Dakin and Latarche — The Plankton of Lough Neagh. 77 



The zooplankton of fresh water differs greatly from that of such an area 

 as the Irish Sea, not only in the entire absence of certain groups, but in the 

 marked absence of larval forms. Very many marine organisms belonging to 

 the bottom fauna have pelagic larvae which aid in distribution, and this is, 

 above all things, characteristic of the plankton of seas like the Irish, Baltic, 

 and North Sea. The suppression of a free-swimming larva is, however, one 

 of the characteristic features in the life-history of fresh- water animals, and 

 hence one great change in the character of fresh-water plankton. Another 

 feature in such plankton as that from Lough Neagh is the part played by the 

 Eotifera. As has been pointed out above, however, the most characteristic 

 feature of the fresh-water zooplankton is the small number of species present 

 compared with the thousands of the sea. Again, we never find large 

 plankton organisms in the fresh-water zooplankton. The zooplankton of 

 Lough Neagh is almost entirely made up of the following species : — 



SCHIZOPODA, Mysis relicta. 

 CoPBPODA, Diaptomus gracilis. 



Cyclops strenuus. 

 Cladoceea, Bosmina obtusirostris. 



B. longirostris. 



Daphnia hyaline var. lacustris. 

 Eotifera, Anurea cochlearis Gosse. 



A. aculeata Hhrenb. 



Notholca longispina Kellicott. 



N. striata Ehrenh. 



Polyarthra platyptera Uhrenb. 



Triarthra longiseta Ehrenh., and other more rare 

 genera. 

 Helizoa, Acanthocystis. 



Flagellata, Dinobryon cylindricum var. divergens (Lnh.) Lemm. 



D. protuberans Lemm. 



D. sertularia var. thyrsoideum Lemm. 



Probably no distribution has been so well discussed as that of Diaptomus 

 in the recent paper of Tollinger's.' From this we can obtain some information 

 as to the geographical distribution of Diaptomus gracilis, which is one of the 

 commonest species of Diaptomus in Europe. It occurs in Eussian Lapland, 

 Sweden, Norway, Finland, Denmark, France, England, Scotland, many lakes 

 in Ireland, Germany, Austria, Switzerland, Tirol, Eussia, Asiatic Eussia. 



' Die geogvaphische Verbreitung der Diiiptomiden. Yon M. A. ToUinger, Zool., Jabr 1911. 



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