1904 - 5 .] The Plankton of Thingvallavatn and Myvatn. 1147 
sample of 16th October, only eight days before the lake is frozen over,, 
contains numerous animals, many of these with parthenogenetic 
eggs or with young ones in the incubating part of the carapace, 
I cannot believe that the Daphnids should die immediately after 
the lake becomes frozen over, even though the sample of 31st 
October does not contain i a single individual. Perhaps a few 
individuals will hibernate, but all in all I suppose that hiber- 
nating females here in Myvatn only play a very inconspicuous 
role in the whole life cycle of D. longispina. My opinion 
with regard to the cycles may be expressed as follows : 
Fig. 8. — 18th Septem- 
ber, small female 
with ephippinm. 
Fig. 9. — 18th Septem- 
ber, big female with 
twelve eggs. 
Fig. 10.— 18th Septem- 
ber, young female. 
From 15th July to 16th October the females of D. longisjpina 
always appear in two different and well-marked sizes ; the one is 
c. P5 mm., the other 2 - 5 mm.; between these two sizes I only 
have found (16th October) a few intermediate stages. The small 
females rarely have parthenogenetic eggs, and perhaps already, a 
fortnight after they have become full-grown, they carry ephippia ; 
the size of the males agrees with that of these females. The large 
females propagate only parthenogenetically, only in one sample (15th 
August) we find a few with ephippia. I received the impression that 
almost all parthenogenetic propagation is restricted to those large 
females, and that the small females mainly are sexual ones, which in 
