MADEEPOEES. 67 



this kind of vegetative reproduction, is merely a continuation of the 

 internal tunic ; and hence it is easy to understand how it may fulfil 

 analogous functions. 



(144.) MADKEPOEID^. Were we to imagine one of the Alcyonidae 

 capable of secreting not merely the calcareous spicula that are mixed 

 up with the softer portions of its body, but abundant quantities of car- 

 bonate of lime, which, being stored tip in the centre of its substance, 

 should form a dense calcareous axis incrusted with the uncalcified part 

 of the living animal, and perforated at its surface so as to form innu- 

 merable cells or lodges adapted to contain the polyps that provide 

 nourishment for the common mass, we should have a good general idea 

 of the structure of the tribe of polyps that next comes beneath our notice. 



(145.) The shallower parts of the tropical seas contain countless 

 forms of madrepores, known to us, unfortunately but too often, only by 

 detached fragments of the earthy skeletons which the beauty of their 

 appearance induces the mariner to bring to our shores. These calca- 

 reous masses generally assume more or less an arborescent appearance, 

 spreading to a considerable extent, so as to cover the bottom of large 

 tracts of the ocean, and not unfrequently they play an important part 

 in producing geological changes, which are continually witnessed in the 

 regions where they are abundant. 



(146.) In the hot climates in which the saxigenous corals abound, 

 they are found to frequent shallow bays and sheltered spots, where they 

 can enjoy the full influences of light and air, unexposed to the agitation 

 of the ocean, which, were it to beat continually upon them, would in- 

 fallibly destroy their delicate substance : in such situations, the sub- 

 marine rocks become gradually incrusted with the calcareous skeletons 

 which they produce ; and if undisturbed, in the lapse of years successive 

 generations will of course deposit such large quantities of calcareous 

 matter as to form beds of considerable thickness. That there are at the 

 bottom of the ocean bold and precipitous cliffs, rising from a depth of 

 1000 or 1200 feet, their broad tops approximating the surface of the 

 ocean, every one will admit, without having recourse to the labours of 

 madrepores to account for their formation, although the sheltered por- 

 tions of the summits of such mountain-ridges afford an eligible position 

 for their increase. In such situations, therefore, they accumulate, and 

 slowly deposit continually increasing masses of earth upon the brow of 

 these submarine mountains, until at last the pile approaches the sur- 

 face of the sea, and even, at low water, remains uncovered by the waves. 

 The further elevation of the rock, as far as the polyps are concerned in 

 its construction, here ceases ; but a variety of causes tend gradually to 

 heap materials upon the newly appearing island : storms, which tear up 

 the bottom of the sea, perpetually throw to the surface sand and mud, 

 which becoming entangled among the madrepores, and matted together 

 with sea- weed, forms a solid bed over which the waves have no longer 



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