1904 - 5 .] The Plankton of Tlmigvallavatn and Myvatn. 1149 
enormous prolificness, and which I have never seen with ephippia. 
In the second part of my plankton work I shall further treat of 
the propagation of the species in these two lakes. 
As far as I know, this is the first time that attempts have been 
made to determine the life cycle of a Daphnid in a special 
locality by means of regular fortnightly collections of plankton 
material. Of course I do not expect the above sketch of the life 
cycle of D. longispina in Thingvallavatn and Myvatn to be exact, 
especially as I have not myself gathered the samples. Further, it 
must be remembered that even if we suppose it to be reliable with 
regard to this particular year, the development of the organisms 
may in other years take a quite different course. Light, tp., and 
the settlement of ephippia, which depend on the direction and 
the force of wind at the moment of settlement, undoubtedly 
exercise their influence upon the course of the cycle. These 
pages must not be expected to present more than an outline 
sketch of my subject to the reader; further explorations must 
test its correctness. 
When comparing the results of the investigations into the life- 
cycle of D. longispina in Thingvallavatn with that in Myvatn, we 
find these very different. In Thingvallavatn D. longispina may 
be regarded as dicyclic, in Myvatn it is monocyclic. As I have 
never had any living material, nor collected the samples myself, I 
do not wish the results to be regarded as conclusive. Besides, if 
this is the case, it becomes quite plain that D. longispina in Thing- 
vallavatn, which in 1902-1903 seems never to have been frozen 
over, might be dicyclic and in Myvatn only monocyclic. I suppose 
that the last autumn brood hibernates in deep water and propagates 
in spring in Thingvallavatn as well as in our own lakes, and that 
the ephippia on the whole play but an inconspicuous part with 
regard to preserving the species in the lake. On the other hand, 
I should think that almost the whole stock of D. longispina in 
Myvatn is hatched from ephippia, and that the hibernating 
parthenogenetic females here only play a minor part i^- the whole 
life-cycle of the species. I venture to express this idea, because it 
agrees with the results of my explorations in Danish lakes, accord- 
ing to which the ephippial eggs of the plankton species of the 
genus Daphnia do not play a prominent part in regard to the life 
