BIOLOGY OF THE SCOTTISH LOCHS 
287 
Differences in the chemical properties of the water may account for 
local differences in the abundance of the Desmids. Most of the lochs 
rich in Desmids are near the sea-board, and those which are farthest 
removed from it are at considerable elevations. The region occupied 
by these lochs partly coincides with the distribution-area of some of 
the more local of the Arctic Crustacea. 
European Species rare or absent in Scotland. — A number of 
species, dominant in the lakes of the European plain, are here rare or 
local, or they are entirely absent. Diaptomiis graciloides is absent ; 
Bosmina coregoni is very rare ; Hyalodaphnia cucuUata is believed to 
be very rare ; Bosmina Jongirostris is very local. 
The blue-green Algas, though often conspicuous enough, are not 
so generally dominant ; and the Diatoms, though present, are unim- 
portant. 
Dr Wesenberg-Lund further regards as belonging to the cosmo- 
politan stock certain Rotifers which are rare or local in the Scottish 
lochs, though common enough in other situations : Nothoica striata^ 
a pond species, rare at the margins of lakes ; Anurcea aculeata, very 
rare, and only in small lakes ; Triarthra longiseta, frequent in lakes, 
but by no means general, and absent from the great lakes. 
Arctic Element. — The Arctic element in the Scottish plankton is 
very important. The Arctic species of Crustacea are Holopedium 
gibbenirn^ Bytliotrephes longimanus^ Daphnia hyalina^ Bosmina 
ohtusirostris { = B. arctica^ Lillje.), Cyclops strenuus, Diaptomus laticeps^ 
D. laciniatus. As several of these species are among the permanent 
inhabitants of the lakes, which persist throughout the year {Daphnia^ 
Bosmina^ Cyclops)^ and some of the others are among the largest 
species, and are generally distributed {Holopedium^ Bytliotrephes), 
the Arctic element is clearly the dominant one in the Scottish 
plankton. 
Distribution of the Plankton. — The distribution of the species 
in the lochs will be considered in a special section. Here the general 
features only, which give the special character to the Scottish plankton, 
will be dealt with. Of the 200 species of animals and plants which 
have been found in the plankton collections, probably about one-half 
should be regarded as casuals. The remaining 100 species are not 
all equally distributed over the country. Many are so rare, or the 
information about them is so scanty, that the study of their distribu- 
tion would serve no purpose. 
About 30 species are generally distributed. Many of the others 
' are confined to the western half of the country, or to the extreme 
north, or they are very local or quite sporadic in their occurrence. 
Diaptomus laciniatus and D. laticeps are irregularly distributed 
over the central, western, and northern counties ; D. wierzejskii is 
