recognize 

 flatworms, nemertines, threadworms, rotifers, and anne- Fiatworms 

 lids, forming a series of phyla. The flatworms are 

 called Platyhelminthes, which is an exact Greek transla- 

 tion of the English term. The Greek word for a 

 worm appears in many other combinations, and a stu- 

 dent of worms calls himself a helminthologist, while a 

 society for the study of worms is a helminthological 

 society. 



The flatworms, as the name suggests, are more or less Groups of 

 flat and usually ribbonlike. They are usually divided flatworms 

 into three classes, the Turbellaria, Trematoda, and Ces- 

 toda; but a fourth division is indicated by a group of 

 peculiar animals living on the outer surface of fresh- 

 water Crustacea, turtles, etc., to which the name Temno- 

 cephaloidea has been given. The Turbellaria or plana- 



1 Our word "worms" is a strict equivalent. There is no "W" in Latin: 

 thus the Latin vallum is our "wall." 



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