268 VERMES. 



(pp. 105-109), the Cestodes are closely united to the Trematodes, and 

 the Acanthocephali to the Nematodes ; and this is not only a super- 

 ficial resemblance, but concerns the development and the anatomical 

 arrangement of the intestinal organs. In certain cases the resemblance 

 is so great that it is difficult to determine with certainty the syste- 

 matic position of the animal. Thus we know of Helminths which 

 seem intermediate types between Cestodes and Trematodes such as 

 Ampliiptyches and AmphUiiie. The connecting links between Ne- 

 matoda and Acanthocephali, though not so perfect, are not wholly 

 wanting (Gordius). 



Thus we can distinguisli two great groups of Helminths which 

 have been named according to their most conspicuous external 

 characteristic respectively Playthelminthes (Platyhelmia) and Nema- 

 thelminthes (Nemathelmia). 



We must not, however, forget that these two groups do not repre- 

 sent definite unities, any more than do the Entozoa or Helminths. 

 The intestinal worms have originated, as we have shown (p. 89 et seq.\ 

 by the adaptation of originally free-living forms to a parasitic environ- 

 ment, and both groups are in various ways connected with free-living 

 relatives. The natural system has attempted to express this relation 

 by associating the Platyhelminthes with the Turbellaria and Hirudinea, 

 and Nemathelminthes with the other free-living worms ; in other 

 words, by distinguishing two classes of worms, which include both 

 free and parasitic species, and which are not unfitly distinguished by 

 their characteristic forms as flat-worms (Platodes) and round-worms 

 (Annelides). I will indeed grant that there may be a difference of 

 opinion as to the limits, and even justifiability, of these two classes. 

 This is especially so in regard to the second class, which includes 

 forms widely differing from one another besides the Nemathelminthes 

 and Chcetognatki (Sagitta), also the Gephyrea and Ckcetopoda. The 

 position of the Hirudinea among the flat-worms has also become 

 doubtful (p. 107). 



But all these are questions of subordinate importance for us here. 

 We have to discuss not the systematic position of worms, but their 

 structure and life-history. The above remarks will serve as a general 

 introduction as to the relations of these form?. 



