34 GUSTAF EISEN, ON THE ARCTIC OLIGOCHiETA. 



Short, slender nerve stenas are nearly always found to be emitted from the front 

 part of the cephalic ganglion towards the cephalic lobe, but sometimes also, as in N. 

 Stuxbergi and Vejdovskyi, towards the segments behind the supra-oesophagial ganglion. 



The muscles fixing the cephalic ganglion to the body-wall are in most species af- 

 fixed to the margin of said ganglion, but in the species of Mesenchytrceus we find them 

 taking their origin near the centre of the ganglion and perfectly covering its entire 

 surface, especially so in M. primcBvus and mirabilis. 



The organs of generation. 



These organs are of two kinds: male and female. 

 The male"ones consist of: 



a. Testes, 



b. Efferent ducts. 



The female ones consist of: 



a. Ovaries, 



b. Receptacles; all the four kinds being found in the same individual. 

 I shall here consider each kind separately and begin with: 



Testes. 



The testes are generally of the same structure and form in the different species, 

 and consist of large bodies, more or less lobated, situated in front of, or even in the 

 segments constituting the cingulum or the segments 11 and 12. In most of the spe- 

 cies said lobes are small and shallow and only occupying the margin of the organ, but 

 in two species, Archienchytrceus profugus and nervosus, we find them very large, each 

 forming a separate spermatozoa-bag. In Mesenchytr. mirabilis some of the testes are 

 entire and others, viz. those in the cingulum, are lobated, somewhat resembling a bunch 

 of very large and compact grapes. 



This lobation of the testes, however, cannot be considered as a character of 

 great importance, as I think it a very variable one, which exists perhaps in old and 

 well developed species. Any lobes exactly like those described by Claparede as be- 

 longing to Pachydrilus verrucosas Clap., were not found in any of the Sibirian species, 

 nor in any others which have come under my notice. 



The spermatozoa are always packed close together, and of different stages of de- 

 velopment in different places of the testes, the least developed ones being those nearest 

 the base, the highest developed again those nearest to the top of the organ. 



The segments containing the testes are, in most of the species, those nearest in 

 front of the cingulum or the 8 th , 9 th and 10 th segments, but sometimes we have to 

 look for them elsewhere. 



Exceptions are, as far as known, but three: 



Neoenchytr. callosus has the testes in the ll th segment, Archienchytr. hyalinus in 

 the ll th and 12 th segment, and Mesenchytr. mirabilis in the 9 th , 10 th , ll th , 12 th , 13 th , 



