WORMS. 



187 



boscis worms, a group which, entirely without justification, was formerly included 

 with the plathelminths. Now intervene the Gephyreans. Finally we reach the 

 annelidan type, beginning with the Carchiannelides and leading off to the Chaetopods 

 and Myzostoma, the Echiurids and JBalanoglossus, and to the Discophori. 



Polyoliseta. 



(Ecliiuridue.)' 



Oligochaeta. 



(Myzostoma.) 



(Balauoglossus.) 

 Discophori. 



ArcLiannelida. 



Gepbyrea. \ 



Clisetosoma. Merinis. 



Desmoscolex. j Nematod, 



Sagitta. 



(Eohinoderes.) 

 Gastrotriolia. 



Nemertea. 



Kotifera. 



Hypothetical Protohelminth. 



Fig. 163. — Diagram of tlie natural affinities of worms. The exact relationship of those forms which 

 are enclosed iu parentheses is doubtful. 



Class I. — PLATHELMINTHIA. 



In fresh and salt waters, and even on moist earth, are found the free living species ; 

 in nearly every animal are found some of the parasitic species of this huge and varied 

 class of worms. The members of the group offer but little resemblance with one 

 another as regards their external form ; one would not be led by their appearance to 

 place a planarian, a fluke, and a tape-worm in the same natural division, but the inter- 

 nal organization of them all most clearly demonstrates the closeness of their natural 

 affinities. A great difference in the ways of life is corresponded to by an equal dif- 

 ference anatomically. On the one hand are the non-parasitic forms, roughly called 

 the planarians, which require, and therefore have, well-developed organs of sense, 

 good means of defence against attacking enemies, perfected means of locomotion, all 

 the paraphernalia of independent existence; on the other hand are the parasites. 



