1904 - 5 .] Study of the Lakes of Scotland and Denmark. 425 
With regard to the geographical distribution, none of the 
plankton-organisms present points of so much interest as the 
Crustacea. It has been mentioned that the plankton-organisms 
have an extremely wide distribution, and may be regarded as 
cosmopolitan ; most of the exceptions belong to the Copepoda 
and Cladocera. Steuer (1901) was the first to draw attention to 
the fact that the Diaptomidae and some of the plankton Cladocera 
seem to have well-marked areas of distribution. Steuer’s views 
have been corroborated and modified or enlarged by the excellent 
investigations of Ekman (1904) in the northern part of Sweden ; 
Ekman’s resrdts fully accord in all the main points with my observa- 
tions in the Danish lakes (1904). Having referred to these 
papers, I shall here restrict myself to those points having special 
reference to the fauna of the Scottish and Danish lakes. 
It may be regarded as a fact that there exists a peculiar associa- 
tion of Arctic plankton Crustacea, mainly restricted to the Arctic or 
North European lakes. This association is characterised by the 
common occurrence of Holopedium gibber urn, Daphnia hyalina, Bos- 
mina obtusirostris , Bythotrephes longimanus, Diaptomus laciniatus , 
the genus Heterocope (perhaps), and certain other species of 
Copepoda. Bosrnina coregoni , as well as B. longirostris and 
Hyalodaphnia cucidlata, are almost entirely absent ; these are the 
particular forms, besides several others, especially Diaptomus 
gracilis and D. yraciloides, Daplmella brachyura, Leptodora kindtii , 
which constitute the huge masses of zooplankton in the Central 
European plains. Of the sub-arctic association, some of the species, 
especially Diaptomus laciniatus , are also common in the alpine 
lakes of Switzerland and other lakes in the Central European alpine 
zone, but most of them ( Holopedium gibberum, Bosrnina obtusi- 
rostris) are never, or only exceptionally, found there. It seems 
to me that these southern alpine lakes are mostly inhabited by 
the same species which are characteristic of the Central European 
plains, and that the arctic elements are on the whole subordinate. 
The following facts may be briefly stated, from the explorations 
of Mr James Murray, and the excellent papers of Mr Scourfield 
and Mr Scott quoted in the Bibliography, as well as from my 
own investigations : — 
Holopedium gibberum is very common, and frequently “so 
