BIOLOGY OF THE SCOTTISH LOCHS 
311 
The Plankton. — There are about 30 species of animals and 
80 plants of common occurrence in the plankton. 15 of the animals 
and 13 of the plants are generally distributed — the others are more 
or less local. Most of the local species are confined to the west and 
north of the mainland, and the islands. The chief characteristics of 
the plankton are the abundance of Desmids, and the predominance 
of Arctic species of Crustacea. 
The seasonal change is but slight, especially in the larger 
lochs. Most of the Arctic species are only present in summer and 
autumn. 
The temperature of the larger lochs has a very small annual range, 
rarely reaching 20^ Fahr., and there is rarely any approach to 
" flowering," but such as there is may occur in winter. 
A diurnal migration of the plankton has been noticed, the larger 
Crustacea coming to the surface after dark. The plankton animals 
are normally abundant in the larger lakes down to a depth of 200 
feet and more. Leptodora appears to make the journey from this 
depth to the surface with great rapidity, as it has been found to 
arrive at the surface immediately after sunset. 
Littoral Region. — The margins of the lakes, though usually some- 
what deficient in higher vegetation, possess in favourable localities a 
very rich microfauna, of Tardigrada, Worms, Rotifera, Infusoria, etc., 
only partly worked out. 
The Abyssal Region. — The muds of the deeper lakes support 
a very sparse population, of about a dozen species — 1 Mollusc, 
3 Crustacea, 3 Worms, 1 Insect, and several Infusoria. Many others 
are casually found in the abyssal region, and in Loch Ness upwards 
of 40 species of animals have been found at a depth of about 
300 feet. 
There are no peculiar abyssal forms in the lochs, unless a few 
Rhizopods found by Dr Penard be considered as such. No relicts of 
a marine fauna have yet been found in the lochs. The physical con- 
ditions characteristic of the abyssal region are total darkness, equable 
temperature, great pressure. The poverty of this region may be 
attributable to a deficit of oxygen in available form. 
Oiigin of the Fauna and Flora. — As no relict fauna has yet 
been found in the lakes, there is no reason for supposing that any part 
of the lake-fauna has had a marine origin, or has come through the 
intermediary of a great inland sea or lake, such as has been postu- 
lated to explain the distribution over the great European plain. 
And as there is likewise no peculiar abyssal fauna, or peculiar forms 
at all in the lakes, the tracing of the origin of the population found 
in them now is comparatively simple. Ordinary migration will 
account for the greater part of it, and this may be extremely rapid. 
