CHAPTER XII 



DISTRIBUTION OF MARINE MOLLUSCA — DEEP-SEA MOLLUSCA 

 AND THEIR CHARACTERISTICS 



Marine Mollusca may be divided roughly into Pelagic and 

 non-Pelagic genera. To the former division belong all Ptero- 

 poda and Heteropoda, and a large number of Cephalopoda, 

 together with a very few specialised forms of Gasteropoda 

 (jLanthina^ Litioj^a, Phyllirrlioe^ etc.). Pelagic Mollusca appear, 

 as a rule, to live at varying depths below the surface during the 

 day, and to rise to the top only at night. The majority inhabit 

 warm or tropical seas, though some are exceedingl}^ abundant 

 in the Arctic regions ; Clione and Limacina have been noticed as 

 far north as 72°.^ 



The vertical range of Pelagic Mollusca has received attention 

 from Dr. Murray of the Challenger^ Professor Agassiz of the 

 Blake and Albatross^ and others. Agassiz appears to have estab- 

 lished the fact that the surface fauna of the sea is limited to a 

 comparatively narrow belt of depth, and that there is no inter- 

 mediate belt of animal life between creatures which live on or 

 near the bottom and the surface fauna. Pelagic forms sink to 

 avoid disturbances of various kinds, to depths not much exceed- 

 ing 150 to 200 fathoms, except in closed seas like the Gulf of 

 California and the Mediterranean, where the bathymetrical range 

 appears to be much greater.^ 



Non-Pelagic Mollusca are, from one point of view, con- 

 veniently classified according to the different zones of depth at 



1 The distribution of some Pteropoda has been worked out by Munthe, Bih. 

 Svensk. Ak. Handl. XII. iv. 2, hy Pelseneer "■Challenger " Bep.^ Zool. xxiii., and 

 by Boas, Spolia Atlantica. 



2 Bull. Mus. C. Z. Harv. xiv. p. 202 ; xxiii. p. 34 f. 



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