PHYLUM III. VERMES (WORMS). 



Under this heading are included a large number of forms 

 commonly known as worms, a group incapable of strict 

 definition. In general it may be said that they have 

 elongate bodies without internal skeleton , without jointed 

 appendages, with a marked bilateral symmetry, and dis- 

 tinct dorsal and ventral surfaces. Further than this we 

 can hardly go in a definition which will at once include all 

 worms and at the same time not include other forms. 

 Indeed, it is probable that the group is not a natural one 

 and that its members should not be associated together. 

 Still in an elementary work it is best to follow a con- 

 servative course and not confuse the beginning student 

 with a number of phyla some of which contain but a few 

 inconspicuous forms. Some worms are terrestrial, some 

 aquatic, ^and some live as parasites on or in other animals. 

 Omitting a number of microscopic forms and small groups, 

 we may divide the Vermes into four classes: Plathel- 



fc/ 



minthes, or natworms; Nemathelminthes, or round worms; 

 Annelids, or segmented worms, and Molluscoidea. 



CLASS I. PLATHELMINTHES (FLATWORMS). 



In the natworms the body is flattened, is without appen- 

 dages or skeleton; the mouth wKen present is on the ven- 

 tral surface and no vent occurs. There is no body-cavity 

 aside from the digestive tract. Some are leaf-like, others 

 are more elongate, and a very few are nearly cylindrical. 

 The free-living and some of the parasites have an alimen- 

 tary canal, but to this there is only a single opening, the 



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