PHYSIOLOGY OF THE INVERTEBRATA. 251 



that bfive fallen from the surface waters, the remainder of 

 the carbonate of lime having been secreted by organisms 

 that live on or are attached to the bottom." 



(/) Sea water collected among coral atolls contained 

 nearly twice as much salts of ammonia as water from oceans 

 where the coral polyp does not live. It appears that the 

 carbonate of ammonia present in sea water arises from the 

 decomposition of animal products, and in the presence of 

 sulphate of lime of sea water becomes carbonate of lime and 

 sulphate of ammonia : 



(NHJ,C03 + CaSO, = CaC03 + (NHJ.SO,. 



" The sulphate of ammonia is in turn absorbed by the 

 marine tlora which form the food of the marine fauna, and is 

 in part resolved into nitrates and free nitrogen." 



" The whole of the lime salts in sea water may be changed 

 by the above reaction into carbonate, and may in this way 

 be presented to the coral and shell builders in a form suitable 

 for their requirements. The temperature of the water is of 

 great importance in this reaction. In cold water, of which 

 the great bulk of the ocean consists, the decomposition of 

 nitrogenous organic matter is greatly retarded ; whereas in 

 tropical surface waters it proceeds with great rapidity. 

 Here, then, w^e have probably the explanation of the massive 

 structures formed by lime-secreting organisms in the coral 

 reef regions, which are also the regions of highest and most 

 uniform temperature in the ocean. In the same way we may 

 account for the great extension of lime-secreting pelagic 

 organisms in the tropical surface currents that flow north and 

 south from the equator. Thus the coral reef-builders and 

 pelagic organisms may not only benefit by the decomposition 

 products arising from their own effete matters, but also from 

 the undecomposed nitrogenous matter carried to equatorial 

 regions from the cold water of the deep sea or from the polar 

 regions." 



{jj) As the quantity of carbonate of lime in sea water is 

 exceedingly small, it was supposed that the lime-secreting 



