. The depths of the ocean; a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. Oceanography. VIII INVERTEBRATE BOTTOM FAUNA 535. Fig. 375. Tridonta borealis, Chemn. (After G. O. Sars.) do not enter the boreo-arctic area to be designated an inter- mediate area. Possibly both Ctenodiscus crispatus and Leptop- tychaster arcticus live 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 a
. The depths of the ocean; a general account of the modern science of oceanography based largely on the scientific researches of the Norwegian steamer Michael Sars in the North Atlantic. Oceanography. VIII INVERTEBRATE BOTTOM FAUNA 535. Fig. 375. Tridonta borealis, Chemn. (After G. O. Sars.) do not enter the boreo-arctic area to be designated an inter- mediate area. Possibly both Ctenodiscus crispatus and Leptop- tychaster arcticus live 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,1 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 (Tridonta) borealis (see Fig. %V^ 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 entomon (see Fig. 376), some worms, and a sea scorpion [Coitus quadricornis), which are mostly to be found in the Baltic, and in a few lakes of North Europe that were
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