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



PTEROPODA. 



37 



much in the phylogenetic development of 

 a species as in the general shape of the 

 body. The heart must have space enough 

 to do its work, and therefore it is always 

 found at the most convenient place. In 

 the broad and flattened Clio falcata good 

 conditions are found under the convex shell 

 of the ventral side of the body, while in 

 species with a narrow and pointed posterior 

 end, or with a circular cross-section of the 

 body, such a transverse position would 

 prove inconvenient, and the heart is found 

 in a position parallel to the long axis of the 

 body (as in Clio pyramidala, Creseis, etc.). 





\^^yvy\Xt/CJL^\bct<' 



A^CH'O/twi/ni^ 



Summing up the results of my in- 

 vestigations into the comparative anatomy 

 of thecosomatous pteropods, I consider it very probable 

 that in the course of time a phylogenetic development 

 has taken place from forms like those still represented 

 by the genus Peraclis in three different directions, viz: 



1) through the genus Procymbulia towards the family 

 Cymbuliidae; 



2) through forms somewhat like Limacina helicoides 

 to the more typical species of the genus Limacina; 



3) through forms like Clio falcata to the family 

 Cavoliniidae. 



In the Cavoliniidae a development seems to have 

 taken place in two divergent directions, corresponding to 

 the two genera Clio and Cavolinia, each including two 

 or more subgenera, of which Euclio and Diacria represent 

 the first steps of development. (See the diagram, text- 

 fig. 31). 



With regard to the generic relations of the genus 

 Cuvierina 1 ) and of the other subgenera of the genus Clio, 

 my material did not allow of any thorough investigation. 



2. Geographical distribution. 



Meisenheimer (1905—06) in his excellent works on 

 Pteropoda has treated so fully the geographical distribution 

 of each species that apparently very little remains to be 

 added by later investigators, and on many points I can 

 merely confirm his results. The special methods used 

 on board the "Michael Sars" in the way of long horizontal 

 plankton hauls at different depths have, however, given 

 results, regarding the numerical occurrence of each species 



QUi> j eJLoXa. - &m.(2fco [fjf ■ Olio. - - - . ) 



<b&n: L/lwie/t-i/rua. 



(*>. ^ . SD.i^xe'vta.-(^Ct^o&/vvla.J 



Fig. 31. 



in various regions and at various depths, which may serve 

 to widen our knowledge of their biology and vertical 

 distribution. 



Horizontal distribution. In discussing the horizontal 

 distribution of pteropods Meisenheimer divided the surface- 

 waters of the ocean into three large zones surrounding 

 the whole globe, viz: a warm circumtropical zone and 

 two cold circumpolar zones. On the borders between 

 these zones are to be found transitional regions, which 

 are geographically and faunistically well defined, and in 

 their essential characters may be considered as belonging 

 to the cold zones. 



As regards the Northern Atlantic, the warm circum- 

 tropical zone has its northern limit in a line running from 

 Cape Hatteras off the American coast (35° N.) in a north- 

 easterly direction towards the European coast (44° N.), 

 thus including all the southern part of the Gulf stream. 

 The transitional region includes, according to Meisen- 

 heimer, the northern part of the Gulf stream, while the 

 cold circumpolar zone stretches its arms southward along 

 the coasts of Greenland and follows the Labrador current 

 along the American coast, covering the Newfoundland banks. 



As will be seen from the chart (textfig. 32) the "Michael 

 Sars" traversed all these three zones, the southern cros- 

 sing of the ocean lying principally within the warm-water 

 zone and the northern crossing within the transitional 

 zone, the cold zone being touched on the way to and 

 from Newfoundland. The material brought home by this 

 expedition should therefore give a good opportunity of 

 testing the correctness of Meisenheimer's results for this 

 part of the ocean. 



') Through later investigations 1 have found a twisting of the oesophagus in Cuvierina very much like that of Diacria and indicating 

 relationship with the archaic species Clio falcata. 



