Dakin and Latakchk — The Plankton of Lough Neacjh. 79 



Now, let us consider briefly the plankton as a community determined by 

 environment — the environment including all external factors whatever that 

 can act upon the organisms. 



The marine plankton is often divided, by those who work at this particular 

 branch of the subject, into oceanic and neritic. Oceanic plankton forms are 

 naturally characteristic of ocean waters far from land. Neritic species are 

 typical of the seas like the Irish Sea. This division, which may appear quite 

 simple, is no simple matter when looked into. Many forms occur both in 

 the oceanic regions, and also in coastal waters and enclosed seas. It is a 

 matter of considerable difficulty to say, with reference to many species, 

 whether a plankton organism is then an oceanic or a neritic form ; if, however, 

 we look at the plankton as a whole, there are many features which are 

 peculiar to the plankton of an enclosed coastal area like the Irish Sea or 

 North Sea. The plankton bears some character which stamps it as belonging 

 to an inland sea (using the term to denote an arm of the ocean). Moreover, 

 a comparison will show that while all seas of this kind have the same general 

 character,, they each bear sub-characters which individualize them. All 

 plankton communities bear a specific character. They are like the human 

 races, but with still greater marks of difference. Now, the difference between 

 an oceanic and a neritic plankton community is a result of environment. 

 That environment is not, however, merely the different nature of the water. 

 The plankton of an area like the Irish Sea is perhaps marked most by the 

 large number of larval forms present — larvae of fixed organisms which in 

 adult life do not belong to the plankton at all. As examples might be 

 mentioned, Balanus larvae, Echinoderm larvae, Crustacean larvae of crabs, 

 lobsters, &e., worm larvae, and so on. Now, oceanic plankton forms, which 

 perhaps could live equally well in coastal water so far as the chemical 

 conditions alone are concerned, would have to enter into competition with 

 those organisms whose special home is in the shallow water. It is probably 

 this very competition that has driven them out in the first case, so that the 

 impossibility of coming back is obvious. Thus we have a particularly good 

 example of the action of other planktonic organisms forming part of the 

 environment for any special form that we may take. 



In fresh water the same holds good. Lough Neagh plankton, examined 

 in general, has a particular character which is peculiar to the lake. It is a 

 property of this community, and renders it different from other lake planktons 

 even though they may contain almost all the species. 



Moreover, this character, which we shall speak of as the Ethos of the 

 plankton, differs for the seasons, as a glance at the photo-micrographs will 

 show. These photos are of slides, each made by taking a drop from the same 



