VT PHYLUM NEMATHELMINTHES 315 



AFFINITIES AND MUTUAL RELATIONSHIPS OF THE 



NEMATHELMINTHES. 



The affinities of all the classes of Nemathelminthes are very 

 obscure, and the propriety of grouping them into a single phylum 

 is extremely doubtful. They all agree in being elongated, cylin- 

 drical worms with a body-cavity, which is sometimes of the nature 

 of a true crelome ; there is a certain resemblance between Nematoda 

 and Chsetognatha in the muscular system ; and the lemnisci of 

 Acanthocephala have been compared with the cervical glands of 

 Nematoda. Beyond these points there is little to unite the three 

 classes ; and, on the other hand, the proboscis of Acanthocephala 

 recalls the rostellum of Cestoda. 



Very various views have been put forward as to the affinities of 

 the Chsetognatha. But, in the absence of adequate evidence of 

 any near relationship with higher phyla, they may be regarded as 

 having their nearest known relatives, even if very remote, in the 

 Nemathelminthes. In connection with this question, the Chseto- 

 somidse, briefly described in the Appendix (p. 313), seem to 

 require consideration. Other possible relationships suggested by 

 the mode of development of the coBlome from hollow diverticula of 

 the archenteron and by other features will be referred to in later 

 sections. 



The three families placed as an Appendix to the phylum present 

 some undoubted resemblance to the Nematoidea : this is especially 

 the case in the reproductive organs of the Ch^tosomatidee, and still 

 more in those of Desmoscolex. But the segmentation of the body 

 in both Desmoscolecidse and EchinoderidaB and the presence of 

 setae show a certain resemblance to higher worms or Annulata, 

 which will be more fully appreciated when that phylum has been 

 studied. 



