GEOGRAPHIC DIVISIONS OF THE PELAGIAL 277 



North Atlantic, amounting in the southwestern portion of the North 

 Sea to 9° even on the bottom, and in the shallow coastal waters of 

 less than 20-m. depth to 15° or more, favor the development of 

 eurythermal animals which may even invade the colder seas and 

 thrive better there on account of the greater uniformity in conditions. 

 In addition to the Arctic area in the North Atlantic, with a tempera- 

 ture below 2°, aboreal area with surface temperature of 2-8° and a 

 temperate area with a surface temperature above 8° have been dis- 

 tinguished, each characterized by dominant forms. It would require 

 too much space to consider these subdivisions. 



Bipolarity. — Between the fauna of the arctic and antarctic seas 

 these are a large number of striking similarities, explicable in part by 

 convergent evolution in similar environments. However, the agree- 

 ment of the two communities goes farther than such a similarity. 

 There are doubtless homologies between them, for the same genera 

 and even identical species commonly appear in both seas. Many of 

 these have a world-wide distribution; in other cases, the genera and 

 species are not present in the surface layers of the transitional warm 

 seas but are found in deeper waters, so that a connected distribution 

 is established; in a smaller number of cases identical species have 

 been found which are not known as yet in the intermediate zones; 

 these are the so-called bipolar species. "Bipolarity" appears when the 

 similarity of the Arctic and Antarctic marine animals due to genetic 

 relationship is greater than their similarity to the intermediate species 

 of the warmer seas.* 



The connection of the fauna of the two polar seas through the 

 deep sea is confirmed by many findings. Of 14 antarctic Foramini- 

 fera, 19 12 also occur in arctic seas; they are partly eurythermal and 

 distributed generally, but they live partly as stenothermal animals at 

 great depths and only in the polar regions do they ascend to within 

 30-70 m. of the surface. Of the radiolarians, 20 a few species are com- 

 mon to both poles which live in the transition region in deep water. 

 The occurrence of the siphonophore, Diphyes arctica, in polar regions, 

 is linked with the finding of this species at great depths in the transi- 

 tion sea; 21 of medusae, scarcely any identical species occur in both 

 seas; but the genera are partly world-wide in distribution, partly 



* The following bipolar species may be mentioned : the annelid worms 

 Priapulus caadatus (in the Antarctic as forma tuberculato-spinosa) and Phascolo- 

 soma margaritaceum ; u species of Cumacea (Crustacea). Eocuma sarsii, Eudorella 

 truncatula, and Campylaspis verrucosa;™ and the barnacle. Balanus biporcalus;™ 

 the salt-water mites, Halacarus drygahkii, IF. alberti;™ the pteropoda Limacina 

 helicina and Clione limacina?* and perhaps a few others. 



