POLYPI. 89 



musica; but the organisation of the polypes, protected by the crim'*on 

 pipes of this beautiful coral, is essentially the same as in the Alcyonium, 

 Gorgo7iia, and Pennatula. 



The most important productions of the apparently insignificant 

 race of Polypi are the accumulations of the calcareous skeletons of the 

 Anthozoci, which form the coral islands and reefs; — the dread of the 

 navigator, — the admiration of the lover of the picturesque, — the 

 subjects of the closest and most interesting speculation to the na- 

 turalist and geologist. 



That masses of rock many leagues in extent should be founded in 

 the depths of the ocean, anv built up to the height of hund^'eds of 

 feet by minute, frail, gelatinous animalcules, is indeed a phenomenon 

 calculated to stagger the unversed in zoological science, and M^hich 

 has demanded the repeated observation of the most accomplished and 

 enlightened voyagers to render intelligible. 



These zoophytic productions have been recently classified by 

 Mr. Darwin * under three heads : ' atolls,' ' barrier reefs,' and 

 ' fringing reefs.' The term Atoll is the name given to the coral- 

 islands, or lagoon-islands by their inhabitants in the Indian Ocean. 

 An atoll consists of a wall or mound of coral rock {^fig. 50. r'\ r^^), 

 rising in the ocean from a considerable depth, and returning into 

 itself so as to form a ring, with a lagoon, or sheet of still water 

 (Jig. 50. 7i) in the interior. The wall is generally breached in 

 one or more places, and when the breach is deep enough to admit 

 a ship, the atoll affords it a convenient and safe harbour. The outer 

 side of the reef usually sinks to a depth of from two to three hundred 

 fathoms, at an angle of forty-five degrees or more : the internal side 

 shelves gradually towards the centre of the lagoon, forming a saucer- 

 shaped cavity, the depth of which varies from one fathom to fifty. 

 The summit of the exterior margin of the reef or wall is usually 

 composed of living species of Porites and 3Iillepora. The Porites 

 form irregularly rounded masses of from four to eight feet broad, 

 and of nearly equal thickness ; other parts of the reef are composed 

 of thick vertical plates of the Millepora complanata intersecting each 

 other at various angles, and forming an exceedingly strong honey- 

 combed mass. The dead parts of these calcareous skeletons are 

 often cemented over with a layer of the marine vegetable xalled Nut- 

 lipora, which can better bear exposure to the air. 



This strong barrier is well fitted to receive the first shock of the 

 heavy waves of the fathomless ocean without; and >vhat at first 

 appears surprising, instead of wearing away at its outer edge, it is 

 here only that the solid reef increases. The coral animals thrive 



* Structure and Distribution of Coral Reefs, 8vo. 1842. 



