44 



KR. BONNEVIE. 



The third theory (maintained for pelagic animals by Chun), 

 assumes that the deep-sea fauna should give the clue to 

 the solution of the bipolarity question. 



The results of my investigations into the comparative 

 anatomy of the thecosomatous pteropods are very much 

 in favour of the last supposition. In the deeper layers 

 of the ocean we have found three species (Peraclis diversa, 

 Limacina helicoides and Clio falcata), all with obviously 

 archaic characters, and we have found a gradual transition 

 with regard to all the important organs of the body between 

 these archaic deep-sea forms and the typical well-known 

 surface species. 



There is therefore every reason to believe that the 

 deep-sea pteropods have retained more of their original 

 characters than the surface species, which have developed 

 in divergent directions according to the varying conditions 

 in the different zones of the surface waters. 



It is of great interest in this connection to point out 

 that among the Cavoliniidae the species which are anato- 

 mically most closely connected with the deep-sea forms 

 (Clio cuspidata, Clio pyramidata and Diacria Irispinosa) 

 are eurythermal species, which may therefore, anatomically 



as well as geographically, be regarded as transitional 

 forms between the stenothermal archaic species of the 

 deep-sea and the equally stenothermal modern species of 

 the surface, now adapted to the conditions of a special 

 and more or less strictly limited temperature-zone. 



Another fact of general interest for a solution of the 

 bipolarity question is the very wide horizontal distribution 

 of the deep-sea forms. Thus Limacina helicoides has 

 been recorded from the coast of Ireland in the North to 

 to the mouth of the Congo and the Cape in the South, 

 while Clio falcata has been found at places so far apart 

 as Davis Strait in the North and the Canary Islands and 

 the South American coast in the South. If these forms 

 and perhaps a few others 1 ) really represent the survivors 

 of an archaic deep-sea fauna, then we should have to seek 

 an explanation of the bipolarity phenomenon in the corre- 

 sponding life-conditions of the arctic and antarctic regions 

 of the ocean. It would harmonize very well with the 

 results of modern investigations regarding heredity and 

 species-formation, if similar life-conditions affecting descen- 

 dants of the same or corresponding ancestors had also 

 produced a similar effect on the organisms. 



') As for example Clio Andrei, Clio sulcata. 



