INVERTEBRATE BOTTOM FAUNA 517 



flourish in some localities in such immense quantities as to 



displace all others, a phenomenon that may certainly be seen 



also now and then in the boreal region, though not to such a 



marked extent. Even when several species occur together the 



specimens appear to be more numerous than is the case in 



the boreal region. On one occasion in the Barents Sea the 



" Michael Sars " brought up in a single trawling over a ton of 



big sponges [Geodea), and near Jan Mayen at another time 



more than a barrelful of Pecten gronlandictis. The prawns 



again are sometimes in myriads, and Sars relates that during 



the Norwegian North Atlantic Expedition the trawl came up 



positively full of the feather star, Antedon eschrichti. One Direct 



reason for such enormous quantities of individuals is that many development. 



of the arctic animals produce their young fully developed, without 



any free pelagic stage, so that in all probability a large proportion 



continue to live where they were born.^ Currents, the nature 



of the bottom, and conditions of nourishment must also be 



taken into account. ''' 



Nowhere perhaps do we find such a marked contrast between 

 the boreal and arctic faunas as when we pass from one of the 

 boreal coast plateaus out into the cold area of the Norwegian 

 Sea. If we trawl on the plateaus, where the temperature does 

 not sink below 6° or 7 C, we find a boreal fauna consisting to a 

 great extent of forms which have migrated into the Norwegian 

 Sea from southern latitudes. As soon, however, as we come to 

 the slope of the deep basin (the cold area), at a depth of say 

 600 to 800 metres,-' where the temperature falls below 0° C, 

 the exclusively arctic element begins to predominate, and we 

 meet with species that are almost entirely foreign to the banks 

 and coasts of the boreal region. 



There is the remarkable Umbellula encrinus (see Fig. 358), Arctic fauna 

 a form belonging to the pennatulids, that may grow several p^Jt^ofX'"' 

 metres high, with large rosette-like polyps at the upper end of cold area of 

 the stalk. Of star-fishes we have the beautiful purple Pontaster ^J^egJ^n'^sea. 



1 Romer and Schaudinn, Fauna antka, Einleitung, p. 48 ; see also Murray, Trans. Roy. 

 Soc. Edin., vol. xxxviii. p. 492, 1896. 



2 At one locality in the North Sea we captured large numbers of snails ( Sipho gracilis and 

 Nepttmea antiqiid) and of a sea-mouse (Spatangits purpureus). The first named deposits its 

 eggs in capsules, from which the young emerge fully developed, a circumstance sufficient to 

 explain their plentifulness, but Spatangus has floating larvae, so that other factors must have 

 come into operation. There may be an aggregation of individuals in a limited area without 

 direct development, provided the larvre are not carried away by currents ; thus our common 

 ascidian {Ciona intestinalis) often forms large congregated masses owing, as far as I could make 

 out, to the fact that the eggs sink in large quantities by the mother's side, and develop in a 

 comparatively short space of time. 



^ The depth at which the temperature falls below 0° C. is liable to variation ; north of 

 Tampen the " Michael Sars" found such temperatures in 1902 at about 550 metres. 



