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 regulated by those conditions which 
affect the general nutrition of the individual. 
The following analyses of recent corals are selected from those made by Silliman :) 
Porites. Madrepora. Pocillopora. Meandrina. Astrea. (Heliastraea ?) 
Carbonate of lime ......... 99°84.) 2.4, 94°807 — ..4)94;583»22.. 693:559) | 2 96°47]... ee 
Phosphates and Fluorides... 2°05 ... 0°745 ... 1:050 ... - 0°910 ... 0802 ...  2°100 
Organiematter i .csncet esac abd ee AAS AB OT Ome b ee 2:72, ne 
The fluorides, phosphates, &c., yielded the following results (per cent. of their 
precipitate) in three examinations. 
ile Pa 3. 
WIMCAE sa s.s)scncsec eee 22°00 12°5 8°70 
BMG 3 3..orieneqetee Nes 13°03 we 79 aia 16°74 
Maenesiauetnedsi Me ae 7°66 2s 4:2 au 45°19 
Fluoride of calcium ...... 7°83 ae 26°3 oe 0°71 
Fluoride of magnesium... 12°48 sioe 26°62 as 2°34 
Phosphate of magnesia... 2°70 oe 8-0 oe 0°34 
Alumina and Iron ......... 16:00 eae 14°84 see 25°97 
Oxidesoftirone:: 2.9, .49s:s. 18°30 
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 
united 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 are liable to be perforated by parasitic borers; and 
the surface of the coral below the soft tissues is often covered with Bryozoa, Ser- 
pul, &e. 
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. Myriads 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. 
1 B. Silliman in Dana’s ‘Structure and Classification of Zoophytes,’ Appendix, p. 124 et seg. 
