PLANKTON 269 



south meet and intermingle." ^ We can now give an oceano- 

 graphic explanation of the facts by showing that no less 

 than three masses of sea-water of different origin and 

 character may enter and affect the British seas in varying 

 quantity, viz. (1) Arctic water, such as normally surrounds 

 Iceland and the east of Greenland, and may extend farther 

 south and eastwards towards Norway, the Faroes, and 

 Shetland ; (2) Atlantic water (Gulf Stream drift), which 

 impinges on the western shores of Ireland and may flood the 

 English Channel, and even extend round the Shetlands and 

 down into the North Sea; and (3) "Coastal" water, such 

 as flows out of the Baltic and, mixed with the other waters, 

 bathes the coasts of N.W. Europe generally, and to a large 

 extent surrounds the British Islands. Each of these bodies 

 of water contains characteristic plankton organisms, and 

 this accounts for much of the variation in our fauna from 

 year to year. 



The Irish Sea, for example, may be regarded as primarily 

 an area of coastal water, which is liable to be periodically 

 invaded to a greater or less extent by bodies of warmer and 

 Salter Atlantic water, carrying in oceanic plankton, and 

 more rarely by Norwegian or Arctic water, causing an 

 invasion of northern organisms. The variations in the 

 nature and amount of the plankton at the same locality in 

 different years depend partly upon the volume and period 

 of such southern and northern invasions, but also upon 

 other factors, such as temperature, sunshine, rainfall, wind, 

 etc., at the time and previously. Of the half-dozen most 

 abundant Copepoda of the Irish Sea, only one, Temora 

 longicornis (Plate XXIII), is a " Neritic " form, native to the 

 locahty. The others are all usually regarded as " Oceanic," 

 that is, as having their true home and centre of distribution 

 somewhere to the north, west, or south in the open Atlantic. 



In many oceanographical inquiries there is a double object. 



1 Natural History of the European Seas, p. 80, Van Voorst, 1859. 

 But this portion was written by Forbes about 1853. 



