156 MR JAMES MURRAY ON 



& tremula, are common in our small lakes; some of the other species may affect 

 more particularly larger lakes ; but no one species is general in the lakes, and it is 

 not by any means the case that any Synchseta is invariably present. Many lakes have 

 normally no Synchseta. I am inclined to regard all the Synchsetadse, like all the 

 Ploesomadse, as local species. 



Triarihra longiseta is more difficult to deal with. It looks a thoroughly limnetic 

 animal ; it has a wide distribution in Scotland ; and, being more frequently seen in winter 

 and early spring, it may have been overlooked in some lochs, and may be commoner 

 than we know. Still, the fact remains that we have only seen it in some twenty-four 

 lochs, and of these only five are moderately deep, while it is absent from all our greatest 

 lochs. It is less common than Gastropus sty lifer and Floscularia pelagica, which are 

 considered local species. While I am not prepared to trace the universal distribution 

 of the species in lakes, I would point out two facts which confirm our experience of it. 



Jennings (26) does not indicate that it is one of the common limnetic species in 

 the Great Lakes, giving only one record, from Sandusky Bay. Zacharias (56) 

 describes a var. limnetica, implying that the type is not limnetic ; but the variety 

 appears to be rare. The species is found in the plankton lists of many European 

 biologists, but it must be remembered that most of the biological stations are established 

 on shallow lakes. 



As to Anursea aculeata, our experience runs quite counter to Dr Lund's. The 

 species has not, to my knowledge, ever occurred in a purely limnetic collection from 

 any lake in Scotland. The type of the species is rare even in littoral collections. 

 Several varieties — A. serrulata, A. brevispina. and A. valga — are of more frequent occur- 

 rence among weeds. Of these A. valga is most nearly limnetic, being abundant in the 

 plankton of a number of small and shallow lakes ; but it also is absent from the larger 

 lakes. As in the case of Triarihra longiseta, Jennings' (26) few records indicate that 

 it is not a common lacustrine species in America. As to its presence in the plankton of 

 many European lakes, the same remarks apply as to Triarihra. 



Besides the five cosmopolitan plankton Rotifers, there have been observed in the 

 Scottish lochs a number of other species, as thoroughly limnetic, but of more local 

 distribution. These are Floscularia pelagica, Floscularia mutabilis, Triarihra 

 longiseta, Polyarthra euryptera, Synchseta pectinata, Synchseta tremula, Gastropus 

 stylifer, Ploesoma, huclsoni, Plcesoma truncatum, Anapus testudo, and Conochilus 

 volvox. 



Proales (Hertwigia) parasita, though not itself limnetic, is carried with its host 

 ( Volvox) into the open water of many lakes, and some even of our great lakes. 



Gastropus stylifer is the commonest of these species. It has been found in about 

 seventy lochs distributed over the whole of the mainland and islands. 



Conochilus volvox may be as common, or even more common ; but, as it is not so 

 easily recognisable when dead and contracted, we have fewer records for it. It is 

 widely distributed. 



