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 cosmopohtan, and beheved to form ' the 

 oldest commimity of organisms on the earth ' (Sir John 

 Miuray). The special feature, however, is the presence 

 here of some Arctic Crustacea, and the absence of certain 

 forms found in other Etiropean 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 foimd that at 

 depths greater than 300 feet the majority of the httoral 

 species disappear, leaving a small group of animals, 

 including one mollusc (a small bivalve called Pisidiwm 

 pusillwm), three Crustacea, three worms, an insect larva, 

 and a few infusoria. These occur also in the httoral 

 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 httoral 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 he but httle above sea-level, a marine 

 crustacean called the opossum shrimp (Mysis) occurs. 

 In the Scottish lochs the species of Mysis found is that 



