POLYPI. 139 



are homologous ; but the axis fig. 64, c, consists of a substance 

 analogous to horn, hardened by phosphate of lime with only a slight 

 trace of carbonate of lime. In one genus of Anthozoa, the external 

 position of the skeleton which characterises the hydi'iform compound 

 polypes is repeated, viz. in the Tiibipora ; but the organisation of the 



polypes, protected by the crimson 

 pipes of this beautiful coral, fig. 67, 

 is essentially the same as in the Al- 

 cyonium, Gorgonia, and Pennatula. 



The most important productions of 

 the apparently insignificant I'ace of 

 Polypi are the accumulations of the 

 calcareous skeletons of the Anthozoa, 

 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 specu- 

 lation to the naturalist and geologist. 



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

 the depths of the ocean, and built up to the height of hundreds of 

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

 calculated to stagger the unversed in zoological science, and which 

 has demanded the repeated observation of the most accomplished 

 naturalists and enlightened voyagers to render intelligible. 



These zoophytic productions are classified 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. 70. r", r"), rising in the ocean from a con- 

 siderable depth, and returning into itself so as to form a ring, with a 

 lagoon, or sheet of still water («), in the interior. The wall is gene- 

 rally breached in one or more places, and when the breach is deep 

 enough to admit a ship, the atoll aflbrds it a convenient and safe 

 harbour. The outer side of the reef usually sinks to a depth of from 

 two to three hundi-ed 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 vai-ies 

 fi'om one fathom to fifty. The summit of the exterior margin of tlie 

 reef or wall is usually composed of living species of Porifes and 

 Millcpora. 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 MiUepora com- 

 planata intersecting each other at various angles, and forming an 

 exceedingly strong honeycombed mass. The dead parts of these 



