1889-90.] Dr J. Murray & Mr R. Irvine on Coral Reefs. 81 
genera as those which build up coral reefs flourish throughout 
colder, and even in polar, seas. In these colder seas the re- 
presentatives of the reef-builders either do not secrete carbonate 
of lime in their body-walls, or if they do so, the shells or skeletons 
are much less massive than in tropical waters. An attentive ex- 
amination of the animals procured by the dredge and trawd from 
all depths shows that in descending into deeper water in equatorial 
regions the amount of carbonate of lime secreted by the animals 
living on the sea bottom becomes less with increasing depth, and 
all the calcareous structures of the organisms become less massive 
with the descent into the deeper and colder water of the abysmal 
regions. This remark does not, of course, apply to the shells and 
skeletons of surface organisms which have fallen to the bottom 
from the surface waters. 
Still another illustration of the same fact is furnished by the 
study of the pelagic organisms collected in the surface and sub- 
surface waters by means of the tow-nets. In the warmest tropical 
waters there are numerous species of Pteropoda, Heteropoda, 
Gasteropoda,, Foraminifera, and Coccospheres and Rhabdospheres 
(calcareous Algse), which lead a purely pelagic existence, and secrete 
carbonate of lime shells. Mr Murray estimates from his tow-net 
experiments that at least 16 tons of carbonate of lime exists in this 
form at any moment of time in a mass of tropical oceanic water 
one square mile in extent by 100 fathoms in depth.* The number of 
species and individuals of these lime-secreting organisms decreases, 
and the shells become less massive, with a wider removal from 
the equator and an approach to the colder water of the poles, 
till we find in the surface waters of the polar regions only one or 
two thin-shelled Pteropods, and one, or at most two/ dwarfed species 
of pelagic Foraminifera. It would appear then that organisms, as a 
whole or individually, are able to, and actually do, secrete more 
lime in regions where there is a uniformly high temperature of the 
ocean water than in those regions where there are great seasonal 
fluctuations of temperature, or where there is a uniformly low 
temperature of the water, as in the polar regions and in the deep sea. 
In temperate seas more carbonate of lime is secreted in the warm 
* Murray, “ Structure and Origin of Coral Reefs,” Proc. Roy. Soc. Edin . , 
vol. x. p. 508, 1880, 
VOL. XVII. 1/4/90 
r 
