THE ROTIFER A OF THE SCOTTISH LOCHS. 155 



for a definite time through the open water of the lake, as far as convenient from the 

 shore. It is advisable to draw them for a time at some distance below the surface, say 

 20 feet, as well as at the surface, because in extremes of weather the animals sometimes 

 retire from the layer close to the surface. They should be examined as soon as possible 

 after collecting, as most of them very quickly die under the changed conditions. While 

 some will survive for a time in the bottles, others, such as Notholca longispina, are so 

 sensitive to change of temperature that they are seldom found alive when the collections 

 are brought home. 



Although very many Rotifera are free-swimming, comparatively few are limnetic, 

 albeit, if the whole world is taken into account, the number is considerable. By 

 limnetic Rotifers is meant such species as normally take up their position, far from the 

 shelter of plants, in the open water of the lake, and extend to every part of it. 



Of the truly limnetic Rotifera, few occur together in any one lake ; their range may 

 be world-wide, but their distribution is local. A species regarded as limnetic in one 

 part of the world may be only known as an inhabitant of the lake-margins elsewhere. 

 It is well to distinguish, among the limnetic species of one lake or district, between 

 these more or less local species, and the others which belong to that universal 

 association of limnetic animals which are present in all lakes offering normal conditions. 



Dr Lund, in the paper above cited (34), gives a short list of species which he 

 characterises as " the cosmopolitan stock of plankton Rotifers." These are Polyarthra 

 platyptera, Synchseta sp., Asplanchna priodonta, Anursea cochlearis, Anuria 

 aculeata, Notholca longispina, Conochilus unicornis, and Triarthra longiseta. 



On the whole, Dr Lund's list embodies the species which we find to be most 

 generally distributed in the Scottish lochs. Inasmuch, however, as it is difficult to 

 avoid generalising from partial data, it may be useful if we examine Dr Lund's list in 

 the light of our experience in the Scottish lochs, and indicate some points to which we 

 must take exception. 



Scotland is pre-eminently a country of lakes. Considering its situation in a 

 temperate region, the great number of its lakes, many of which, though not of great 

 extent, are from their depth to be classed among great lakes, we would be justified in 

 regarding Scotland as favourable for the existence of the cosmopolitan stock of Rotifers. 

 We would expect to find this stock in all our greater lakes ; we would at the least 

 expect that no member of it would be absent or rare. The fact that five out of 

 Dr Lund's eight cosmopolitan species are our commonest limnetic species shows that 

 Scotland is suitable for them. These five most thoroughly limnetic species are 

 Polyarthra platyptera, Asplanchna priodonta, Anursea cochlearis, Notholca 

 longispina, and Conochilus unicornis. 



Let us now consider the three species which do not live up to their cosmopolitan 

 character in Scotland. 



Synchseta sp. is unsatisfactory, as Dr Lund does not name the species which he 

 regards as cosmopolitan. Various species of Synchseta, especially S. pectinata and 



