1094 Proceedings of Royal Society of Edinburgh. [sess. 
5. Euflagellata.— Of these the species of Dinobryon are the only 
real plankton organisms in the larger lakes. The remainder are 
all tycholimnetic, their home being in the smaller lakes or ponds. 
6. Rhizopoda. — It seems that we have in the European lakes, 
and especially in the clearer and colder ones, a very peculiar but 
slightly studied fauna of Rhizopoda , consisting of but a few and 
very fragile species. Further investigations upon this point will 
presumably increase the number of species as well as give more 
detailed information regarding the periodicity and biology of these 
interesting animals. 
7. Infusoria. — 'These are of hardly any importance in the pelagic 
region of the greater lakes, all having a surprisingly short and 
clearly defined max., which rarely extends beyond a longer period 
than one or one and a half months, and which generally occurs 
during spring. ( Dileptus track elioides, Tintinnidium jluviatile , 
Staurophrya elegans.) The only perennial Infusorium is Codonella 
lacustris. 
8. Rotifera. — These may he referred to two groups. One of 
these contains cosmopolitan poly- or di-cyclic perennial species in 
larger lakes. They have two max. and two sexual periods, one in 
spring and another in autumn. The other, containing monocyclic 
periodical species, has its max. and sexual period in summer at the 
highest tp. of the water, and is of a more restricted distribution. 
The poly- or di-cyclic group generally attains its greatest max. in 
ponds, but may also reach a considerable max. in larger lakes. 
The max. reached by the monocyclic group is commonly rather 
small. All in all, the Rotifera in the pelagic region of the 
greater part of the larger lakes play but an inconspicuous part. 
9. The association of plankton Crustacea in Danish lakes does not 
differ in any way from that of the lowland lakes of Central Europe. 
As shown by the explorations of Lilljeborg (1900), G-. 0. Sars 
(1861-1901), and by the considerations of Steuer (1901), it 
seemed probable, when I wrote my work, that a closer examination 
of the arctic regions would prove the existence of an association 
of plankton Crustacea, which differed from those inhabiting 
warmer countries. This association was, however, at that time 
but inconsiderably known. 
10. During the last four years I have been occupied by the study 
