Obrázky na stránke
PDF
ePub

racter. The internal cavity is a simple tube without radiating lamellæ or special organs of reproduction, and the gemmules grow out singly or in bunches from the sides of the animal. The Hydra, an animal a line or two in length, consists of a tubular body, with a mouth at one extremity, surrounded by a circle of tentacles; and the structure of the animal is so simple that it may be turned inside out, and still live and eat; it may be cut into forty or more parts, and from the dissected body will grow as many distinct Hydræ. The Hydroidea are all minute, and act no important part in reefmaking.

The Hydroidea were long considered mature animals. But recent investigations have shewn that part at least develop Medusa, which are properly the adult individuals, since these alone produce true ova. This diyision has therefore been recently removed from the zoophytes, and placed with the Acalephs or jelly-fishes.

The Bryozoa.—The Bryozoa are other coral-making species; but they are related to certain molluscs called Ascidiæ rather than to zoophytes. In habit and size, they much- resemble the Hydroidea. From a minute cabin-like cell, they extend a circlet of slender arms or tentacles, and expand into a delicate goblet-shape flower, seldom over a line in dianheter. These polyps differ, both from the Actinoidea and Hydroidea, in having two extremities to the alimentary canal-an anus, as well as a mouth; the intestine curves around and termi.nates in the disk. They are widely removed from true zegophytes, both by this character and also by having the tentacles furnished with vibratile cilia,—that is, minute appent dages resembling short hairs, which are kept in nearly constant vibration. Some species of Bryozoa form thin crusts over rocks or sea-weeds, consisting of united cells, scarcely distinguishable unless magnified. The corolla of other species are branching or thin foliaceous; and these also consist of series of minute cells.

2. Texture and Composition of Corals. The texture of calcareous corals is in general quite porous or cellular. Small stars or rounded depressions are scattered

over the surface, and sometimes these stars form the centres of small prominences, called calicles (little cups). Besides the polyp cells, which mark the position each of a separate polyp, there are pores or cellules penetrating the texture of the coral mass; yet in some zoophytes, the coral secretions continue increasing in the animal till the pores are almost or quite obliterated, and the texture is nearly compact, the polyp cells alone remaining. In many species, wherever there are concavities of much depth in the surface of a zoophyte, the coral of these concavities is loose or more spongy than elsewhere, for the reason, apparently, that the polyps in such parts have a poorer chance for securing food and fresh portions of water.

In the Gorgoniæ, and other species forming a distinct axis to the branches, this axis is solid, without a trace of a cell, and usually with faint evidences of a concentric structure. It is thus that the red coral of commerce used in jewellery, differs from the Madrepore or common white coral; it is the axis of a species of Corallium; and the polyps constituted a layer about it, in the same manner as the polyps of Gorgoniæ cover the horny axis of these species.

In hardness, the common calcareous corals are a little above ordinary limestone or marble, the degree being represented in the mineralogical scale of Mohs, by 35 to 4, while in limestone it is about 3. The ringing sound given when coral is struck with a hammer, indicates this superior hardness. It is a common error of old date to suppose that coral when first removed from the water is soft, and afterwards hardens on exposure. But, in fact, there is scarcely an appreciable difference; the live coral has a slimy feel in the fingers, but if washed clean of the animal matter, it is found to be quite firm. The waters with which it is penetrated, may contain a trace of lime in solution, which evaporates on drying, and adds slightly to the strength of the coral, but the change is hardly appreciable. A branched madrepore rings on being struck when first collected; and a blow in any part puts in hazard every branch throughout it, on account of its elasticity and brittleness. Its specific gravity varies from 2:5 to 2:8: 2:523 was the average from fifteen specimens examined by Professor B. Silliman junior. *

In composition, the common reef corals, of which the branching Madrepora and the massive Astræas are good examples, consist almost wholly of carbonate of lime, the same ingredient which constitutes ordinary limestone. In 100 parts, 90 to 96 parts are of this constituent; of the remainder, there are 3 to 8 parts of organic matter, with some earthy ingredients, amounting in certain species to 2 parts, though often less than 1. These earthy ingredients are silica, magnesia, alumina, oxide of iron, phosphate of magnesia, and fluorides of magnesium and calcium. The following is the result of one of Mr Silliman's analyses from those made by him for the Report on Zoophytes.t The specimen was a Porites from the Sandwich Islands. It afforded,- Carbonate of lime, 95-84; Phosphates, fluorides, &c. 2.05; Organic matter, 2:11. The various earthy ingredients are included in the second line of the analyses, in this species, amounting to 2:05 per cent. One hundred parts of the same, subjected to exact analyses, gave the following result :—Silica, 22.00; Lime, 13-03; Magnesia, 7.66 ; Fluoride of calcium, 7-83; Fluoride of magnesium, 12:48 ; Phosphate of magnesia, 2:70; Alumina (and iron), 16:00; Oxide of iron, 18:30. In other analyses, similar results were obtained, with sometimes a larger proportion of fluorides.

The horny corals (axes of Gorgoniæ and Antipathi) were found by Hatchett to have nearly the constitution of ordinary horn. I

The sea-water and the ordinary food of the polyps are evidently the source from which the ingredients of coral are obtained. As coral is an animal secretion, there is no good reason for the surprise with which this subject is sometimes approached. The same powers of elaboration which exist

* Report on Zoophytes, p. 713. On page 711, it is suggested by the author that the high degree of hardness, which characterises the corals and also the shells of many molluscs, may arise from the structure of the calcareous secretions being like that of Aragonite, instead of common calc spar. The hardness is near that of Aragonite, though sometimes a little exceeding it.

† Op. cit., p. 712, and this Journal (2), 1–189. | Report on Zoophytes, p. 56.

[ocr errors]

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 zb to z 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.f 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.

D

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.

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

* Trans. Roy. Soc. of Edin., xvi., 145, 1846; Amer. Jour. Sci., 2d Ser., ii., 114, 1846.

† See also Darwin, Op cit., p. 60.

« PredošláPokračovať »