30 BRITISH FOSSIL CORALS 



others, and the tissues in contact with the membranes are the least resisting. The 

 calcareous and other salts which form the sclerenchyma are derived from the matters 

 assimilated by the coral during its digestive and respiratory processes ; their deposition is 

 a vital and not a mechanical process, and its amount is regnlated by those conditions which 

 affect the general nutrition of the individual. 



The following analyses of recent corals are selected from those made bv Silliman ■? 





Porites. 



Madrepora. 



Pocillopora. 



Msandrina. 



Astrjea. 



(Heliastraea?) 





95-84 



94-80/ 



94-583 



,. 93-559 .. 

 .. 0-910 .. 



. 96-471 



. 0-802 



... 91-782 

 2100 



Phosphates and Fluorides . . . 



2-05 . 



.. 0-745 . 



1-050 . 



Organic matter 



2-11 . 



.. 4-448 .. 



,. 4-397 .. 



.. 5-536 .. 



. 2-727 



... 6-118 



The fluorides, phosphates, &c., yielded the following results (per cent, of their 

 precipitate) in three examinations. 





1. 



2. 



3. 



Silica 



22-00 



12-5 



8-70 



Lime .... 



13-03 



7-G6 

 7-83 



7-5 



4-2 



26-34 



1674 



Magnesia 



45-19 



Fluoride of calcium 



0-71 



Fluoride of magnesium... 



12-48 



26-62 



2-34 



Phosphate of magnesia... 



2-70 



8-0 



0-34 



Alumina and Iron 



1000 

 18-30 



14-84 



25-97 



Oxide of iron 





Silliman arrived at the following conclusions respecting the proportions of the 

 phosphates, fluorides, and other salts : — " Fluorine is present in much larger proportion than 

 phosphoric acid. The silica exists in the coral in its soluble modification, and probably is 

 iinited to the lime. The free magnesia existed as carbonate, and was thrown down as 

 caustic magnesia by the lime-water." 



The dead and living tissues arc liable to I)e perforated by parasitic borers ; and 

 the surface of the coral below the soft tissues is often covered with Bryozoa, Ser- 

 pulse, &c. 



The inner membrane develops the buds, and it has an absorbing as well as a 

 depositing power. 



Food is obtained by living corals through the agency of the tentacules, the spiral 

 threads, the cilia of the disc, and the lips. It consists of Animalcula, small Crustacea, the 

 ova of Mollusca, and the spores of Alga^ and smaller marine plants. Mj-riads of 

 organisms may be seen in every small glass of water taken from the tropical seas, and the 

 growth and nutrition of the coral-polypes can be readily accounted for. 



' B. Silliman in Dana's ' Structure and Classification of Zoophytes,' Appendi.x, p. 124 et seq. 



