Dakin and Latarche — The Plankton of Lough Ncagh. 81 



In Lough Neagh we have a plankton which differs considerably from the 

 lake planktons of the English lakes and Scottish lochs, and resembles that 

 of a district far removed— Denmark. At the same time there are many 

 differences between these two plankton communities. The resemblances are 

 no doubt due to the en-\dronment. The lakes have the same shallow water's 

 and large surface-area, the same shelving banks. The presence of Mysis 

 relicta, no doubt, is due in both cases to biological factors. The possibility of 

 Mysis reaching the waters of a lake is not sufficient alone to account for its 

 presence now. It must have found the conditions suitable for its continued 

 existence. 



Messrs. West have shown how the Desmid characters of the English and 

 Scottish planktons are due to certain geological factors. We have emphasized 

 the complex interaction of all the organisms in the lake-waters. A profound 

 study of the plankton in this way should lead finally to the enunciation of 

 certain laws, and, given the climate, physical features of a lake and certain 

 predominant organisms occurring there, it may ultimately be found possible 

 to write off at once a list of organisms that sliould be found to occur in the 

 plankton, with their respective abu7idance and seasonal changes. 



Mysis relicta. 



This report would perhaps not have been considered incomplete if no 

 mention were made of the interesting relict Schizopod for which Lough Neagh 

 has become famous. 



Mysis relicta abounds in Lough Neagh waters, and has been collected by 

 numerous naturalists, amateur and professional, by using a light dredge on the 

 bottom. Nowhere in the literature is it spoken of as a common plankton 

 form ; and it has certainly been regarded by most workers as a creature living 

 on the bottom or rising at times a few inches above the mud. 



It only occurred in a whole year's daylight-plankton catches on one date. 

 On that occasion there was a very heavy sea running, and the bottom net 

 brought up quantities of mud. 



Paradoxical as it may appear, Mysis relicta is as important a plankton 

 form as any other species of plant or animal caught in the lake. In fact, one 

 might say it was the most common Crustacean in the plankton. Most surprising 

 feature of all, it is present in the plankton of the surface-ivater for probably the 

 whole year, and in considerable quantity. Why, then, has Mysis relicta been 

 practically entirely absent from the catches ? Why have not observers 

 commented upon the fact of a large shrimp-like creature being present in ■ 

 considerable quantities in the water ? The answer is that plankton-workers 



