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James D. Dana, Esq., on Coral Reefs and Islands. of a lake prevail within, affording safe navigation for the tottling canoe sometimes through the whole circuit of an island ; and not unfrequently, ships may pass, as by an internal canal, from harbour to harbour around the island. The reef is covered by the sea at high tide, yet the smoother waters indicate its extent, and a line of breakers its outline. Occasionally a green island rises from the reef, and in some instances a grove of palms stretches along the barrier for miles, where the action of the sea has raised the coral structure above the waves.
The sketch annexed conveys some idea of the peculiar features presented by a Pacific island, and its encircling reefs, though in order to fill out the scene, the jagged heights and deep gorges of the islands should be covered with forests, and the shores with groves and native villages. The coral platform which borders the shore is represented with its usual uneven line,-its broad harbours with a narrow entrance, -and to the left, an irregular ship-channel running between the inner or fringing reef, and the outer or barrier. At a single place the sea is faced by a cliff, and here, owing to the boldness of the shores and the depth of the waters, the reef is wanting. To the right there is only a fringing reef.
Coral Islands.-Coral islands resemble the reefs just described, except that a lake or lagoon is encircled instead of a mountainous island. A narrow rim of coral reef, generally but a few hundred yards wide, stretches around the enclosed waters.
In some parts it is so low that the waves are still dashing over it into the lagoon; and in others, it is still verdant with the rich foliage of the tropics. The coral-made land, when
highest, is seldom elevated over eight or ten feet above high tide.
When first seen from the deck of a vessel, only a series of dark points is descried just above the horizon. Shortly after, the points enlarge into the plumed tops of cocoa-nut trees, and a line of green, interrupted at intervals, is traced along the water's surface. Approaching still nearer, the lake and its belt of verdure are spread out before the eye, and a scene of more interest can scarcely be imagined. The surf beating loud and heavy along the margin of the reef, presents a strange contrast to the prospect beyond,—the white coral beach, the massy foliage of the grove, and the embosomed lake with its tiny islets. The colour of the lagoon water is often as blue as the ocean, although but fifteen or twenty fathoms deep; yet shades of green and yellow are intermingled, where patches of sand or coral knolls are near the surface; and the green is a delicate apple-shade, quite unlike the usual muddy tint of shallow waters.
The belt of verdure, though sometimes continuous around the lagoon, is usually broken in some parts into islets, which are separated by varying intervals of bare reef; and through one or more of these intervals a ship-channel occasionally opens into the lagoon. The larger coral islands are thus a string of islands arranged along a line of coral reef. The King of the Maldives bears the high sounding title of “ Ibrahim Sultan, King of the thirteen Atollons and Twelve Thousand Isles ;” which Captain W. F. W. Owen, R.N., remarks is no exaggeration.
The usual features of these islands are presented in the above sketch. The narrow belt is seen to consist of several patches of vegetation, and within are the quiet waters which offer a retreat for vessels whenever there is an opening to the lagoon.
A few small coral islands are simple reefs without lagoons. In some cases they are bare banks of coral; but generally, the usual vegetation of the islands has obtained a foothold, and afford some protection against the glare of the coral sand.*
* For further observations on Coral Reefs and Islands vide Dana's Memoir in Silliman's Journal, May 1851.
II. Structure, Growth, and Habits of Coral Zoophytes.
1. Structure and Growth of Zoophytes. A singular degree of obscurity has been thrown around the growth of coral zoophytes and coral formations, through the various speculations which have been offered in place of facts; and to the present day, the subject is seldom mentioned without the qualifying adjective mysterious expressed or understood. Some writers, scouting the idea that reefs of rocks can be due in any way to “animalcules,” talk of electrical forces, the first and the last appeal of ignorance. Others call in the fishes of the seas, suggesting that they are the masons, and work with their teeth in the accumulation of the calcareous material. Very many of those who discourse quite learnedly on zoophytes and reefs, imagine that the polyps are mechanical workers, heaping up these piles of rock by their united labours; and science still retains such terms as polypary, polypidom, as if each coral were the constructed hive or house of a swarm of polyps, like the honeycomb of the bee, or the hillock of a colony of ants.
It is vain to hope to understand fully the works of Him who is himself infinite and incomprehensible. The scrutinizing eye of science penetrates with far-reaching sight the system of things about us, and in the dim limits of vision reads everywhere the word mystery. All life, animal and vegetable, and all that is inanimate, declare it; surely there is no special reason, except such as may arise from want of study and consideration, for attributing it pre-eminently to the humblest grades of existence.
It is not more surprising nor a matter of more difficult comprehension that the polyp should form coral, than that the quadruped should form its bones, or the mollusc its shell. The processes are similar, and so the result; in each case it is a simple animal secretion, a formation of stony matter from the aliment which the animal receives, produced by certain parts of the animal fitted for this secreting process. This power of secretion is the first and most common of
those that belong to living tissues; and though differing in different organs according to their end or function, it is all one process, both in nature or cause, whether in the animalcule or in man. Coral is never, therefore, an agglutination of grains made by the handiwork of the many armed polyps ; for it is no more an act of labour than bone-making in ourselves. And, again, it is not a collection of cells into which the coral animals may withdraw for concealment, any more than the skeleton of a dog is its house or cell; for every part of the coral of a polyp in most reef-making species is enclosed within the polyp where it was formed by the secreting process.
It is important that this point should be thoroughly understood, and fully appreciated. That error may no longer be perpetuated, the words polypary and the like, have been rejected by the author in his volume on Zoophytes, and the more familiar term corallum has been used instead.t With this introductory explanation we proceed.
a. Structure of Coral Animals or Polyps.--A good idea of a coral polyp may be had from comparison with the garden aster; for the likeness in external form and delicacy of colouring is singularly close. The aster consists of a tinted disk bordered with one or more series of petals; and in exact analogy, the polyp flower, in its most common form, has a disk often richly coloured, fringed around with petal-like organs called tentacles. Below the disk, in contrast with the slender pedicel of the plant, there is a stout cylindrical pedicel or body,
* It is not perhaps within the range of science to criticise the poet; yet we may say in this place, in view of the frequent use of the lines even by scientific men, that more error in the same compass could scarcely be found than in the part of Montgomery's Pelican Island, relating to coral formations. The poetry is beautiful, the facts nearly all errors—if literature allows of such an incongruity. For ourselves, we think the poet transcends his appropriate limits, when false to nature.
† See page 15 of the Report on Zoophytes. The term corallium has been set aside by authors because of its being used for a genus of corals. Corallum is an old form of the same word, as particularly explained on the page just referred to, and is not liable to this objection. The true nature of calcareous corals was first pointed out by Milne, Edwards, and Ehrenberg.
often as broad as the disk itself, and usually not much longer, which contains the stomach and internal cavity of the polyp; and the mouth, which opens into the stomach, is placed at the centre of the disk. Here, then, the flower animal and the garden-flower diverge in character, the difference being required by the different modes of nutrition in the two kingdoms of nature.
There are many species of polyps, which have all the external and internal characters of coral polyps, yet secrete no lime or coral. Our descriptions of structure may be best drawn from them, and afterwards the single peculiarity of the coral-making polyp—its secretion of coral—will come under consideration. The species here referred to are called Actinice in science, in allusion to the radiated or aster-like flower which forms the summit of the animal.* There is the same allusion in the common appellation Sea Anemone. The richest anemones, daisies, and tulips of our gardens would not rival them in beauty, neither will they exceed them in the size of their flowers; for a breadth of two and three inches is common. The polyps here alluded to, along with the coral polyps allied, constitute the order or division of zoophytes called Actinoidea.t
The Actiniæ are entirely fleshy, and usually live attached by their lower extremity to the submerged rocks of the shores. The mouth, at the centre of the flower-like disk forming the summit of the animal, is a simple opening without teeth or appendages of any kind. The tentacles—the petals of the flower—are tubular organs, and communicate internally with the interior cavity of the animal. The animal contracts when disturbed, and conceals the flower by rolling inward over it, the margin bearing the tentacles ; and in this state it seems like a lifeless lump of animal matter. Left quiet for a while, it again expands and appears as before. This expansion is produced by receiving water into the interior from without, mostly through the mouth, and thus filling the tentacles and swelling out its fleshy body. They are generally found ex
* From agriv, a ray of the sun. † This term alludes to their general resemblance to Actiniæ.