402 EXISTENCE AND DISTRIBUTION OF MARINE ORGANISMS. 



the open ocean, are absent or but sparingly represented in tow-net 

 gatherings near the land. 



All the carbonate of lime secreting organisms are much more abun- 

 dant in the warmer than in the colder waters, and we have seen that 

 their dead shells are much more abundant at the bottom within the 

 tropics than toward the polar areas j indeed, through these organic 

 processes the lime present in solution in the ocean, probably in large 

 part originally derived from the disintegration of the continental rocks, 

 is at the present time being accumulated toward the tropical regions 

 of the earth. In the Tropics there are in the surface waters over 

 twenty species of pelagic foraminifera which secrete thick carbonate of 

 lime shells. These mostly disappear as the colder waters of the polar 

 regions are approached, and are there represented by two dwarfed 

 species of globigeriua. In the same way many species of shelled ptero- 

 pods, heteropods, and pelagic gasteropods live iu the warmer waters of 

 the tropics, but disappear or are represented only by small thin-shelled 

 limacina? or naked species in polar waters. The calcareous coccospheres 

 and rhabdospheres of the tropical and warm waters give place in polar 

 waters to species of alga?- which secrete no lime. 



This abundant secretion of carbonate of lime in the warm waters of 

 the Tropics is apparently due to chemical rather than physiological con- 

 ditions. When neutral ammonium carbonate is added to sea water at 

 a high temperature — 80° to 85° F. — the lime salts other than carbonate 

 present in sea water are quickly decomposed and an immediate precipi- 

 tate of carbonate of lime having the properties of aragonite is formed, 

 while if the same experiment be carried out at a low temperature — 40° 

 to 45° F. — the carbonate of lime separates out very slowly and in doing 

 so takes the form of calcite. The abundant secretion of carbonate of 

 lime in the warm waters of the Tropics at the present day, as well as 

 the feeble development of carbonate of lime organisms in cold polar 

 regions, are interesting facts when we remember that coral reefs flour- 

 ished within the Arctic Circle during Palaeozoic and even later times, 

 and from the manner in which the lime is secreted we may safely con- 

 clude that the polar waters in these ancient times must have had a 

 temperature of about 70° F. (21° 1 C). 



Not only is the number of species of lime- secreting organisms in the 

 surface waters of the Tropics greater than in the cold water of the 

 polar regions, but the same holds good for the radiolaria and nearly all 

 other classes of pelagic organisms, the characteristic of the pelagic 

 organisms of the polar areas being a relatively small number of species 

 and a great abundance of individuals. Another peculiarity of the tow- 

 net gatherings in the Arctic and Antarctic areas is the almost complete 

 absence of pelagic larvae of benthos animals, which are so abundant in 

 the surface waters of the Tropics. A comparison of two tow-net gath- 

 erings conducted under precisely similar conditions, one in the cold 

 waters of the Antarctic and the other in the warm waters of the Tropics, 



