416 Proceedings of Royal Society of Edinburgh. [sess. 
organisms from the Littoral region find their way to the central 
parts of the Pelagic region, and that many of the forms peculiar to 
the central parts of the larger ponds, especially many Chlorophycea, 
may he carried by the rivers into the Pelagic region of the lakes ; 
still, the majority of these organisms never play a prominent part 
in the composition of the plankton. 
Out of about 150 plankton organisms which have been 
recognised in the Danish lakes, very few appear in such vast 
quantities as to give the plankton a monotonous character, or to 
influence the life-conditions of the lake during the greater part of 
the year. Among these are Melosira crenulata and granulata, 
Asterionella gracillima, Aphanizomenon flos aquce, Ceratium liirun- 
dinella , the species of Diaptomus , Daphnella brachyura , 
Hyalodaplinia cucullata , Bosmina coregoni , and Leptodora 
Idndtii. Prom April to December there are in almost every 
lake, besides the above-mentioned species, others which may pre- 
dominate during a shorter period. Among these I would mention 
Fragilaria crotonensis , and other Diatoms, Coelosphcerium kiitzin- 
gianum, Polycystis , and a few other Cyanophycea, a very few 
Chlorophycea and Protozoa, some Rotifera, and of Crustacea 
especially Cyclops oithonoides , Bosmina longirostris , and Daphnia 
hyalina. Besides those organisms whose home is in the littoral 
zone, or in the central parts of ponds, which are always rare in 
the Pelagic region of the lakes, there are other rare forms found 
in this region that only appear in the summer months. These 
organisms, as far as I know, have apparently reached or nearly 
reached their northern limit with us; this applies especially to 
some Rotifers, Cyanophycea, etc. 
Though the life-conditions in our lakes do not vary very much, 
still there is a good deal of difference in the plankton of the 
different lakes : this refers mostly to the Diatoms and Cyano- 
phycea, those two great groups of organisms which, in my opinion, 
affect more than any other the common life-conditions of the lake. 
As a general rule, we may say that these two groups rarely attain 
their maximum development in the same lake or simultaneously. 
Most of the fresh- water Diatoms reach their highest development 
at a relatively low temperature (below 12° or 10° C. = 54° or 50° F.) 
and in the colder of our lakes ; on the other hand, the Cyano- 
