CHAPTER X. Madrepores, Corals, and Sponges

MADREPORES, CORALS, AND SPONGES

PLATE XLIII

MADREPORES (1)

In some ways these curious creatures are very much like sea anemones, and if you were to find one with its tentacles spread you would be almost sure to think that it was a small anemone. But if you touched it you would find that you had made a mistake, for instead of closing itself up into an almost shapeless lump of jelly, as the anemones do, it would just draw back its tentacles, and leave a kind of flinty skeleton still standing up. For madrepores are really much more like the wonderful little creatures which make coral. They suck lime, in some strange manner which nobody quite understands, out of the sea-water, and build it up round and underneath their own bodies. And if you startle them in any way they draw themselves down inside this shelly covering, and disappear from sight altogether; so that all that you can see is a number of thin plates standing upright on their edges, and looking rather like the lower surface of a mushroom turned into stone.

Madrepores feed on very tiny animals, such as the fry of small fishes, and the zoeas of shrimps and prawns. And they catch their victims by means of a number of fleshy tentacles, which are very much like those of the sea anemones, except that they always have little round knobs at the tips. These tentacles are set with numbers of tiny cells containing slender poisoned darts, just as those of the anemones are.

If you want to find madrepores, you must look for them among the rocks near the water’s edge when the tide is at its lowest. But they are not very common, and on many parts of the coast they are never found at all.

PLATE XLIII

THE SEA FINGER (2)

If you walk along the shore as the tide goes out, you may often find a soft, pink, fleshy object which has been thrown up by the waves. And if you search among the pools at low-water, you are nearly sure to see other soft, pink, fleshy objects just like it growing upon their rocky sides, or upon the stones and shells which lie at the bottom. They are often known as “dead men’s fingers,” or “dead men’s toes.” But as those are not very nice names, we will call these objects “sea fingers.”

Now if you pick up one of these sea fingers and look at it carefully, you will see that its surface is pierced all over with numbers of tiny holes. And if you take a good strong magnifying-glass, and look at one of the holes through that, you will see that it is shaped like a little flower with eight petals, or a star with eight rays.

The fact is that the sea finger is the home of a most curious animal; or perhaps one should rather say that it is the home of hundreds of most curious animals. Indeed, it is not at all easy to know which is the right way to describe it. For if you were to take a living sea finger, and to put it into a vessel of clear sea-water, you would very soon notice that a little tiny star-shaped animal had poked itself out of each little star-shaped hole. There would be hundreds of these little animals--or “polyps,” as they are called--altogether. But yet they would only have one body between them, for they are joined together in such a wonderful way that the food which is caught and eaten by one polyp nourishes all the others as well as itself!

1. THE MADREPORE.

2. THE SEA FINGER.]

PLATE XLIV

THE TUFT CORAL (1)

Nearly all the coral-building animals are found in the tropical seas, for they can only live in water which is quite warm all the year round. But there are just a very few which are sometimes found off our own shores, and one of these is the Tuft Coral. It looks rather like a tree which has just been “pollarded” by having all the small branches taken away and all the big ones cut quite short; and sometimes it weighs as much as six or even seven pounds.

People sometimes say that the curious substance which we call “coral” is made by “coral insects.” But the little animals which make it are not related in any way to the true insects. They are really tiny polyps, very much like those of the sea finger; and they suck up lime out of the water, and build it up underneath and round their own bodies, just as the madrepores do.

If you were to place one of these tuft corals in a vessel of clear sea-water, and to watch it carefully, you would soon see the little polyps poking themselves out, and spreading their tiny fleshy feelers, or “tentacles.” The coral which they make is pearly white in colour, with just a faint tinge of rosy red, and the polyps themselves are partly white, and partly fawn, and partly chestnut brown.

One does not often find a tuft coral, however, for the polyps like to live in rather deep water. But when there is a very high spring-tide, as there generally is about the end of March and the end of September, the waves retreat afterwards a good deal farther than usual. And then, if you go right down to the water’s edge, you may perhaps find a tuft coral fastened to the rocks.

PLATE XLIV

THE BREAD-CRUMB SPONGE (2)

I dare say that you will be rather surprised to hear that nearly three hundred different kinds of sponges have been found in the British seas. You will not be able to find very many of these, however, for they nearly all live in deep water, and have to be scooped up by means of the dredge. But the Bread-crumb Sponge is easily found, for it lives in shallow water, and you are nearly sure to find it if you look for it in the rock-pools.

1. THE TUFT CORAL.

2. THE BREAD-CRUMB SPONGE.

3. THE GRANTIA SPONGE.

4. FORAMINIFERA.]

But I hardly think that anybody, on seeing it for the first time, would take it to be a sponge at all. For it is not in the least like a bath sponge. It is just a kind of fleshy crust, sometimes greenish in colour and sometimes yellow, which grows round the stems of sea-weed, or covers the surfaces of rocks and stones. And the odd thing about it is that when it clings to sea-weeds its surface is quite smooth, with a number of large holes in it, but that when it grows on rocks it is covered all over with little projections which look just like the craters of volcanoes.

It is rather difficult to describe the animal which lives in the sponge, for it really consists of a large number of tiny animals all joined together in one common mass, very much like the polyps of the sea finger. But they are so very small that unless you examine them by means of a good strong microscope they only look like a mass of brownish jelly.

These little creatures obtain their food in a very curious way. If you look at the surface of the sponge through a magnifying-glass, you will see that it is pierced by a great many very tiny holes as well as by a number of bigger ones. Now water is always passing in through the small holes and out again through the big ones; and as it does so the little creatures manage to suck out all the tiny atoms of animal and vegetable matter which were floating about in it.

PLATE XLIV

THE GRANTIA SPONGE (3)

This is quite a small sponge, which you may often find by hunting about in the rock-pools just above low-water mark. Sometimes it clings to sea-weeds, and sometimes it hangs down from the surfaces of the rocks; and when you find one you are almost sure to find several others close by.

In appearance, they are rather like little flat white bags, or purses; and when they reach their full size they are generally about an inch long and an inch and a half wide.

PLATE XLIV

FORAMINIFERA (4)

“Foraminifera!” That is rather a long name; isn’t it? But if we cut it in two, and strike out one of the letters, we shall see what it means. Foramin-(i)-fera. Now the first part of the name is a Latin word which means “a hole,” and the last part is another Latin word which signifies “bearers.” So “foraminifera” means “hole-bearers,” and this title has been given to certain very tiny creatures which live in the sea because they inhabit shells, which are pierced all over by numbers and numbers of still tinier holes.

These foraminifera are so very small that numbers of them can live in a single drop of water! Yet, strange to say, all the chalk in the world is made of their shells! For in days of old--thousands and thousands of years ago--they were found in the sea in millions of millions of millions. And as they died their empty shells sank down to the bottom of the sea in such enormous numbers that at last they formed a layer hundreds of feet thick. Then suddenly one day there came a great earthquake, and a great deal of this vast layer of shells was forced up above the surface in the form of what we now call chalk. So that “the chalk cliffs of old England” are really made of nothing but shells, so very small indeed that you cannot see them without the help of a very strong microscope!

There are a great many different kinds of foraminifera. But if you look at them through a good microscope you will always see that their shells are pierced by the tiny holes from which they take their name.