196 8 



All the investigations have almost exclusively been carried on in ponds not in lakes. 

 It is a well-known fact that the Rotifera really form a rather prominent part of the 

 lake plancton. But the investigations have also shown that from the plancton of 

 lakes we only know extremely few, perhaps not a single Rotifer, which is not 

 also to be found in the pond plancton: even Ploesoma Hiidsoni; Gastropus sty lifer 

 are pond forms; here in the ponds the maxima are much more pronounced than 

 in the lakes. It is also a well-known fact that many of the colonies of the most 

 pronounced plancton Rotifers from the pelagic region of larger lakes only rarelj' carry 

 resting eggs, and that it seems that we often have to do with acyclic colonies. The 

 pelagic region therefore is not the right spot for those who wish to studj- the males 

 of the Rotifera. 



The Rotifer life in ponds may of course be divided into a series of rather 

 different societies or associations. Any one who wishes to get an idea of the Rotifera 

 of a pond need only reckon with three associations viz. the Rotifer life in the central 

 part of the pond, free from vegetation, "the pelagic" fauna; the creeping or swimming 

 Rotifer fauna of the vegetation, and the fixed Rotifer fauna. Each of these faunas 

 maj' be collected by special apparatus, and in different ways, the first by means of 

 plancton net Mullergaze No. 20, the second with a plancton net Müllergaze No. 15, 

 provided with a sieve to keep back the algæ. These two nets are provided with a 

 cord of about 10 meter and are used as throwing nets; the fixed fauna is best 

 gathered by means of a net, placed upon a 1 meter long stick. — For the study ot 

 the "pelagic" fauna a boat is always desirable. — A special Rotifer fauna strongly 

 associated with ponds of quite a special type cannot, according to my experience, 

 as a rule be pointed out; the single exception is the Rotifer life in ponds which 

 dry out early and only possess water for a month or two after the ice has melted; 

 especially if these ponds are simultaneously polluted by dark water from dunghills, 

 we shall find quite a special fauna with Hydatina and some species of Notommatidæ 

 as the most characteristic Rotifer forms in them. A special Rotifer fauna only as- 

 sociated with peaty water does not exist; some species, viz. Pedalion, occur mainly in 

 localities of this nature, but this species may also be found in ponds with clear 

 most probably chalky water. It is of interest that the ditïerent holes in a peat moor 

 often contain a very different Rotifer fauna simultaneously. 



Any one who wishes to become acquainted with the Rotifer fauna of a localitj', 

 must further remember alwaj's to take a series of samples, as far as possible at 

 regular intervals, best everj' week; a single sample has only verj' little scientific value. 

 The Rotifer fauna differs from week to week; this especially holds good with regard 

 to the "pelagic" Rotifer fauna; for any one who wishes to stud}' the periodicit}', 

 the temporal variations, the occurrence of sexual periods etc., this manner of pro- 

 ceeding is of course a conditio sine qua non. In the time from 1900 to 1920 regular 

 observations have been carried on in very many ponds or smaller lakes; the re- 

 searches which are now going to be published, make use of twenty-two series of ob- 



