INVERTEBRATE BOTTOM FAUNA 557 



low temperature depends upon their finding the conditions 

 necessary for reproduction, namely, higher temperatures during 

 a portion of the year. 



With regard to vertical distribution, it should be noted stenothermal 

 that the deeper a species lives the more uniform is the tempera- ^°'^'^^^' 

 ture to which it is exposed (stenothermal forms). This is true 

 especially of the boreal areas, whereas in arctic tracts there is, 

 as a rule, less difference between the temperatures in deep and 

 in shallow water. It is not so much the depth as the tempera- 

 ture which regulates the distribution of animals. Another 

 factor affecting distribution is salinity. Many forms, particularly Euryhaiine 

 the littoral ones, can stand a considerable variation of salinity Jafine^fo°ms, 

 (euryhaiine species), while others are limited to water varying 

 little in salinity (stenohaline species) ; the former includes 

 those littoral forms which are as much at home among the 

 skerries as far up the fjords or even in the mouths of the rivers, 

 while the latter are only to be found off the coast or at 

 considerable depths. 



I have already tried to make it clear that no arrangement of 

 vertical faunal zones applies to the whole of the Norwegian Sea. 

 Forms which near the coast inhabit the littoral zone may be met 

 with, normally apparently, out on the plateaus, in the sub-littoral 

 zone, or perhaps in the deep-sea zone. Thus in the northern 

 portion of the North Sea the trawl brought up from a depth of 

 180 to 190 metres Ophiothrix fragilis and large specimens of 

 Ejipagnrus bei'nhardiis — forms which are distinctly littoral along 

 the Norwegian coast, and on the Faroe plateau we found these 

 and a number of others at no metres. When we compare the 

 North Atlantic with the Norwegian Sea we find still more strik- 

 ing differences, some of the species belonging to the Norwegian 

 Sea occurring at far greater depths in the Atlantic. Now if we 

 remember that the physical conditions in the medium in which 

 a species lives are largely responsible for its vertical distribution, 

 we may assume that in the littoral zone of the coastal waters and 

 in the deeper parts of the Norwegian Sea and Atlantic there are 

 at any rate certain identical conditions — temperature is most 

 decidedly not one of them — which permit these species to live 

 impartially in these areas. If it were merely a question of 

 adaptation to quite different conditions, we might expect them 

 to adapt themselves also to the deeper water-layers along the 

 coasts. 



Light is unquestionably one of the principal factors affecting Effect of light. 

 vertical distribution. During the Atlantic Expedition of the 



