CTENOPHORA. 



93 



appears not to occur in the Greenland Seas, as also Romer (Op. cit. p. 76) and Damas & Koefoed 

 (Op. cit p. 413) state that it does not occur under high-arctic conditions. The more remarkable is the 

 fact that it occurs in the antarctic regions under conditions corresponding to the most arctic (Moser. 

 Ctenoph. d. deutsch. Siidpolar-Exp.) --so remarkable, indeed, that one can scarcely help thinking that 

 it may, perhaps, not be really this species. 



While these three species are thus of little or no value for judging of the currents, the case 

 is quite different with Mertensia ovum. This is a true arctic form, not occurring outside the polar water, 

 and if it is found in some place, where it does not occur usually, it affords the proof that arctic water 

 has intruded and I would even be inclined to reverse the statement, viz. that where Mertensia ovum 

 does not occur, there is no polar water. 



This has an important bearing upon the question, whether the polar water reaches the North 

 European Seas, as is maintained by Aurivillius 1 ) and Chun 2 ). "Im Laufe des Winters kann das 

 Plankton der kalten Stromgebiete sich so weit nach Suden vorschieben, dass von den vereisten Fjorden 

 West-Gronlands in dem Smith-Sund und in der Baffinsbai bis nach Helgoland und in die westliche 

 Ostsee eine einheitliche arktische Fauua die Oberflache bevolkert". (Chun. Op. cit. p. 8). The proof of 

 this Chun finds in the occurrence in these waters of such forms as Pleurobrachia pileus, Bolina in- 

 fimdibulum and Beroe cucumis, the Siphonophore Diphyes arctica, and the Appendicularians Oikopleura 

 labradoriensis and Fritillaria borealis, while Aurivillius mentions as "arktisch im engen Sinne": 

 Sagitta arctica, the Crustaceans Calanus liyperboreus, Euchata norvegica, Metridia longa, Euphausia 

 inermis and the Pteropod Clione limacina (Op. cit. p. 87 91). That the Ctenophores named are really 

 without value as a proof of the presence of arctic water, is already sufficiently explained, and the 

 same appears to hold good of all the other forms. Of Diphyes arctica Damas & Koefoed (Op. cit 

 p. 414) state that "sa distribution est independante de la temperature, de la salinite et de 1'origine des 

 eaux dans lesquelles il se trouve"; Fritillaria borealis is known also from the Mediterranean and the 

 Bismarck Archipelago. Without entering on a more detailed discussion of the geographical distribution 

 of all the forms mentioned above -- (most of them are given in "Nordisches Plankton" as distributed 

 over the North Atlantic down to ca. 50 60 N.) I must express my conviction, in perfect accord- 

 ance with Damas & Koefoed (Op. cit), that not one of them is really and exclusively arctic and 

 thus cannot be taken as a proof of the intrusion of arctic water into the North-Sea and Skagerak. 

 Mertensia ovum, if definitely proved to occur there, would give the indubitable proof of the intrusion 

 of arctic waters here ; but there is no proof that this species really occurs in these Seas (Comp. above, 

 p. 65). I quite agree with Moser (Ctenoph. d. deutsch. Siidpolar-Exp. p. 180) that the fact of Mertensia 

 ovum not occurring here is a proof that the arctic waters do not reach so far South in the North 

 European Seas, "denn andernfalls ware nicht einzusehen, warum Mertensia ovum nicht zeitweise auch 

 hier, wie an der amerikanischen Kiiste, weiter siidlich vordringeu sollte". - This result is in accord- 

 ance with the view of the hydrography of these seas set forth by Martin Knudsens) where the 

 polar stream is stated not to reach the North European Coasts. 



') Carl W. S. Aurivillius. Vergleichende thiergeographische Untersuchungen iiber die Plankton-Fauna der Skage- 

 raks in den Jahren 18931897. Kgl. Svenska Vetensk. Akad. Handl. Bd. 30. 1898. (p. 133 135). 



2 ) C. Chun. Die Beziehungen zwischen dem arktischen und antarktischen Plankton. 1897. 



3) Martin Knudsen. Havets Naturlaere. Hydrographi, med sserligt Hensyn til de danske Farvande. Skrifter udg. 

 af Komniissionen for Havundersogelser. Nr. 2. Kobenhavn 1905. 



