INVERTEBRATE BOTTOM FAUNA 



535 



Fig. 375. 



Tridoiita borealis, Chemn. 



(After G. O. Sars. ) 



do not enter the boreo-arctic area to be designated an inter- 

 mediate area. Possibly both Ctenodiscus crispatiis and Leptop- 

 tychaster arcticus Hve chiefly in isolated basins, where the tem- 

 perature for part of the year sinks lower than in the other parts of 

 the fjord, though I do not know that 

 this has actually been confirmed. 



Occasionally too we find in far 

 more southern areas a few forms 

 that must be considered purely 

 arctic, although they are quite accli- 

 matised and plentiful. They are 

 survivals (relicts), and date from the 

 glacial age when the northern seas 

 were inhabited by an arctic fauna. 

 The milder climate which succeeded 

 the glacial period brought about the 

 elimination of all those species that are now purely arctic, and 

 such forms are at present practically limited to arctic tracts. 

 Only a few were able to adapt themselves to the altered con- 

 ditions,^ and are to be found to this day 

 in isolated areas, located outside the 

 range of this chapter, though owing to 

 the interest attached to them, they 

 may be briefly alluded to. 



There is, for instance, the mussel 

 Astarte {Tridonfa) borealis (see Fig. 

 375), large quantities of which are 

 found in the arctic tracts from Lofoten 

 northwards. In the south we do not 

 find it till we come to Oresund, The 

 Belts, and the Western Baltic, where 

 it is very plentiful. In the interven- 

 ing waters it is merely a stray guest, 

 having been found once or twice in 

 the neighbourhood of Bergen. The 

 survival forms include also a few crus- 

 taceans, for instance, the isopod Idotea entoinon (see Fig. 

 376), some worms, and a sea scorpion {Cothis quadricorms), 

 which are mostly to be found in the Baltic, and in a few lakes 

 of North Europe that were connected with the sea in the 

 glacial age. 



^ On the other hand there are, as already stated, a number of forms from the glacial age 

 which became thoroughly acclimatised, and, in contradistinction to the relict-forms, are 

 widely distributed throughout both regions. 



Fig. 376. 

 Idotea entomon, L. (After Stuxberg. ) 



