INVERTEBRATE BOTTOM FAUNA 551 



environment without any considerable changes in their bodily 

 structure, as for example the decapod crustaceans Hippolyte 

 seciirifj-ons (boreal) — Hippolyte spiims (arctic), Sabinea sarsi 

 (boreal) — Sabinea septenicarinata (arctic). These forms are so 

 alike that I cannot help thinking they must have had some 

 phylogenetic connection in a geologically not very remote past. 

 Other forms of the same category have no near relations in the 

 arctic region, and cannot, therefore, be of arctic origin. That 

 these species lived in the Norwegian Sea in late glacial times, 

 when more boreo - arctic conditions prevailed, seems evident 

 from their normal distribution nowadays in boreo-arctic areas, 

 but it is impossible to decide whether they migrated into the 

 Norwegian Sea from the American or the European side, or 

 are derived possibly from southern species which have become 

 morphologically so altered in their new home that the specific 

 differences are unmistakable. 



There are other species in the Norwegian Sea which, so 

 far as is known, are strictly confined to the boreal and boreo- 

 arctic areas, extending neither southwards nor to the coasts 

 of North America in the west. They are, however, not very 

 numerous. Like the forms just mentioned they could not 

 have lived in the Norwegian Sea during the Glacial Age, 

 and have probably originated there in post-glacial times, 

 through development from southern immigrants that have been 

 morphologically altered by adaptation to their environment. 

 Several of them are closely allied to species known outside the 

 Norwegian Sea. In some cases there would seem to have been 

 a variation from the immigrated species, and we find inhabiting 

 the Norwegian Sea both the primitive form and its descendant, 

 like the crustaceans Pag2Lrus chir'oacanthus (a purely boreal 

 endemic species) — Pagurus l(svis (immigrated primitive form), 

 Cheraphilus (purely boreal endemic) — Crangon or Pontophilus 

 (immigrated primitive form), Virbius fasciger (purely boreal 

 endemic) — Virbiiis varians (immigrated primitive form). We 

 may take it for granted, in view of what we know nowa- 

 days regarding the larger invertebrate forms, that the majority 

 of these species have not a widespread distribution either 

 southwards or westwards, and this might give grounds for 

 believing that they had immigrated in their present form. 



I have already mentioned that the littoral and sub-littoral Distributional 

 faunas differ greatly in different areas of the Atlantic, and we ^'^^^• 

 find similar differences when we compare the Atlantic and 



