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in other animals belong to polyps; for this function, as we have remarked, is the lowest attribute of vitality.
Neither is it at all necessary to inquire whether the lime in sea-water exists as carbonate or sulphate, or whether chloride of calcium takes the place of these. The powers of life may make from the elements present whatever results the functions of the animal require.*
Various waters were collected in the vicinity of the coral islands, and at different distances from them, for the purpose of analysis, in order to compare the constitution of the sea in different parts; but they were lost with the Peacock on the bar of the Columbia river. The proportion of lime salts which occurs in the water of the ocean is about av to it of all the ingredients in solution. Professor Forchammer has ascertained that around the West Indian seas, where corals abound, lime is not as abundant as elsewhere in the ocean, the proportion, according to five analyses, being 247 to 10,000; while in the Kattegat, where the rivers of the Baltic carry it in considerable quantities, the proportion, from four analyses, is 371 to 10,000.+ Schweitzer obtained the following results in water taken from the British Channel :IWater 964:74 grains; chloride of sodium, 27.06; chloride of potassium, 0-77; chloride of magnesium, 3-67; bromide of magnesium, 0.03; sulphate of magnesia, 2.29; sulphate of lime, 1.41; carbonate of lime, 0.03=1000.00.
Recently Dr G. Wilson has detected fluorine in sea-water,
* If a drop of sea-water be slowly evaporated under a microscope of high power, crystals of selenite (sulphate of lime) are produced, having the most common forms presented by native crystals of this mineral, as stated in works on mineralogy. On adding more water, they are again dissolved; and this may be repeated indefinitely. These results would seem to indicate that the lime was mostly in the state of a sulphate. Mr Darwin states the remarkable fact, described by Mr Webster (Voyage of the Chanticleer, ii., 319), that a deposit of salt and gypsum two feet thick occurs on the shores of Ascension, which was formed by the dash of the waves. Beautiful crystals of selenite were obtained by the writer in logs of half decomposed wood in the shore cliffs near Callao, which were of similar origin.
† On Comparative Analytical Researches on Sea-Water, by Prof. Forchammer, Rep. Brit. Assoc. for 1846, p. 90.
| Lond. and Ed. Phil. Mag. for July 1839, xv., 51; Amer. Jour. Sci., xxxviii., 12. VOL. LII. NO. CIII.-JANUARY 1852.
shewing that all the ingredients of coral are actually contained in the waters of the ocean.*
It has been common to attribute the origin of the lime of corals to the existence of carbonic acid springs in the vicinity of coral islands. But it is an objection to such a hypothesis, that, in the first place, the facts do not require it; and in the second, there is no foundation for it. The islands have been supposed to rest on volcanic summits, thus making one hypothesis the basis of another. Carbonic acid springs are by no means a universal attendant on volcanic action. The Pacific affords no one fact in support of such an opinion. There are none on Hawaii, where are the most active fires in Polynesia ; and the many explorations of the Society and Navigator Islands have brought none to light, Some of the largest reefs of the Pacific, those of New Holland and New Caledonia, occur where there is no evidence of former volcanic action.f
The currents of the Pacific are constantly bearing new supplies of water over the growing coral beds, and the whole ocean is thus engaged in contributing to their nutriment. Fish, molluscs, and zoophytes are thus provided with earthy ingredients for their calcareous secretions, if their food fails of giving the necessary amount; and by means of the powers of animal life, bones, shells, and corals alike are formed.
The origin of the lime in solution throughout the ocean is an inquiry foreign to our present subject. It is sufficient here to shew that this lime, whatever its source, is adequate to explain all the results under consideration.
3. Causes influencing the Growth of Coral Zoophytes. Marine zoophytes generally require pure ocean water, and they abound especially in the broad inner channels among the reefs, or the large lagoons, and in the shallow waters outside of the breakers. In these channels, at the Feejee group, there are species of every genus, and they grow in the greatest luxuriance, exceeding in profusion and display all that was elsewhere seen in the Pacific. Here are found the huge Astræa domes, the Meandrinas, Porites, the leafy clusters of the Merulinæ, numerous Madrepores:-indeed nearly all the Pacific corals described in the Report on Zoophytes, exclusive of those from the Tahitian and Hawaiian Islands, were obtained from the inner reefs of the Feejees.* It is therefore an assertion wide from the fact, that only smaller corals grow in the lagoons and channels, though true of lagoons and channels of small size, or of such parts of the larger channels as immediately adjoin the mouths of freshwater streams.
* Trans. Roy. Soc. of Edin., xvi., 145, 1846; Amer. Jour. Sci., 2d Ser., ii., 114, 1846.
† See also Darwin, Op cit., p. 60.
There are undoubtedly species especially fitted for the open ocean; but as peculiar conveniences are required for the collection of zoophytes outside of the line of breakers, we have not the facts necessary for an exact list of such species. From the very abundant masses of Astræas, Meandrinas, Porites, and Madreporas thrown up by the waves on the exposed reefs, it was evident that these genera were well represented in the outer seas. In the Paumotus, the single individuals of Porites lying upon the shores were at times six or eight feet in diameter. Around the Duke of York's Island the bottom was observed to be covered with small branching and foliaceous Madrepores (Manoporæ), as delicate as any of the species in more protected waters.
Species of the same genera grow in the face of the breakers, and some are identical with those that occur also in deeper waters. Numerous Astræas, Meandrinas, and Madreporas grow at the outer edge of the reefs, where the waves come tumbling in with their full force. There are also many Milleporas, and some Porites and Pocilloporas in the same places. But the weaker Manoporas, excepting incrusting species, are found in stiller waters, either deep or shallow. The Nullipores, properly calcareous vegetation, flourish best along the line of breakers, and form thick accumulations upon the reef.
* The author's observations on the species of corals were not commenced till reaching the Feejees, where we were among the inner reefs. Previous to that time, this department in zoology was in the hands of Mr J. P. Couthouy.
† Porites and Milleporæ, according to Mr Darwin, prevail on the surf-reef of Keeling's Island. Chamisso states that the large Astræas live and grow in the breakers.
Again, the same genera occur in the shallow waters of the reef inside of the breakers. Astræas, Meandrinas, and Pocilloporas are not uncommon, though requiring pure waters. There are also Madreporas, some growing even in impure waters. One species was the only coral observed in the lagoon of Honden Island (Paumotus), all others having disappeared, owing to its imperfect connection with the sea. Upon the reefs inclosing the harbour of Rewa (Viti Lebu), where a large river, 300 yards wide, empties, which during freshets enables vessels at anchor two and a-half miles off its mouth to dip up fresh water alongside, there is a single porous species of Madrepora (M. cribripora) growing here and there in patches over a surface of dead coral rock or sand. In similar places about other regions, species of Porites are most common. In many instances, the living Porites are seen standing six inches above low tide, where they were exposed to sunshine and to rains; and associated with them in such exposed situations, there were usually great numbers of Alcyonia and Xeniæ. Porites also occur in the impure waters adjoining the shores; and the massive species in such places, commonly spread out into flat disks, the top dying from the deposition of sediment upon it.
The exposure of six inches above low tide, where the tide is six feet, as in the Feejees, is of much shorter duration than in the Paumotus, where the tide is less than half this amount; and consequently the height of growing coral, as compared with low-tide level, varies with the height of the tides. The powers of endurance in some coral zoophytes cannot surprise us, for it is well known that these animals are often very tenacious of life. The hardier species belong mostly to the genera Porites and Pocillopora, besides the family Alcyonidæ.
The small lagoons, when shut out from the influx of the sea, are often rendered too salt for growing zoophytes, in consequence of evaporation,-a condition of the lagoon of Enderby's Island.
Coral zoophytes sometimes suffer injury from being near large fleshy Alcyonia, whose crowded, drooping branches lying over against them, destroy the polyps, and mar the growing mass. But Serpulas and certain species of bar
nacles, constituting the genus Criseis, fix themselves upon the living Astræa, Millepora, and other corals, and finally become imbedded by the increase of the zoophyte, without producing any defacement of the surface, or affecting its growth. Many of the Serpulas grow with the same rapidity as the zoophyte, and finally produce a long tube, which penetrates deep within the coral mass; and, when alive, they expand a large and brilliant circle or spiral of delicate rays, making a gorgeous display among the coral polyps. Instinct seems to guide these animals in selecting those corals which correspond with themselves in rate of growth; and there is in general a resemblance between the markings of a Criseis and the character of the radiations of the Astræa it inhabits.
The effects of sediment on growing zoophytes are strongly marked, and may be often perceived when a mingling of fresh water alone produces little influence. We have mentioned that the Porites are reduced to flattened masses by the lodgment of sediment. The same takes place with the hemispheres of Astræa; and it is not uncommon that in this way large areas at top are deprived of life. The other portions still live unaffected by the injury thus sustained. Even the Fungiæ, which are broad simple species, are occasionally destroyed over a part of the disk through the same cause, and yet the rest remains alive. Wherever streams or currents are moving or transporting sediment, there no corals grow; and for the same reason we find no living zoophytes upon sandy or muddy shores.
The influence of temperature on the development of animal life, and the distribution of species, is well known. But in no department is it more strikingly displayed than in that of zoophytes. In a former report we have considered the general influence of temperature on the several divisions of this order of animals. The remarks which follow are consequently confined to the reef-forming species. We reserve for still another page the influence of this cause on the distribution of reefs, since we are occupied here with zoophytes as animal species, and not with reefs,--a result from the growth of corals.
The temperature of the ocean in which reef-corals grow