Fig. 57
Sea-anemones and corals
An examination of any typical coral fragment, living or dead, massive or branched, shews that the surface of the stone is shaped into little cups, which may be large (half an inch or more in diameter) or very small; they may entirely cover the surface or be scattered at intervals, be sunk below the general level or project boldly from it (see the figures of corals on [Plate XXVI]). In any case, all ordinary corals consist of a multitude of such cups borne upon a common mass of the same stony material, and each cup may be regarded as the remains of one individual “polyp” as the coral animal is called.
It will be a simpler way of gaining an idea of what a coral polyp really is to take a least specialised member of its class, one in which each individual lives disconnected from its fellows and does not secrete the complicated skeleton characteristic of the stony corals. [Fig. 52] shews a “sea-anemone” in no material respect different to those which Gosse has so beautifully figured from our own shores. It is seen to be a graceful translucent cylinder fixed by its base to some chance stone or shell. This extreme simplicity of form gives the living animal a by no means inconsiderable beauty.
The free end of the cylinder bears a circle of what look like hairs, but which are found on testing to be highly sensitive to touch and are surprisingly adhesive for organs of such transparent delicacy. They are consequently better named the tentacles, their function being to adhere to and close down upon any little water-flea or such-like animal that has the bad luck to swim against them. They are also provided with stinging cells which are used for paralysing their prey and for defence. In the centre of the disc is the mouth, a simple opening, a mere hole in the top of a sac.
Our polyp, or anemone, has no organs of locomotion, no organs of sight, taste or hearing, indeed no brain. It has at any rate the rudiments of a nervous and muscular system, for it is able to move its tentacles all towards that one which has captured prey and assist it in conveying the food to the mouth. The body is sensitive all over, for any part when touched will contract, and a violent shock causes the tentacles to fold together and the whole animal to close down to a shapeless hemispherical mass. This is the limit of its sensory and muscular powers, two or three muscular movements, a few of the simplest reactions to outside influences, nothing that could be called even the sense of touch.
Internally again the principal interest of the organism is its extreme simplicity. There is no stomach or gut, no heart or veins, no lung or gills, no kidneys and no brain. The animal is in fact a simple sac, the inner walls are much folded to increase their area, but there is but one body-space to serve for everything. In this space the food is digested. There is but one opening into it from the outside, so the indigestible remnants of the food are voided by the same opening as that they entered by.
Simplicity of organisation could scarcely go farther; we have here an example of one of the lowest forms of life. Lowly organisms of this kind shew an astonishing indifference to the separation of one part from another. No cutting or mutilation does any permanent harm. Chop the beast to fragments, and not only will each piece remain alive, but it will grow until it encloses a new sac, forms a new mouth, tentacles, and adhesive base, and behold a number of new and complete polyps. This possibility has been taken advantage of by nature, and numbers of these lowly forms of life propagate themselves in this way. A projection arises on the side of the animal, is automatically amputated, grows missing organs, and becomes a complete and independent animal. The process is exactly like the planting of rose-cuttings, one of the cases of asexual reproduction in the animal kingdom.
Now in many cases, where propagation by buds takes place, the buds undergo their full development into complete polyps while remaining connected with the parent. An example of a “colony” of sea-anemones thus formed is given by Plate XXV, Figs. [53] and [54]. An allied form, known as Palythoa, is common in the Red Sea as little star-like rings of tentacles, of a beautiful deep, yet bright, green, carpeting the sand and stones in shallow water. Each star, or polyp head, measures about a quarter-inch in diameter, so that when a few dozen occur together they form a quite conspicuous patch of colour.
As already stated the corals are similar “colonial” organisms, the numerous cups on their stony branches representing each one polyp head. But how the polyps are connected with the stony material is best explained by, as before, taking the simplest possible case, that of a solitary non-colonial polyp, which is exactly like our simple sea-anemone, but has a stony cup like one of those of the ordinary corals.