1904 - 5 .] Study of the Lakes of Scotland mid Denmark. 419 
(chains) being different from that of the colonies in the pelagic stage 
(stars). These remarks may prove of some importance, inasmuch as 
future investigations may show how littoral organisms become trans- 
formed into pelagic organisms, and as they support the hypothesis, 
now commonly adopted, that the fresh-water plankton is derived 
from the common microscopical littoral and bottom fauna and 
flora, very few organisms having immigrated directly from the sea. 
As a character common to all our plankton, I may add that the 
seasonal variations of the organisms are very conspicuous, and 
more especially those of Daphnia ( Hyalodaphnia ) cucullata , 
Bosmina coregoni , Asplanchna priodonta , Ceratium Mrundinella , 
Asterionella gracillima , Melosira crenulata , Fragilaria crotonensis , 
Pediastrum, etc. I shall return to the investigations on this point 
after treating of the plankton of the Scottish lakes. 
I may point out that the vivid red colour characteristic of many 
Crustacea in other countries is not with us very conspicuous ; 
several Copepoda do, as a rule, in winter, change from yellowish- 
white into a deep red colour. 
With regard to the vertical distribution of the plankton, I only 
venture to remark that the greatest profusion of plankton is to 
be found in the upper layers of water. Like most of the 
naturalists who have studied the plankton in the lakes of the 
northern part of Central Europe, I have not been able to dis- 
tinguish any vertical wanderings at different hours of the day ; 
I venture to think that such wanderings are rather inconspicuous 
with us, but further investigations with improved appliances will 
be necessary to decide this question. 
3. The Abyssal Region. — In my paper on the bottom-exploration 
of the Danish lakes (1901), I have pointed out that there are 
reasons for fixing the limit between the Littoral region and the 
Abyssal region at about 9 or 10 metres (30 or 35 feet). In speaking 
of our shallow lakes we cannot, of course, strictly use the term 
“abyssal region”; the principal conditions laid down by Forel 
regarding this region, especially the uniformity of all the life- 
conditions, are never fully realised in the Danish lakes. Still, it 
may be maintained that we can speak of an abyssal fauna, 
inasmuch as this is quite different from the littoral fauna, and 
apparently similar to the abyssal fauna in deep alpine lakes. 
