278 THE BIOLOGY OF HYDRA : 1961 



played by photosynthesis in facihtating the deposition of calcare- 

 ous matter in a wide variety of hermatypic organisms, irrespective 

 of the possibility that the mechanisms concerned may be very 

 different. 



Comparison of the results summarised in the first two columns 

 of Table I shows that skeletogenesis rates calculated from Ca^' 

 uptake are much higher than those calculated from the simul- 

 taneous C^^ carbonate uptake. In CaCO... the stoichiometric mass 

 ratio of calcium to carbon is 40 12 or about 3.335. This ratio should 

 apply to the mineral constituent of the coelenterate and algal skel- 

 etons which is mostly CaCO:., though some of the algae may con- 

 tain traces of dolomite in addition to calcite and aragonite (18). 

 However, the ratios calculated from our data are nearly all higher 

 than the theoretical value, and they vary over a wide range. This 

 either indicates that the organisms are secreting a skeletal mineral 

 greatly enriched in calcium, or that the specific activities of the C^^ 

 and Ca^*" labelled percursors change with respect to the external 

 medium, and to each other, during the process of deposition. As 

 there is no experimental evidence for calcium enrichment we are 

 inclined to explain the apparent carbonate deficit shown in our 

 data on the basis of the second alternative. 



The transfer rates given in Table I were calculated on the as- 

 sumption that during CaCO;, deposition the specific activities of 

 the Ca*'' and C^^ labelled percursors do not change with respect 

 to the sea water or to each other, a condition that would occur only 

 if the system were in isotopic equilibrium. However, this was not the 

 case in our experiments which were run over sufficiently short 

 periods of time that it was impossible for the test colonies to achieve 

 isotopic equilibrium. Therefore it is to be expected that the specific 

 activities in the newly formed skeletal CaCO;. would be less than in 

 the dissolved Ca+^ and HCO^ of the medium if the labelled ex- 

 ogenous atoms were to exchange with intracellular stores of un- 

 labelled atoms to final deposition into the skeleton. 



Given that the molar fluxes of calcium and carl:>onate are equal 

 and linked by some common pathway, and using the specific activi- 

 ties of the precursors dissolved in the sea water as a reference 

 base, the calculated deposition rates will be the higher for that 

 component which has suffered the least isotopic dilution, e. g. cal- 



