m PHYLUM NEMATHELMINTHES 321 



between Nematoda and Chaetognatha in the muscular system ; 

 and the leinnisci 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 ot 

 the Chaetognatha. 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 

 Nemathelminlhes. In connection with this question, the Chaeto- 

 somidae, briefly described in the Appendix (p. 319), seem to 

 require consideration. Other possible relationships suggested by 

 the mode of development of the ccelome 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 Chaetosomidae, and still 

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

 in both Desmoscolecidae and Echinoderidae 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. 



VOL. I 



