38 



KR. BONNEVIE. 



[REP. OF THE "MICHAEL SARS" NORTH 



Fig. 32. The route of the "Michael Sars" Expedition < 



. Boundary 



line be*ween the circumtropical and the tranzitional zones Boun- 

 dary line between the transitional and the circumpolar zones 



The horizontal distribution of the thecosomatous ptero- 

 pods from the "Michael Sars" expedition is indicated in 

 the two tables on p. 46 and 47, the first table showing 

 the number of individuals of each species at the different 

 stations, while in the second table the stations are grouped 

 according to their geographical position, and the number 

 of individuals summed up for each of these groups. The 

 European-African coastline is divided according to its 

 position within the warm-water and the transitional zones, 

 while the stations (71 to 79) in the cold zone off New- 

 foundland form another group. The open ocean 

 is divided into four parts, the stations belonging 

 to the eastern half and the western half of each 

 crossing being taken as a separate group (SE, 

 SW, NW, NE). 



A division of the ocean like that just men- 

 tioned into a western and an eastern half might 

 perhaps at first seem arbitrary and inappropriate 

 for a study of organisms principally living in 

 the upper layers of the ocean, and therefore 

 independent of the sea-bottom, but a glance at 

 table I (p. 46 — 47), will at once justify such a 

 division, the material from Stations 52 to 84 

 in the western part of the ocean being very 

 different from that obtained at Stations 45 to 

 50 and 87 to 92 in the eastern part. The 

 stations off the Azores must with regard to 

 their pteropod-fauna be considered as belonging 

 to the western half of the ocean. 



The question as to the effect of temperature upon 

 the distribution of pteropods may best be answered by 

 comparing the northern and southern crossing. (Table II, 

 p. 46—47). 



The opinion that the pteropods are a group con- 

 sisting principally of warm-water animals is strongly 

 supported by the fact that the material brought home 

 from the southern crossing of the "Michael Sars" expedi- 

 tion is richer than that from the northern crossing, not 

 only with regard to the number of species (22:15), 

 but still more so with regard to the number of 

 individuals (5050 : 950); during the southern crossing 

 more than five times as many were taken as during 

 the northern crossing. 



Considering the relations between the different forms 

 taken during the two crossings, we find a striking diffe- 

 rence between the Limacinidae on the one hand and 

 the Cavoliniidae on the other, most of the Limacinidae 

 being represented in the material from both crossings 

 while most of the Cavoliniidae were taken during the 

 southern crossing only. 



The Limacinidae include, according to Meisenheimer, 

 a series of species found in all the temperature-zones, 

 but each species is absolutely typical of its own zone. 

 Thus Limacina helicina is a representative of the cold 

 arctic sea, while Limacina balea (reiroversa Meisenheimer) 

 belongs to the transitional zone, and the other species of 

 Limacina to the warm-water zone. 



With a few exceptions the results of the "Michael Sars" 

 expedition agree very well with Meisenheimer's views, 

 and even the exceptions serve on the whole in confirm- 

 ing them. 



Fig. 33. 



