38 GUSTAF EISEN, ON THE ARCTIC OLIGOCHiETA 



All except that of A. Levinseni are furnished with small glands at the base. 



M. falciformis has the lower part somewhat differentiated, the epidermis or cuti- 

 cula being thicker and striated. 



In M. primcevus we also meet with a receptacle on a low stage of development 

 and of a remarkably minute size. Its middle part, however, is furnished with two 

 lobes or follicles, not further differentiated. 



The higher developed forms of the receptacle are characterized by its upper or 

 middle part being in some way or other differentiated, enlarged or furnished with fol- 

 licles for the reception of spermatozoa, or by having its base to a greater extent sur- 

 rounded by glands. 



In some species the receptacle is furnished with follicles or bladders at the middle 

 of the receptacle proper. Such species are M. mirahilis, N. durus and Ratzeli, and 

 I consider their receptacles higher developed than those of any other species. 



Some receptacles are simply enlarged in the middle. We hnd the same in A. na- 

 sutus, ochraceus, afjinis and N. callosus. Other species have the enlargement at the top 

 as A. lampas, Dicksoni and g emmatus. A peculiar and also very characteristic form of 

 receptacle is found in A. nervosus, N. Stuxbergi, Vejdovslcyi and A. profugus. The top 

 is here very large and eggshaped and the narrower tube is surrounded by a heavy 

 stratum of glands. It is peculiar and interesting to note that all the four above men- 

 tioned species were caught in or near water, or under seaweeds at the shore. "Si- 

 milar external influences apparently have had here similar effects on the development 

 of the receptacle. And it is only natural to suppose that the peculiar shape of the 

 receptacle and especially the numerous glands round its base or tube must in some 

 way or other be protective, or exsude a secretion, protective to the spermatozoa more 

 or less exposed to the sea water. 



Segmental organs. 



The segmental organs in the different species are all constructed according to the 

 same general plan. Every segmental organ can be said to consist of three different 

 parts: 1° one middle lobe in which the bulk of the tube is round, 2° one outer tube 

 connecting the former with the body-wall and having in its extremity the outer ori- 

 fice, and 3° one inner tube connecting with the perigastrium and having its inner 

 opening or extremity inserted in the dissepiment or septa between the segments. 



The middle lobe only consists of a fold of the tube, surrounded by glan- 

 dular matter through which the windings of the tube can sometimes be plainly 

 seen, but generally very difficult to folloAv. The inner and outer tubes also are 

 surrounded by glandular matter, but to a less extent than the middle lobe. Said 

 lobe is generally single or not lobed, but in the species of Mesenchytrteus there is a 

 tendency to duplication. In M. primaivus are found the most developed segmental or- 

 gans and three large middle lobes instead of one, M. mirahilis has only two large middle 



