English summary. 
In the summer of 1923, a scholarship from Dansk-Islandsk Fond, 
enabled me to visit Iceland for the purpose of study. During this 
journey I made a collection of fresh water Entomostracians from 
125 localities in the most different parts of Iceland (see the map 
pag.84).This material I have now examined partly, from an ecological, 
and partly from a zoogeographical point of view. The material was 
taken with a throwing-net almost always used from the beach. A 
material collected in this way is not fit for direct quantitative in- 
vestigations, but it can be used for a calculation of the relative 
frequency in which the species are occurring in the waters. 
In order to come to an understanding of the biology of the 
species, through a material collected in this way, it is necessary to 
know something of the conditions under which they are living. 
The ecological factors, which I have been able to control, are 
the depths of the waters, their vegetation, the nature of the bottom, 
and the height above the sea-level, on which the temperature depends. 
A more exact examination of the Chemical and physical factors will 
no doubt result in a far more penetrating understanding of the biology 
of the species than I have been able to obtain by the means at my 
disposal. 
One of the factors of the greatest importance to the Entomo¬ 
stracians is the length of the period during which the waters are 
icefree, as this is the time of the year when the species grow 
and propagate. By making a comparison in different localities be- 
tween informations at hånd as to the length of the icefree period 
and the temperature of the air, I have come to the result that 
the waters are icefree just for so long a time as the 
temperature of the air is above + 4® C. This however 
only holds good for countries, where the climates are resembling 
those of the localities, named in the table (pag. 88). 
The Entomostracians I have dealt with in this paper are the Eu- 
phyllopoda, the Cladocera and the Copepoda, with the exception of 
the Harpacticidae. 
The species I have found, 29 in all, are put down in table 3, 
