332 Frits Johansen. 



they have reached the size of the adult. It is thus only the winter- 

 eggs which hibernate, whilst the animals themselves die in Sep- 

 tember; from the eggs the young ones hatch out in early summer 

 and these grow in the course of the summer. 



Ostracoda. 



These animals are also met with in the littoral region especially 

 by the banks of lakes where there is plenty of vegetation, and they 

 are extremely common in bogs and pools. As a rule they are 

 collected in groups at the bottom of the lake, where they creep 

 about in the fine mud with slow movements; in general because 

 there is some kind of food or other there, e. g. a dead Apiis glacialis. 

 I have myself seen (Hvalrosodden, August 1906), how they collect 

 about such an animal and creep in under the carapace ; taking this 

 up we can see, how the Ostracods eat away its ventral half (on one 

 Apus I found about 10 Ostracods). And even living Apiis are not 

 safe from them; it is however only at the end of the summer, when 

 a number of the adult Apus are dying (and therefore weak) that we 

 find them infested with the Ostracods. These appear therefore as a 

 kind of carrion-feeder on the bottom. It is characteristic that, like 

 Apus but in contrast to Daphnia pulex, they are found both in still 

 and running water; as bottom-animals they are also better able to 

 keep their place in the water in spite of the currents round about. 

 They are fairly indifferent towards being dried up; thus on ^'^/7 1907 

 I have found living Ostracods in a quite dried-up pool at Skibs- 

 havnen; they had buried themselves down into the sligthly damp mud 

 and thus sought to escape the dangers of evaporation their resting 

 places are exposed to in the course of the summer. In August I 

 have observed two forms of Ostracods, on the one hand numerous, 

 larger (a pin's head in size) and greenish black animals and others 

 which were fewer in number, only half as large and reddish brown. 



When the thin ice begins to form in September, the Ostracods 

 become more rare and less active; a portion have probably buried 

 themselves down in the mud, but most of them are dead, and on 

 the banks at many places we find numerous empty shells (collected 

 in groups like the living animals), and these are also found again 

 when the banks of the lake thaw in the early summer. If we closely 

 examine the banks (Skibshavnen ^^/it 07), we find the dead Ostracods 

 on the mud-covered stalks of herbage (e, g. of moss) frozen into the 

 ice; inside the open shells we then find a number of light-red grains, 

 the hibernating eggs. When the ice melts along the banks of the 



