414 Proceedings of Royal Society of Edinburgh. [sess. 
A stranger unacquainted with our lakes on reading these lines 
might form the impression that the shores of our lakes were for 
the most part inhabited by the common fresh-water fauna to be 
found in every shallow pond with rich vegetation. This impres- 
sion would be incorrect, for a closer examination would certainly 
show that, while many species are common to ponds and to the 
vegetation zone of the lakes, still it would appear that most of the 
Phryganidse, Libellulidse, Ephemeridse, some of the Crustacea, 
many Planaria, some Oligochseta and Rotifera are quite peculiar 
to the lake-shores, and rarely appear in ponds. Further, it would 
seem that several species of snails common to the ponds and the 
shores of the larger lakes are represented in the lakes by special 
forms differing from those found in ponds. I cannot in this short 
paper discuss this point in greater detail, but will content myself 
by remarking that the fauna of the littoral zone of our lakes is 
on the whole very different from that of our ditches and ponds. 
In winter the greater part of this rich fauna disappears. In 
November and December many of the organisms, especially snails 
and some insect larvae, migrate into deeper water before the shores 
are covered with ice ; other organisms, for instance many insect 
larvae, go ashore and burrow holes in the ground, while a great 
many other species, especially Daphnids and Rotifers, make resting 
organs * and, by means of them, survive the freezing in the ice. 
Still, there are numerous organisms which appear to live in winter 
beneath the ice as they do in summer in water having a tempera- 
ture of about 25° C. (77° F.) ; for example, Planaria, Phryganidse, 
Amphipoda, Nepkelis , etc. 
2. The Pelagic Region . — With regard to the plankton, I may 
refer to my Plankton studies (1904), and restrict myself in this 
place to the following brief remarks. Our lakes are nearly always 
extremely rich in plankton, so much so that throughout the greater 
part of the year — from April to December — it affects the colour 
and transparency of the water, and is doubtless one of the main 
factors in determining the varying amounts of oxygen and carbonic, 
acid dissolved in the water. It will thus be understood that the 
plankton of our lakes — its composition and its abundance — must 
necessarily greatly influence the other organisms in the lakes. 
* Hibernating buds, ephippia, or eggs. 
