THE OHJITOGNATHA, OR PRIMITIVE MOLLUSOA. 359 



Gegeubaur (1856) aud of Pterhelminthes of Harting 

 (cii-c. 1865). 



In 1857 a fantastic theory of Vertebrate affinity was put 

 forward by Meissuer. It has, however, failed to find further 

 support than that accorded to it by Haeckel. Metschnikoff 

 (1867) included the Chtetognatha in his new group of 

 Ambulacrata, together with the Echinodermata, Brachiopoda, 

 and Bnteropneusta. This view has been supported by 

 Biitschli, who, while believing the Chgetognathato have some 

 affinity to the Annelida, considered that the method of the 

 development of the hypoblast is a sufficient justification for 

 regarding them as being near akin to the Echinodermata 

 and to the Tunicata; still more recently (1876) he has 

 proclaimed the Brachiopoda to be their nearest relations. 



A revival of the Molluscan theory is due to Langerhaus 

 (1878), who re-investigated the nervous system, and dis- 

 covered the circumoesophageal nerve ring. 



In more recent times, while the Annelidan theory has 

 found some support, notably by Giard (1875) and Gourret 

 (1884), it has also found a vehement opponent in Grassi, 

 wbo maintains that among the Cheetognatha there is not to 

 be found any trace either of the metamerisation of the body, 

 or of setse, or of parapodia. On turning to recent zoological 

 text-books we find that the majority of writers content them- 

 selves with expressing their uncertainty concerning the 

 exact position of the group in the Zoological System, but, 

 on the whole, there is a distinct feeling in favour of an 

 association with the " Vermes." 



Introductory. 



The theory that the Chgetognatha are primitive Moll u sea 

 is on first thoughts open to the criticism that they do not 

 possess features which have hitherto been generally accepted 

 as essential to members of the Molluscan phylum, and 

 that the obvious points of resemblance are the results of 

 convergence rather than indications of phylogenetic affinity. 



