The Animal World, A Book of Natural History Part 62

However, a limpet is not gripping the rock all the time with such vigor; he would literally be tired to death, and starved to death, too, if he didn't ease up most of the time. It is only when he is alarmed by a touch that he clamps down. If you want to get him free, just wait till he loosens up, then hit him a sudden sharp blow on one side with a stick or stone, and knock him off. Then you will be able to examine the soft body and see how he is built.

Limpets are vegetable-feeders, and when the water is still, or absent, they creep slowly about the rock, nibbling the tiny vegetation on its surface. Another interesting fact in limpet-life is told on page 421.

Another kind of limpet is very common on those rocky sh.o.r.es, which is shaped somewhat like a loose round-toed slipper or a French _sabot_. This is the slipper-limpet, or half-deck, as fishermen call it.

On the lower rocks near the water, and hidden in among the wet seaweeds, lie many small spiral gastropods which we call periwinkles. Two of the commonest kinds are littorinas, marked with fine lines and colors in various ways. Another, reddish with chestnut bands, is named _Lacuna_; and you may pick up several kinds of small blackish ones, such as _Bittium_, or of light-colored ones, as _Rissoa_, which is prettily mottled; while numerous in some places is the purple-sh.e.l.l or _Purpura_, which is interesting because it belongs to the European sh.o.r.es as well as to ours, and because from it the ancients gathered some of their purple dye, although another mollusk (the murex) furnished most of it. But in old times the coast people, both of old England and New England, obtained from this little mollusk an indelible violet ink with which to mark their clothes.

Would you like to see a little of this dye?

Very well, you can easily do so. Look! Hold the purpura over this sheet of white paper, and give the animal a little poke with the head of a pin. There! It has squirted out a drop of liquid upon the paper. It does not look much like purple dye, does it? It looks very much more like curdled milk. But lay it in the sunshine and notice what happens. Do you see? It is turning yellow. Now a blue tinge is creeping, as it were, into the yellow, and turning it to green. The blue gets stronger and stronger, till the green disappears. And at last a crimson tinge creeps into the blue, and turns it to purple.

Another curious thing about the purpura is the way in which it lays its eggs. It fastens them down to the surface of the rock by little stalks, so that they look like tiny egg-cups with eggs inside them; therefore when these eggs hatch, several little purpura come out of each cup.

All the small periwinkles feed upon the algae, but with the purpura, which seems to live mainly on young barnacles, we come to a lot of flesh-eaters--small mollusks of prey, as we might say.

There are several spiral sorts, mostly from one to two inches long, whitish and heavily ribbed, which are sometimes called dog-whelks; but the worst one, which lives by thousands on the beds of planted oysters scattered all along the sh.o.r.e of Long Island Sound, is known to the oystermen as the drill, or borer. It is particularly fond of the flesh of oysters, and cares nothing for their sh.e.l.ls, as it carries in its mouth a drilling instrument (see page 419) by which it can bore a round hole through the poor oyster's armor. In this way it destroys many thousands of dollars' worth of valuable oysters every year.

It was pretty certain we should find a starfish down near low-water mark, and here is a fine one.

Starfishes are among the oddest of sea-animals; for one reason, because they have so many legs. Perhaps you did not know they had any legs at all; certainly you can see none when you pick up a dead specimen on the beach. The fact is that a starfish keeps its legs inside its body, where there are a lot of organs protected by its hard, limy hide; and when it wants to use them it pokes them out through little holes on its under or grooved side, and fills them with water.

You would like to see its legs, no doubt. Very well; you shall. This starfish is still alive: we can easily see that, for when we pick it up its rays stand stiffly out; but if it were dead they would be quite soft and flabby, and would hang down. So we will put it into a shallow pool of clear sea-water, and see what happens. There! did you notice that it moved one of its rays? See, the one in front is being slowly pushed forward. Now the rays behind are being drawn up; and now that they have taken a fresh hold the front one is being pushed forward again. The starfish is really walking! What will it do when it comes to a stone?

Why, walk over it! What will it do when it comes to rock? Why, climb up it! Now take the starfish out of the water. Turn it over on its back.

There! do you see? On the lower surface of every ray are hundreds of little fleshy objects waving about in the air. Those are its "feet," or at least its means of walking; and each has a sort of cup at the end which acts as a sucker. By means of these the starfish can cling tightly to the surface of a stone. So by using first the little sucker-legs on one or two of its rays, and then those on the others, the starfish is able to crawl about quite easily.

The starfishes live upon animal food--mainly other mollusks, which they kill in a very curious manner. When, in crawling about, they come upon a whelk or clam or oyster, they creep over it and clasp it in their five arms in a murderous embrace from which there is no escape. Even if the creature can move off, its captor clings to it with its hundreds of tiny suckers, and rides along with it like that Old Man of the Sea in Sindbad's story.

Now if you look again at our specimen you will see on its under side, a small pit in the center of its body, closed by five points. This is the mouth, and the points are sharp. As soon as the starfish has a grip upon its victim the mouth opens and there is gradually pushed out a strong membrane which is the creature's great loose stomach. This envelops the animal, sh.e.l.l and all, or as much of it as possible, and soon begins actually to digest the flesh. When the meal is finished the starfish draws back its stomach and leaves only the empty sh.e.l.l of its prey.

These voracious starfish are a worse enemy to the cultivated oysters than are the drills; and, having an abundance of food on the thickly planted beds, they become extremely numerous, so that it costs the owners of the beds much money each year to gather them off the beds by means of a sort of great rake called the tangles. Otherwise the oysters would soon be wholly destroyed. The men used simply to tear to pieces what they caught and throw them overboard again; but they soon learned that this was worse than useless, because each half, or even a single arm, would not only go on living but would reproduce all the missing parts; so that in trying to kill one starfish they had brought to life two or perhaps even five, which was very discouraging. Nowadays, therefore, all captured starfishes are brought ash.o.r.e and left there, and often are made use of by being ground up with oyster-sh.e.l.ls, fish-bones, etc., into an excellent fertilizer.

What is that greenish-gray object covered all over with spikes? It is clinging in a little hollow of the rock, half hidden in seaweed of the same color.

Ah! that is a sea-urchin, and although it looks so very unlike them it is really a kind of first cousin to the starfishes. Here is a dead one from which the spines have been knocked off. Just look at it carefully, and you will see that it is very much like a starfish rolled up into a ball. See, you can trace the five rays quite easily, and if you look at it through a strong magnifying-gla.s.s you will find that its surface is pierced in hundreds of places with tiny holes through which it can poke out little sucker-feet, just as the starfishes do.

Look again at the sh.e.l.l from which the spines have been knocked away. Do you see that it is covered all over with little pimples? Now on every one of these pimples a spine was fastened by a kind of ball-and-socket joint, the pimple being the ball, and the socket lying inside the base of the spine; and by means of special muscles the animal could move the spines about, just as though it were a kind of hedgehog. In fact, this is the reason why it is called sea-urchin, for urchin is an old name for hedgehog. So, when a sea-urchin crawls about, it does so partly with its sucker-feet, and partly with its spines as well.

Sometimes, however, these creatures use their sucker-feet for quite a different purpose. They poke them out as far as they can from among their spines, and then take hold of little stones, small pieces of broken sh.e.l.l, and other bits of rubbish which they find at the bottom of the sea, and cling to them very tightly. The consequence is that you cannot see the animal at all, for it is quite concealed by this curious covering, and unless you were to take it out of the water, you would never have the least idea what it really was.

Now look at the mouth of this spiky sea-urchin. You will find it in the very middle of the lower part of the body. Do you see what great teeth it has? There are five of them arranged in a circle as in the mouth of a starfish, and they are made in just the same way as the front teeth of a rat or a rabbit, that is, they never stop growing all through the life of the animal, so that as fast as they are worn away from above they are pushed up from below, and thus always keep just the proper length and sharpness.

Sea-urchins are rather few and small along the sh.o.r.es of southern New England, but more numerous northward, and on rocky bottoms offsh.o.r.e. On the offsh.o.r.e bottom there lives also a queer sort whose sh.e.l.ls are often cast up and are well known to the children as sand-dollars.

These are about the size and shape of one of mother's cookies, and are covered with a stiff brown fur of short spines. On one side--the under one--is the little mouth, and around it the faint outlines of five radiating arms, each sketched, as it were, by a double row of "pin-p.r.i.c.ks" where the almost invisible feet are pushed out. These sand-dollars are creeping about at the bottom in myriads where the water is a few fathoms deep; and storms cast up thousands upon the beaches or into the tide-pools, where very likely we may find some in the course of our next visit to the ocean-side.

IV

BETWEEN TIDE-MARKS

We must start early on our walk today, as soon as the tide falls away from the piece of rocky sh.o.r.e we have in mind, so that we may have plenty of time; for the field which we have left until the last is the richest the seaside naturalist has to explore.

As the sea sinks away it uncovers not only the weedy ledges which we studied the other day, but also s.p.a.ces between them of low rocks and loose stones half sunk in mud and sand. There is much to interest the botanist, too, but he will have to look out for himself. We have more than enough to do to look after the animals.

Many dead sh.e.l.ls are lying about, showing the various species of sh.e.l.l-fish which inhabit this sh.o.r.e or the waters of the offing. Some of them we already know, and others we can never expect to get alive except by dredging. Such are the scallops, which rarely come up as far as low-water mark, in spite of their wandering habits; and the jinglesh.e.l.ls or goldsh.e.l.ls, although these, like the young oysters to which they are closely related, may usually be found clinging to stones, where they seem swollen scales or "blisters" of thin amber, or gold-colored horn.

There is one--let us examine it. We can't pick it off, or even pry it off; but when we slip a knife-blade slowly beneath it, it comes loose, and we discover that this queer creature is a bivalve mollusk looking (and tasting) like an oyster, and with a small flat sh.e.l.l underneath the bulging top one. In this undersh.e.l.l is a large hole, through which pa.s.ses a stout stony stalk which anchors this creature as firmly as an oyster is fixed by the cementing of its undersh.e.l.l to whatever it has attached itself when young.

The jinglesh.e.l.ls are extremely numerous all along the coast south of Cape Cod, wherever the water is no more than about seventy feet deep, especially in Long Island Sound; and the oystermen gather them from the beaches and from their dredgings, and scatter their sh.e.l.ls over the floor of the sound as "seats" for young oysters. They are especially useful for this purpose because they are so slight and brittle that when, as often happens, two or three minute oyster-larvae settle down on one of these sh.e.l.ls, they will, as they grow, break it apart by the strain, and then each oyster, relieved from the crowding of its mates, will form a round, nicely shaped sh.e.l.l instead of a narrow or misshapen one, and consequently be more valuable when it comes to be dredged up, after a couple of years or so, and offered for sale.

This rough s.p.a.ce between tide-marks is a fine place for crabs. We have seen some of these creatures already, elsewhere; and our book (see Chapter x.x.xV) has already instructed us as to the general characteristics of crustaceans. Here, scrambling about the ledges just under water, are big rock and Jonah crabs, but not so many of them as you might see in Maine. Both are eaten when "soft-sh.e.l.ls," but are not so good as the blue crab. Here, too, are lively and pugnacious fiddlers and some green or stone crabs, wonderfully active little creatures, which in England are sent to market, but on this side of the ocean are used only for bait.

Still more comical and interesting is one of the spider-crabs, which may be called thornback. It has a little body, but very long legs, so that a big male thornback might cover eighteen inches in the stretch of its legs.

Do you see how long his great claws are, and how his back is covered all over with tiny hooked spines? It is quite easy to understand why the name of thornback was given to him. But how is it that all those tufts of seaweed are growing on the upper part of the sh.e.l.l?

Well, the answer is a very odd one. The crab planted them there himself!

The fact is that when he is lying down at the bottom of a pool he does not want to be seen, for fear that the animals upon which he preys should take alarm, and escape before he can catch them. So he actually pulls up a number of little sprigs of seaweed, and plants them on his back one after the other, pressing the roots down with his claws till at last they are held quite firmly by the little hooked spines with which his sh.e.l.l is covered! Then as long as he keeps quite still he is perfectly invisible, and his victims may even crawl over him without suspecting that they are in any danger.

Stranger still, if a thornback crab which has covered his back with seaweeds should be placed in a tank in which sponges are growing, he will soon find out that he is not nearly so well hidden as he would like to be, and will get very uneasy. Before long he will discover what the reason is, and will actually pull all the sea weed off his sh.e.l.l, and plant sponges on it instead.

Here, too, scampering and rattling about among the pebbles, are lots of hermit-crabs, dragging after them the sh.e.l.ls in which they have ensconced their soft hind bodies, as is described on page 402. And under the stones--turn them over and you will see--are dozens of strange little half-transparent creatures which you might easily believe were insects, but which really are diminutive cousins of the crabs and crayfish named amphipods and isopods, and so forth. You may find under some stone one of the tubes made by a certain species, composed of grains of sand glued together by sticky threads much like spiders' silk.

These minute crustaceans exist in vast mult.i.tudes near the surface of the ocean at certain seasons, and form the princ.i.p.al food of the whalebone-whales, which gulp them down wholesale. Some of them, also, are parasitic on fishes.

But what is the curious little creature clinging flat upon this rock among the weeds? It looks like some sort of pill-bug half an inch long, doesn't it?

Ah! that is a chiton. It is really a kind of sh.e.l.l-bearing mollusk, like the whelk and the periwinkle; only instead of having its sh.e.l.l made all in one piece, it has eight sh.e.l.ly plates on its back, which overlap one another just like the slates on the roof of a house. Just touch it with your finger. There! Do you see? It has rolled itself up into a ball, just like those pill-millepedes which you may find in the garden. It always does this if it is frightened. And its sh.e.l.l is so stout and hard that as long as it is rolled up it is quite safe from nearly all its enemies.

If you were to hunt about among the rocks quite close to the water's edge when the tide is at its lowest, you would most likely meet with a number of chitons, and you would be surprised to find how much they vary in color. Some are ashy gray all over; but a great many are streaked and spotted with brown, and pink, and orange, and lilac, and white. But the strangest thing of all about chitons--there are far larger ones in the warmer parts of the world--is that some of them have nearly twelve thousand eyes scattered about all over their sh.e.l.ls!

But we are lingering too long by the way, for our real destination to-day is that fine pool over there. It is a basin among the ledges, filled with quiet sea-water left by the retreat of the tide, half-floored with sandy mud, and its edges fringed with feathery seaweeds, corallines, and hydroids. Here is a capital home for the little folk of the sea, where there is always fresh clear water, but where only a part of the time do the surges pound, and then never with full force; furthermore, a wall of rocks protects the nook, and enemies can rarely enter to destroy the peaceful society.

In warmer parts of the coast, as in the Gulf of Mexico, or upon the Pacific coast, or most of all in some of the tropical islands which now belong to the United States, such a pool would be brilliantly carpeted with sponges, sea-anemones, coral-polyps and corallines, of which you may read on pages 431 to 435. The water of the North Atlantic, and the winters of its American coast, are too cold, however, to allow any but a very few hardy species of these lowly sea-flowers to grow in our pool; but there are quite enough to keep us busy during the hour or two left before the returning tide creeps over the jagged rim of the basin and drives us away.

Here, for instance, is half an oyster-sh.e.l.l looking as if it had been bored full of holes with bird-shot. It could hardly have been any boy's target though; for, see, we can find many such fragments. There is one under water. Take it out and you will find every one of the hundreds of little pits filled with a yellow spongy material. It is real sponge, called the boring-sponge, because it riddles all sorts of old sh.e.l.ls until they fall to pieces. This is a good thing, for then they are gradually ground to powder and dissolved in the water, and so help to keep it supplied with the lime needed by living animals for their sh.e.l.ls.

But other sponges help in this work. One is a brilliant crimson, and spreads a velvety mantle over the sh.e.l.l, from which rise branches as big as your fingers. We may probably discover among others here the pretty urn-sponges, like cl.u.s.ters of yellow or gray goblets about half an inch high. On the reefs of the Gulf coast of Florida, you know, several sorts of sponges grow to great size and are gathered and prepared for use--a trade which furnishes employment to hundreds of men.

But this clear pool holds more beautiful things than sponges. If we are fortunate we may find a sea-anemone. Do not fancy from its name that it looks anything like the pretty pink and white anemones that delight you in the woods in the spring. It does, indeed, look something like a clove-pink, or some sorts of chrysanthemum, when it is fully expanded, yet it is not a flower at all, but a true animal.

Its body is shaped like a barrel, or sometimes more like a tube, with a large throat leading into a big stomach which is held in position in the center of the body by six part.i.tions radiating like the spokes of a wheel from the stomach to the tough outer skin. Between these are other shorter part.i.tions extending inward from the skin, but not reaching the stomach.

This is the type of structure in the polyp family, which the sea-anemones represent; and the stony coral-polyps are built on the same plan, only there the outer wall and the radiating inside part.i.tions become hardened plates of lime as the animal grows, and form, when many grow into a solid ma.s.s, the immense coral reefs described on page 433.

The New England coast has several small sea-anemones, and one handsome one, sometimes as big as a teacup, a few of which dwell in our pool.

Just come, very quietly, over to this side, and gaze down through the clear water upon that reddish block of stone. Do you not see that large brown tuft, quivering and moving like a chrysanthemum each petal of which was alive? That is the brown sea-anemone; but some specimens show much brighter tints.

Ah!--did you notice how that minnow turned and fairly flew as he felt a touch of one of those waving petals? No wonder he was in such a hurry to escape from its clutches, since he knew quite well that the grasp of those arms means death. For every one of them is set with scores and scores of tiny oval cells, made in such a way that they spring open at the slightest touch. And inside each cell is a slender poisoned dart, which leaps out as soon as it is opened.