THE ROTIFERA OF THE SCOTTISH LOCHS. 159 



Digltna, Diaschiza, Diurella, Philodina, and Callidina. These genera constitute the 

 characteristic Rotifer-fauna of lake-margins ; other genera, though common enough, 

 are more casual in their occurrence. There are several species of each of these genera 

 common in the littoral region, though none of them are confined to lakes. 



It is in the littoral region that the richest Rotifer-fauna is found ; in fact, the whole 

 Rotifer population of a lake may be ascertained from the marginal collections, as the 

 limnetic and the abyssal species here meet and mingle with the proper littoral forms. 

 Including the casual as well as the permanent inhabitants, a large number may occur 

 in any one lake. We observed 148 species in Loch Ness — undoubtedly far under the 

 true number — and Stenroos noted 157 in Nurmijarvi-See. 



From sixty to eighty of these species may be considered as of ordinary occurrence 

 in lakes, and likely to be found in any lake which is carefully examined. The others 

 are more local and uncertain. 



Although by far the most densely peopled part of the lake, the littoral region has 

 not the most marked lacustrine character. It is the few limnetic species which are 

 most truly characteristic of lakes. Although the limnetic Rotifers also occur in ponds, 

 their special characteristics are such as fit them for lake life. These characteristics — 

 spines, transparency, free-swimming, etc. — have probably had their full development in 

 lakes, though the animals now often extend into smaller waters. 



The littoral Rotifers are none of them confined to lakes ; they may be found in 

 moist places anywhere — in ponds, bogs, streams, and among moss. Nevertheless, even 

 the littoral region has a certain lacustrine character. 



Leaving out of account some very shallow lochs and certain bodies of contaminated 

 water near towns, the water of our lochs is, on the whole, pure, if peaty, and the 

 genera given above as most common in lakes are those which have a preference for 

 clear water. 



A small number of species may be cited as pre-eminently characteristic of pure lakes, 

 though not exclusively lacustrine. Most of them are Bdelloids. They are Microdina 

 paradoxa, Philodina Jiaviceps, P. brevipes, Furcularia reinhardti, and Euchlanis lyra. 



The shallow, weedy bays of the larger lochs, such as Inchnacardoch Bay in Loch 

 Ness, afford much the same breeding-grounds for Rotifers as ponds and bogs, and it 

 is in such bays that most of the casual species occur. Here we find swamp Rotifers, 

 Rotifers from streams, and moss-dwellers casually introduced, all flourishing together. 



There is one important distinction between such bays and ponds or swamps, which 

 probably accounts for the number of casual species being smaller than might be 

 expected. So long as these bays are in open communication with the deep water of 

 the lake, a moderate temperature is maintained. Inchnacardoch Bay was never more 

 than a trifling degree warmer than the centre of Loch Ness. 



The distinction drawn by Jennings (26) between swamp and lake Rotifers is as 

 clearly marked here, when such bays become in dry seasons completely cut off from the 

 loch. Such a case is found in an extensive swampy stretch in Burlom Bay, Loch Ness. 



TRANS. ROY. SOC. EDIN., VOL. XLV. PART I. (NO. 7). 22 



