THE ANIMALS OF LAKES AND RIVERS 197 
abyssal fauna is sometimes virtually absent, e.g. in the 
Caspian, owing to the paucity of oxygen, and its members 
rarely if ever show the specialization seen in the 
animals of the ocean abysses. This seems to be partly 
because lakes are necessarily temporary phenomena, 
destined to be filled up after a longer or shorter period, 
and therefore there is no time for a special abyssal 
fauna to develop. As considerable differences exist in 
the fauna of different lakes, it seems better to give 
some account of a few types rather than to make 
general statements in regard to lakes as a whole. 
The fauna of the lochs of Scotland has been studied 
in great detail by the members of the Scottish Loch 
Survey, so that it is possible to make a considerable 
number of general statements in regard to it. 
The members of the Survey studied in all 562 lochs, 
and, exclusive of vertebrates and of insects and their 
larvae, 440 species of animals were found. Most of the 
lochs studied are shallow, but Loch Morar has a maxi- 
mum depth of over 1,000 feet, and Loch Ness of over 
750 feet, rendering an abyssal fauna possible. The 
members of the Survey did not especially investigate 
the fishes. We know, however, from other sources 
that the special feature is the number of members of 
the salmon family, which run into many species or 
varieties; cf. the Loch Leven trout, which is peculiar 
to that lake. 
Of the invertebrates the majority may be described 
as littoral in that they occur in shallow water, near 
the margins of the lochs. Here are found many insect 
larvae, and not a few adult insects adapted for life in 
the water, e.g. water-beetles and water-boatmen. Here, 
too, are found nearly all the few molluscs, including the 
large mussels and the small fresh-water snails. Small 
