70 GUIDE TO THE CORAL GALLERY. 



The majority of the specimens belong to the group of stony corals 

 or Madreporaria. The stoniness is clue to the secretion of carbonate 

 of lime by the skin of the lower part of the polyp. As the body 

 grows against this unyielding surface it has to give way, and forms 

 folds ; the skin of these folds again forms plates of carbonate of lime ; 

 and thus we get the outer " wall " and the inner septa. The extent 

 and proportion to which these are developed vary greatly, especially 

 in the colonial forms. In these the nutrition of the colony is effected 

 by a system of canals ; the appended figure (Fig. 11) is Dr. G-. H. 

 Fowler's representation of the canal system of Rhodopsammia. When, 

 as in the genus Madrepora and its allies, this system perforates the 

 substance of the coral (Fig. 12) the coral is said to be perforate. 



The result of budding, long continued and extending layer over 

 layer, is the formation of large solid masses, which go to form coral 

 reefs ; the final fate of reef -corals is well indicated in Case G b, where 

 the specimens selected for the Museum by the late Mr. Darwin are 

 shown. The size to which colonial masses of coral may grow may 

 be judged from the two enormous specimens of TurUnaria peltata, 

 which cover an area 16 feet and IG feet 8 inches round, and weigh 

 12 cwt. and 13f cwt.^ respectively (Fig. 13). Sometimes a coral is 

 by the force of the waves carried away from its resting-place ; if it 

 be dead, and its substance filled with air, it may float ; if so, it will 

 become, like the large mass of Favia (Fig. 14) shown at the entrance 

 to the Shell Gallery, the sport of the waves, and may at last find its 

 home on an island, where the species to which it belongs is never 

 found in the living state. 



Naturahsts experience great difficulty in determining pieces of 

 coral ; the reason for this is to be seen in the photographs of Tvr- 

 hinaria, where marked differences in appearance (Figs. 15, 1(>) are 

 easily apparent. The causes of these differences are seldom easy 

 to discover, and the guesses of stay-at-home naturalists are of little 

 service. It seems certain that maddiness of water may be an 

 important influence, as a deposit of sediment would kill the centre 

 of a cup-shaped coral ; here and there indeed there are indications of 

 spouts by which water may run ofl'. The extraordinary differences 

 seen in the large mass of " Brain coral," Maeandrina cerehriformis 

 (Fig. 17), which is placed in the adjoining corridor, are due, it is 

 suggested by one experienced in coral reefs, to a marked difference in 

 the amount of sunlight which could reach the two halves of the mass. 



1 1 cwt. = 112 lbs. ; 60 kilograms is about equal to 1 cwt. 



