THE ROTIFERA OE THE SCOTTISH LOCHS. 157 



Plcesoma, of one species or another, is of general occurrence all over the country, but 

 we were unable to identify the species in so many of the lochs that the distribution 

 cannot be traced. P. hudsoni was the commonest species in the islands, but was also 

 found here and there on the mainland, where, however, some smaller species were 

 commoner. 



Floscularia pelagica is widely distributed on the mainland, and occurs in Shetland, 

 but was not observed in the Hebrides. Though only recorded from some thirty lochs, 

 it is probably much commoner, as it would readily be overlooked in preserved collections. 

 Triarthra longiseta was noted in some twenty-four lochs in Caithness, Sutherland, 

 Ross, Inverness, Edinburgh district, and Galloway. It was not seen in the islands. 



We have thus some sixteen species of truly limnetic Rotifers. Many other more 

 or less pelagic species have been found, but they are confined to little lochs or the 

 weedy bays of the larger ones, and cannot with us be called limnetic. A large number 

 of littoral species have occurred casually in the plankton collections, even to such 

 heavy-bodied creepers as Philodina rugosa and P. laticeps. 



Mr Hood, in a recent letter, gives some information as to the seasons when the 

 species of Plcesoma are found, and mentions two species which were not identified in 

 any of the Lake Survey collections. The two additional species are Anarthra aptera, 

 Hood (19 and 21), and Plcesoma lenticulare (18). Plcesoma hudsoni he finds from May 

 to August, P. truncatum from June to October, P. lenticulare from July to September. 



Many Continental naturalists give longer lists of plankton Rotifers from limited 

 districts or single lakes than are recorded for the very numerous lakes of Scotland. It 

 is well to bear in mind that most of the biological stations are situated on the shallower 

 lakes, and that the plankton lists include species which are not limnetic in this country. 



Dr Lund (32) records twenty-four Rotifers from the plankton of the Danish lakes, 

 ten or twelve of which are littoral species with us. Apstein (1) records twenty-three 

 species from the lakes of Holstein, eight or ten of which are not limnetic in Scotland. 



On the other hand, Forel (14) has noted just fifteen species in the great Lake of 

 Geneva ; Stenroos (48), eight in Nurmijarvi-See ; and Jennings (26), twelve species in 

 Lake Erie, all of which are limnetic according to our definition. 



The limnetic Rotifers, in common with the other pelagic organisms, extend through 

 all the open waters of the lake, right in to the shore, and frequently occur in the 

 washings of the littoral plants ; they also often occur in ponds. The limnetic region is 

 not characterised by the possession of any species peculiar to itself, but rather by the 

 absence of the majority of the littoral forms, even such as swim freely, and the extension 

 into it of a limited number of species which are especially independent of shelter. The 

 limnetic and the abyssal regions have this in common, that they are in Scotland 

 distinguished by negative rather than positive characters. 



How far the limnetic Rotifers extend beneath the surface of the lake is unknown ; 

 we have no data as to the vertical range of the plankton organisms, except for some of 

 the larger Entomostraca. 



