198 THE ANIMALS OF LAKES AND RIVERS 
crustacea are numerous, especially those forms popu- 
larly called water-fleas, from their jumping movements. 
Many rotifers occur, together with such small animals 
as water-mites, water-bears (Tardigrada), and so forth. 
Only some thirty species are typically pelagic and 
thus constitute the plankton. These comprise fourteen 
small crustacea, twelve rotifers, and four protozoa. 
Of these thirty species most are very widely distributed, 
the animals which constitute the fresh-water plankton 
being all but cosmopolitan, and believed to form ‘ the 
oldest community of organisms on the earth ’ (Sir John 
Murray). The special feature, however, is the presence 
here of some Arctic crustacea, and the absence of certain 
forms found in other European lakes, both no doubt 
due to the northern position of the lochs. 
As has been just stated, the lochs are mostly shallow, 
and therefore an abyssal fauna is rarely developed. 
Of the two deep lochs, only in Loch Ness were successful 
deep-water dredgings made. Here it was found that at 
depths greater than 300 feet the majority of the littoral 
species disappear, leaving a small group of animals, 
including one mollusc (a small bivalve called Pistdium 
pusillum), three crustacea, three worms, an insect larva, 
and a few infusoria. These occur also in the littoral 
region, and the forms from deep water show no special 
feature, so that it would be more correct to say that 
a few only of the littoral forms can live in depths 
greater than 300 feet than to say that a special deep- 
water fauna exists. 
Before leaving the fauna of these lakes, one interest- 
ing point may be noticed. In certain of them, especially 
those which lie but little above sea-level, a marine 
crustacean called the opossum shrimp (Mysis) occurs. 
In the Scottish lochs the species of Mysis found is that 
