36 GUSTAF EISEN, ON THE ARCTIC OLIGOCH^ETA. 



The curious windings or loops of the tu be, as figured by Clapaeede and many 

 others, do not exist, as I, nearly always, succeeded when trying to pull out the tube 

 to its entire length, which should certainly not have been the case, if it had been in 

 any way entangled. 



The windings in certain species are very characteristic, being in some species 

 contracted to a ball, in others (N. Vejdovskyi) extending far backwards through the 

 segments. 



The form of the vesicle (»Samentrichter» — »entonnoir») is varying, even in the 

 same species, and is very seldom regular enough absolutely to characterize the species. 

 In A. nasutus the opening of the vesicle is surrounded by a curious ring, about the 

 true nature of Avhich I am not certain. It looks like a finely striated ring of some 

 chitinous substance, but I am inclined to believe it to be the tails of the sperrna- 

 tozoa which are thrown backwards, and of which perhaps some have become stationary 

 (see Pl. VI, fig. 10, f and g). In the closely allied species A. afjinis and ochraceus 

 this ring does not exist, or at least has not yet been found. 



Seen from the side the outlines of the vesicle of the Gen. Mesenchytrceus are those 

 of a more or less irregular flag, but seen from the front the vesicle seems more regu- 

 lar and bottleshaped. But in every instance the illustrations will better give an idea 

 of the different shapes, than any lengthy descriptions. 



In M. falciformis the lower end of the vesicle is furnished with a small peculiarly 

 twisted and striated sacklike organ, the functions of which I do not at all recognize. 



One pair of efferent ducts is the rule, but in one specimen of M. mirabilis I 

 found two ducts on one side, and one on the other side of the ventral nerve-ganglion, 

 an irregularity not found elsewhere. 



The spermatozoa in the species of Archienchytrceus och Neoenchytrceus are always 

 observed protruding from the interiör opening of the vesicle, but in Mesenchytrceus 

 they are encysted and not free before entering the receptacle. In N. callosus the tails 

 are extremely long, longer than the efferent duct itself. 



The penis consists of two parts, one external and one internal. The latter is the 

 largest and nearly always of a globular form. The external partor the penis proper, is 

 generally very small and globular, except in Archiencliytrceus nervosus-, where it is lar- 

 ger and hookshaped (Pl. VIII. fig. 16, f). 



The size of the interiör part of the penis differs in different species, being very 

 small, in some, as in M. primcevus, in others, as in A. profugus, as large as or even 

 larger than the vesicle itself. 



Ovaries. 



The ovaries are generally found in the ll th and 12 th segments, the size and form 

 varying slightly in different species, but never sufficiently to furnish any real charac- 

 teristics of the species. 



The exceptions from the general rule are very few, and worthy of notice. 



