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 speciahzation 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 pecuKar 

 to that lake. 



Of the invertebrates the majority may be described 

 as httoral 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 



