1144 Proceedings of Royal Society of Edinburgh. [sess. 
hirsuticornis, Alona sp., Acroperus leucocephalus , Chydorus sphcericus, 
and several species of Cyclops. Of these bottom forms the sample 
of 18th September contains C. sphcericus in great numbers, as well 
as many Macrothrix hirsuticornis. Of the genus Bosmina only 
B. obtusirostris appears in May and June, but only a very few 
individuals of this species may be found. Of plankton Crustacea 
Myvatn has only one species, Daphnia longispina, which in its 
short lifetime fills the water with huge masses. I have tried to 
give its life-history in the following pages. 
Most of the samples contain a lot of detritus, and the samples of 
16th June and 1st July contain hardly anything but empty skins of 
pupae of Chironomus, which colour the samples black. If this 
sample gives a true picture of the limnetic region of the lake, it must 
at that time have been teeming with pupa skins, and the great 
swarm must have been hatched by this time. 
Owing to the absolute absence of all phytoplankton, the 
zooplankton is almost wholly reduced to depend upon detritus for 
its main sustenance. It is most interesting to see how the genus 
Diaptomus , which in a far greater degree than all other plankton 
Crustacea exist upon phytoplankton and more especially upon 
Diatoms, is wholly wanting, and that among the plankton Crustacea 
the main form of the plankton is also here a Daphnia , which is 
everywhere known to feed upon detritus. Without any attempt 
at generalising, I still suppose we may conclude that further 
investigations probably will prove the phytoplankton of the arctic 
lakes to play by no means the prominent part in the composition of 
the plankton as in lakes of the temperate zone. 
3. The life cycle of Daphnia longispina , O.F.M. 
D. longispina in Myvatn presents but a very slight seasonal 
variation. All the individuals from May to June belong to the 
typical microcephala form ; later on the head becomes a, little higher, 
and its ventral edge is curved slightly more inward ; further, the 
rostrum is more prominent. All in all, the head is highest in those 
females which only propagate sexually, and lower in those which 
only propagate partlienogenetically. 
On 30th May we only find a few quite young ones ; on 16th June 
