CHAPTER VI 



TYPE IV. VERMES 



The term Vermes (Lat. vermis, a worm), or worms, no longer 

 has a definite scientific significance. Tt is nothing more than a 

 popular name given to more or less widely different animals, 

 which generally have a shape commonly called vermiform, and 

 it is only in this popular sense that we can use the word in 

 zoology to-day. Used in this way, however, it is a convenient 

 term for keeping together several groups which, at present at 

 least, are better held in this loose relationship than scattered 

 about. At the same time it should be remembered that many 

 zoologists discard the name entirely, while others apply it to a 

 part of the group. A worm is pretty readily distinguished from 

 the animals we have considered up to this point by its striking 

 bilateral symmetry, and its sharply differentiated anterior and 

 posterior ends, — the head containing a mass of nerve cells, 

 which is commonly called the brain. From the types which we 

 shall study after the Vermes, this type is distinguished chiefly by 

 negative characteristics, i.e. by the absence of structures which 

 occur in the others. The Vermes, through one or another of 

 their several subtypes, show relationships to almost all the larger 

 groups of animals, relationships in part structural, but for the 

 most part embryological, — by means of their larvae. Beyond 

 these few points little can be said in behalf of the type Vermes 

 as a whole, and it is necessary to divide it into several subtypes, 

 which by many zoologists are considered distinct types. 



SUBTYPE I. PLATYHELMINTHES 



The Platyhelminthes (Gr. irXarix;, flat, and eXfiiv^, worm) are 

 for the most part distinctly flattened worms, although there are 

 some flat worms which we can no longer place in this group. 



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