XX.-0LIG0CHJ1T0L0GICAL RESEARCHES. 



By GUSTAT ElSEN. 



INTRODUCTION. 



In the beginnmg of the present year (1879) I had the honor to pre- 

 sent to the Academy of Sciences of San Francisco, Cal., and also to the 

 Eoyal Academy of Sciences of Stockholm, Sweden, preliminary reports 

 on the same subject of which I now propose to treat more extensively. 

 Neither of these reports, however, is as yet published, and this will ex- 

 plain why in the following- no mention of them has been ma^le, and no 

 citations from them quoted. 



The theories advanced in them I have had little opportunity to 

 change. Only in few instances have further investigation and access to 

 a greater number of specimens necessitated any material modification 

 of my former views, and whenever any such change has been made, I 

 have always called special attention to the same. 



My investigations have been confined principally to the characteriz- 

 ing of the species and to the anatomical structure of the generative 

 organs. The following treatise, therefore, has no pretension to being 

 exhaustive, even if such a word could properly be applied to any 

 work in natural science ; but, on the contrary, I must freely confess that 

 many an important point pertaining to this subject has here been only 

 lightly touched, or even not at all enlarged upon — mostly the result of 

 a limited time and library. 



The many new forms here described for the first time may perhaps 

 to some degree compensate for the above defect, and, as I trust, call a 

 more general attention among zoologists to the fresh- water oligochaeta 

 — a class which, I am sure, will in the near future be found to contain 

 thousands of forms. 



My field of investigation has been limited to Sweden and California, 

 and in both countries to comparatively few localities. 



In the following I propose first to give a description of all the species 

 found by me, and afterwards to treat more minutely of some points in 

 their anatomy and physiology. 



[13 879 



