INVERTEBRATE BOTTOM FAUNA 459 



wegian Sea, and the littoral fauna naturally accords with its 

 surroundings. This is true also of the archibenthal area (that 

 is to say, the steep continental slopes) and the abyssal region. 

 The temperature at 1000 metres may be as high as 6' or 8' C, 

 and 2^ or 3° C. at still greater depths. Here, again, the fauna 

 conforms to its surroundings. In addition to the vast central 

 abyssal plain, the boreal region of the Atlantic includes the 

 coast plateaus off Europe and North -West Africa, and the 

 southern slopes of the ridges extending from the Shetlands to 

 Greenland, that is to say, practically the whole of the eastern 

 portion of the Atlantic. Arctic currents, on the contrary, 

 prevail in the western portion of the Atlantic, and cause 

 hydrographical, and therefore faunal, dissimilarities at different 

 parts of the coast. In the coastal areas south of Cape Cod 

 (about lat. 42" N.) we find Gulf Stream water and a character- 

 istic warm-water fauna ; but north of Cape Cod we meet with 

 an icy polar current descending from higher latitudes, so that the 

 stretch of coast from Cape Cod to the north of Newfoundland 

 must be looked upon as boreo-arctic. More genuinely arctic 

 conditions prevail off the coasts of Labrador and Greenland. 



Boreal Region of the Norwegian Sea 



The boreal coastal area may be divided into three vertical The coastal 

 zones, distinguished by different physical, topographical, and bo*rea°iVeg1on 

 biological conditions. The uppermost is the littoral zone, which of the 

 extends from the shore down to a depth of 'X)'^ or 40 metres — sea."^^^'^" 

 that is to say, almost as far down as there are sea-weeds. The 

 physical and topographical conditions characterising the littoral 

 zone are : periodic changes in temperature and salinity (the 

 temperature of the water being directly affected by that of the 

 air), strong light, and a great variety in the materials at the 

 bottom, such as loose stones, solid rock, sand with or without 

 coarse or fine fragments of different kinds of shells, mud, 

 and " mixed mud " — that is to say, sand, mud, and stones all 

 mixed together. Here we find the whole vegetation collected, 

 consisting of fucoids, green and red algae, Laniinaria, and 

 Zostera, all of which, as a rule, form big interdependent com- 

 munities that are very often arranged in belts. 



The lower limit of the sttblittoral zone on the west coast 

 of the Scandinavian peninsula may be put at about 150 

 metres. It differs from the preceding in being without 

 vegetation, as well as in having more uniformity in the bottom- 



