ATLANT. DEEP-SEA EXPED. 1910. VOL. III]. 



PTEROPODA. 



43 



Peraclis diversa from its wide horizontal distribution 

 might be considered an eurythermal species, but textfig. 37 

 shows it to be strictly stenothermal; except for two or 

 three individuals out of 48 it was taken in waters having 

 a temperature of 6° to 10° C. and a salinity of 35-o to 

 35-5 pro mille. 



Similar conditions are found with regard to Limacina 

 helicoldes, which, as shown by tables I and II, was taken 

 during both the northern and southern crossings, but only 

 in the western part of the ocean while, as shown by 

 table III, most of the individuals were taken in relatively 

 shallow water (500 to 750 m.). This distribution is readily 



dark coloured, and two of them (Limacina helicoides and 

 Clio falcata) have on the tentacles peculiar white end- 

 plates, which are most probably organs of light-production 

 or of light-perception. 



"Bipolarity" of pelagic animals. The discontinuity in 

 the distribution of the same or of corresponding species 

 known only from the arctic and antarctic regions of the 

 sea is a fact well known in the case of pelagic animals 

 but many questions have still to be answered before a 

 full understanding of this fact is reached. 



This "bipolarity" has been explained in three essen- 

 tially different ways (see Meisenheimer 1905, p. 87—92): — 



93 C6 93 



Fig. 38. Hydrographical section along the northern route of the ''Michael Sars" Expedition. The positions where 



Limacina helicoides was taken are indicated by crosses. 



understood when the hydrographical conditions are taken 

 into consideration, as shown in textfig. 38; it is seen that 

 Limacina helicoides was found in a cold water-layer 

 (4° to 10° C), having a salinity below 35-o per thousand. 

 In the western part of the ocean this layer, at the time 

 of the "'Michael Sars" expedition, occurred at a consider- 

 ably lesser depth than in the eastern part. 



The third deep-sea species, Clio falcata, seems to 

 occur under similar hydrographical conditions as the two 

 species just mentioned, its optimum temperature being 

 also below 10° C. 



These three species, which live under conditions so 

 different from those at the surface, though belonging to 

 different groups of pteropoda have certain features in 

 common. They are all characterized by a series of archaic 

 characters in their anatomy (broad and folded plate-like 

 foot, a right-sided position of the pallial cavity, long 

 intestine, symmetrical visceral gangliar mass), they are all 



1) the corresponding forms at both poles represent 

 the survivors of a fauna previously equally distributed all 

 over the sea (Pfeffer, Murray) ; 



2) they originated in a continuous fauna occupying 

 a certain zone between both poles, and then gradually 

 withdrawing to the colder regions of the arctic and ant- 

 arctic water (Meisenheimer); 



3) the apparently discontinuous faunas at the poles 

 are still connected with each other through the cold-water 

 fauna of the deep-sea. (Ortmann, Chun). 



A decision between these different interpretations must 

 be based not only upon the geographical data given by 

 the authors mentioned, but also upon a knowledge of 

 the anatomy of the species living in different parts of the 

 ocean. The more archaic species, according to Pfeffer 

 and Murray might be excepted to exist near the poles, 

 while in the theory of Meisenheimer the more original 

 forms should be found in the warmer regions of the ocean. 



