Dakin and Latahche — The Plankton of Lough Neagh. 61 



Murray finds that D. hyalina is general in Scotland, where it may be 

 perennial. In some of the higher lochs it dies out in the winter. Kofoid 

 records B. hyalina fi'om the plankton of the Illinois Kiver. The individuals 

 appeared in spring and attained their maximum in Jime, and disappeared in 

 July. . Apstein finds that D. hyalina occurs in the plankton of the Baltic 

 lakes from September to July, attaining its maximum in November to 

 January. 



The period of reproduction differs very considerably in "different districts. 

 In the arctic regions it is monocyclic, while in the low-lying Swiss lakes it 

 is acyclic. It has been known to reach its maximum, in spring, in winter, or 

 in both, and may be perennial or present in the plankton during a few months 

 of the year only. The species is common all over Europe, the arctic and the 

 tropical zones. In the Baltic lakes it is less common than in the arctic 

 lakes, and occurs more frequently as a pond form than in the pelagic region 

 of the large lakes. In L. Neagh at the time of its maximum it is the most 

 common crustacean in the plankton. 



Bosmina. 



Bosmina is one of the plankton forms that has been most studied from 

 the point of view of seasonal variation. More interesting still, however, is 

 the study of the distribution. The species of Bosmina that are common seem 

 to be very definitely distributed, so much so, in fact, that definite regions in 

 Europe can be mapped out by the presence or absence of certain forms. 



In L. Neagh Bosmina appears to be our most abundant Cladoceran 

 caught in the net-catches. The records for these forms in Ireland seem to be 

 very thin ; and in Lough Neagh the form recorded by Kane is Bosmina mixta 

 (stated by him to be not previously known from the British Isles). This 

 species is certainly one of the lesser characteristic forms; and it renders it all 

 the more curious that Kane has not recorded Bosmina ohtusirostris from Lough 

 Neagh — a form which is somewhat closely approached by B. mixta. We have 

 found two species in large numbers, viz. Bosmina ohtusirostris and Bosmina 

 longirostris. This, of course, does not preclude the existence of other species 

 in small numbers. As mentioned before, the work has not been of a detailed 

 systematic kind. 



We have discussed in another part the remarkable character — Bosmiiut 

 ohtusirostris and B. longirostris occurring together in considerable nimibers 

 and their relations from the point of view of distribution. In the table the 

 two species have been lumped for convenience in counting. In Scotland the 

 most common species appears to be the arctic form B. ohtusirostris. In Lough 

 Neagh B. longirostris appears to occur in the greatest numbers, although both 



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