Pelagic Organisms in Scottish Lakes. 59 



it corresponds very closely with the area of greatest rain- 

 fall. A connection between this fact and the abundance of 

 Desmids might be supposed in two ways : granting that 

 peaty-water is favourable to Desmids, we may suppose that 

 the heavier rainfall washes out more peaty matter from the 

 bogs and carries it into the lochs ; or that it prevents the 

 accumulation of other substances in solution which would 

 be deleterious to Desmids. 



Considering the fact that the Desmids of the western type 

 have a distribution entirely littoral, occupying only those 

 countries bordering the J^orth Atlantic on both sides, I 

 think it highly probable that the conditions determining 

 their limits will be found to be mainly climatic, and that 

 the proximity of the ocean is one of these, whether by its 

 effect on the mean temperature of the adjacent fresh-water 

 lochs, or by affecting the rainfall. 



If the western Desmids have had their origin in North 

 America, and have reached our shores from that source, it is 

 difficult to account for their failure to spread farther over 

 Europe, unless there is some climatic check upon their 

 advance. 



A prominent feature of the Scottish plankton is the Arctic 

 character of its Crustacea. Dr Lund cites the following 

 species as belonging to the Arctic association of plankton 

 Crustacea : — Holopeclium gilherum, Bythotrejohes longimanus, 

 Diaptomus laciniatus, Da2Jhnia hyalina, and Bosmina oh- 

 tusirostris. All of those except the Dapinia and Bosmina are 

 summer species, and die off in winter. They form about one- 

 half of the total number of species of Crustacea in the Scottish 

 plankton, which is very poor in Entomostraca ; in quantity 

 they greatly preponderate during the summer. The other com- 

 mon Crustacea in the plankton are Diajptomns gracilis, Cyclops 

 strenuus, Leptoclora kincltii, and Polyphemus pediculus. 



Diaphanosoma hrachyurum is characteristic of our smaller 

 lakes, rarely occurring in the open water of the larger ones. 

 Leptoclora, Polyphemus, and Diaphanosoma are also summer 

 forms. Diaptomus laticeps and D. wierzejskii, though not 

 included by Dr Lund in the Arctic association, must also, 

 I think, be considered as purely northern species. 



