VERMES (WORMS). 



Under this heading are included a large number of forms 

 commonly known as worms, but which are incapable of 

 strict definition. In general it may be said that they have 

 elongate bodies, without internal skeleton, without appen- 

 dages, with a marked bilateral symmetry, and distinct 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. Some of these 

 worms are terrestrial, some aquatic, and some live as para- 

 sites on or in other animals. Omitting a number of micro- 

 scopic forms and small groups, we may divide the Vermes 

 into four classes : Plathelminthes or flat- worms, Nemathel- 

 minthes or round-worms, Annelids or segmented worms, and 

 Molluscoidea. 



CLASS I. PLATHELMINTHES (FLAT-WORMS). 



In the flat-worms the body is flattened, is without appen- 

 dages or skeleton; the mouth when 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 

 mouth. Aside from the digestive cavity, the body is solid 

 throughout, there being no such body-cavity as we have seen 

 in all forms hitherto studied. The nervous system consists 

 of a centre or ' ' brain, " always in the dorsal front portion of 

 the body, from which nerve-cords run to various parts, there 



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