194 6 



1893 — 1900 just at the period when mine should have been printed; as the results of 

 the researches were almost quite congruent, I provisionalh' retracted most of mj' paper, 

 but have carried on mj' investigations to this very day. Every year it became more and 

 more obvious how difficult it really was to get a clear understanding of the phenomena 

 and how necessary it was to extend the investigation over as long a time as possible. 

 During the long period from 1898 to 1920 these investigations have not always 

 been in the foreground. Those who know the publications from this laboratory, 

 will be acquainted with a series of investigations, which have nothing to do with 

 the study of Rotifera. As however verj' manj' of the investigations have been carried 

 on in ponds and moors, and I have almost always had a Rotifer net with me, a 

 lot of rather casual observations have been gathered. It was noted when rather rare 

 Rotifers were observed in the different ponds, and when maxima or sexual periods 

 were detected. The material which was collected in this way and which was 

 derived from more than twenty years of observation, originated from many hundreds 

 of ponds, Ij'ing partly in North, partly in Middle Seeland, mainh' with a radius of 

 only a few kilometers from the laboratory. 



Fii'stly my attention was directed towards the study of the periodicity of the 

 Rotifera; simultaneously herewith, I had ample opportunity of getting to know a 

 long series of males, and I saw how manj' of them had never been described. I 

 therefore made a great manj^ notes with regard to their occurrence. Simultaneously 

 herewith a great many cursorj' drawings and notes relating to the males were made. 

 When all this material was collected and overlooked in 1919, it was clear that it 

 contained many new males and many new observations relating to males hitherto 

 slightly known. A monographical treatment of the males of the Rotifera was then 

 planned. From the many separate casual observations from the foregoing years 

 I was able to begin the investigations upon a long series of fixed localities and 

 upon fixed times. From the earlier observations I knew exactly when and where 

 I might expect to find the males of very manj' Rotifers. In the spring of 1920 these 

 studies were the chief aim of the laboratorj'. — In very many cases my calculations 

 were correct, but there were of course also cases where it was impossible to find 

 the species seen many years before. What has been in a very high degree unfor- 

 tunate for the investigation is the devastation of so very many small ponds, either 

 lying in the fields or in the forests, and which cultivation has either totally closed 

 or filled with material that has killed the fauna. However, if I had not had the 

 preliminaiy investigations of the years 1900 to 1920 as a support, it would un- 

 questionably have been quite impossible for me in the course of onlj^ two years of 

 observation to procure sufficient material for drawing about fifty Rotifer males. 

 Most of the authors who have studied the group for a long series of years have 

 commonly only seen a glimpse of the males of very few species. The now published 

 investigation gives camera-drawn figures of 45 males; of these males more than 

 twenty are quite new to science and eight belong to families in which males have 

 hitherto been wholly unknown. Manj' of the other males here described and figured 



