POLYPIFERA. 



33 



MADREPORID^E : MADREPHYLLID.-E. The 

 next group of Polypiferous zoophytes may 

 very properly be called Madreporygenous, 

 seeing that it is by their agency that vast 

 masses of calcareous matter are constantly in 

 process of deposition, which by their immense 

 accumulation not only form coral reefs and 

 islands in tropical seas, but have powerfully 

 contributed to modify the face of our planet. 

 The manner in which these huge territories 

 of newly formed land are constructed by the 

 silent labours of these humble beings is now 

 tolerably well understood.* In climates 

 where the heat is intense, in enclosed and 

 tranquil bays, the saxigenous corals are found 

 to grow upon submarine rocks, which they 

 encrust to a considerable depth. It is upon 

 gentle declivities and where the sea is shal- 

 low, that the largest masses of madrepores 

 are met with. In quiet water they spread 

 extensively, otherwise they only construct 

 small masses formed by species which suffer 

 least from the agitation of the waves. 



It is asserted that some reefs rise from 

 immense depths like perpendicular walls, but, 

 although it is true that such reefs exist, they 

 are not formed exclusively of madreporic 

 rock, for the madreporygenous polypes can 

 only exist at depths where they enjoy the 

 influences of light and air, and consequently 

 could not possibly grow at 1000 or 1200 feet 

 below the surface. The sea, which breaks 

 furiously upon such reefs, would inevitably 

 destroy them, if they alone composed the cliffs; 

 but the fact is, that sheltered iron) the waves 

 in the hollows of pre-existing rocks, they con- 

 tribute to increase their bulk. 



Corals found upon elevated tracts of the 

 South Sea islands and Australia have no 

 doubt been thrown up by volcanic agency, 

 which raised the bottom of the sea where 

 they were formed. 



When, under the shelter of submarine rocks, 

 polypes have raised their abodes to the surface, 

 they remain uncovered for a little time during 

 the lowest tide. Storms turn up from the bot- 

 toms of the shallow waters sand and mud, which 

 become entangled and fixed in the interstices 

 of the madrepores, so that the summit of this 

 new island comes to remain continually above 

 the surface, and the waves can no more de- 

 stroy what they have contributed to construct ; 

 its circumference gradually enlarges, and its 

 edges grow higher by the addition of fresh 

 sand. According to the direction of the 

 winds or currents it may remain long bar- 

 ren, but if by the action of these two causes, 

 the germs of vegetation are brought to it 

 from neighbouring coasts, it becomes covered 

 with verdure, by the gradual decay of which 

 vegetable soil accumulates, until at length it 

 becomes fitted for the abode even of man 

 himself. 



To describe all the various forms of the 

 madreporygenous zoophytes would be useless, 

 even were it possible within the limits per- 

 mitted to us ; we shall therefore content our- 



* Quoy et Gaimard, Voyage de TUranie. 



VOL. IV. 



selves with selecting those genera which have 

 been most attentively examined, and from 

 their history the reader will have little diffi- 

 culty in obtaining a clear insight into the 

 economy of the rest. Throughout the entire 

 series the vital agent will be found to present 

 itself in the form of a soft, gelatinous crust 

 wherewith the exterior of the polypary, what- 

 ever may be its shape, is closely invested, or 

 more correctly speaking, the calcareous par- 

 ticles are gradually deposited in its cellular 

 interstices, and thus moulded into form. Ex- 

 ternally this living cortex is generally studded 

 with polypes lodged in cells excavated in the 

 polypary, the forms of which are indicative of 

 the species. 



The Ftmgite, although their calcareous 

 skeletons are to be met with in every cabinet, 

 have never, as far as we are aware, been 

 brought to our shores in their recent state; 

 and accordingly their living condition has 

 been very erroneously described by several 

 modern writers. The dried framework of the 

 Fin/gia agaric/formis owes its name to its si- 

 militude to a mushroom, which it closely re- 

 sembles. In shape it is a circular disc, the 

 inferior surface of which is flat, and rough r 

 and granular, while superiorly it is convex, its 

 upper surface being arranged in broad cal- 

 careous lamellae, which radiate from the centre 

 to the circumference of the mass (fig. 38). 

 According to Forskal*, when in a living 

 state, the whole superficies of the Fungia 

 is covered over with a thin gelatinous layer, 

 which dipping in between the radiating la* 

 mella? coat every part of the calcareous sur- 

 face, but without any polype or appearance of 

 tentacula. The gelatinous coat, indeed, seems 

 exactly to represent the living crust of the 

 sponges, being entirely destitute of anything 

 like a stomachal cavity, and apparently nou- 

 rished altogether by its general surface, which 

 must appropriate nutriment from the sur- 

 rounding water. The living film which thus 

 encrusts the Fungia is the only agent em- 

 ployed in the construction of the beautiful 

 calcareous basis that supports it, each particle 

 of which, as it is derived from the circumam- 

 bient element, is added, by interstitial deposit, 

 to the growing fabric, which is thus built up 

 in the regular form belonging to the species. 

 The upper surface of the living animal has 

 been observed to be provided with bubbles of 

 aii% apparently secreted by the living film in 

 which it is imprisoned. These bubbles seem 

 to have no regularity of arrangement, but 

 nevertheless play an important part in pre- 

 serving the Fungia from destruction ; for the 

 mass being in its adult state unattached to 

 any foreign body, is of course quite at the 

 mercy of every passing wave, which, taking it 

 up, might capsize it, and thus bury its upper 

 surface in the sand; but the air bubbles 

 placed there, as it were in anticipation of 

 such an accident, acting the part of floats, 

 always by their buoyancy keep the living 

 side uppermost, and allow the creature to 



* Flora (Egyptiaco-Arabica ; Hauniae, 1775. 



D 



