42 



GUSTAF EISEN, ON THE ARCTIC OLIGOCHiETA. 



being very short. In the other two genera the tube is much longer and less variable 

 and the whole organ seems to be in a higher stage of development. The duplication of 

 the organ can only be said to be abnormal, being hitherto found in M. mirabilis alone- 



The ovaries and testes in the different species are all built after the same plan, 

 and the slight variations they are subject to are scarcely restricted to certain species. 

 A form which is commonly met with in one species is sometimes also found, but rarely, 

 in another. 



Several species evidently seem to have been developed from the same related ancestor. 

 Such is the case with N. durus and Hatzeli, both inhabiting Norway and Sweden. 

 The former resembles more an Archienchytrceus, its supra-oesophagial ganglion being 

 less emarginated behind; in other respects the two species are very much alike. Why 

 one of the species (N. durus) has, to a certain extent, retained its ancestral character, 

 and the others been developed to a higher degree is difficult to say at present as we 

 know so very little of their geographical distribution. 



The following diagram gives some idea of my opinion of the »phylogenetic tree» 

 or the general direction of the development of Enchytrceus. 



Neoenchytraus. 



Mesenchytrteus. 



Archienchytrcsus. 



I 



