70 
GUIDE TO THE CORAL GALLEKY. 
The majority of the specimens belong to the group of stony corals 
or Madreporaria. The stoniness is due 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 6 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 Turbinaria peltata , 
which cover an area 16 feet and 16 feet 8 inches round, and weigh 
12 cwt. and 13f cwt. 1 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. 
Naturalists experience great difficulty in determining pieces of 
coral ; the reason for this is to be seen in the photographs of Tur- 
binaria , where marked differences in appearance (Figs. 15, 16) 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 muddiness 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 off. The extraordinary differences 
seen in the large mass of “ Brain coral,” Maeandrina cerebriformis 
(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 cwt. = 112 lbs. ; 60 kilograms is about equal to 1 cwt. 
