Chapter XXXVIII: Annelids and Coelenterates (3)

Well, the reason is that a limpet clings to a rock by turning the whole lower surface of its body into one big sucker; it presses it tightly against the rock and then lifts the middle part. The consequence is that a chamber is formed in which there is nothing at all--no water, not even air; and, as happens when you lift a brick with a small leather sucker, the weight of the atmosphere presses down upon it so strongly that no force you can bring to bear will pull it off.

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 shores, 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-shell or _Purpura_, which is interesting because it belongs to the European shores 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 algæ, 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 shore 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 shells, 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, shell 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 shell 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 ashore and left there, and often are made use of by being ground up with oyster-shells, 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-glass 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 shell 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 shell, 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 shores of southern New England, but more numerous northward, and on rocky bottoms offshore. On the offshore bottom there lives also a queer sort whose shells 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-pricks" 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 shore 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 spaces 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 shells are lying about, showing the various species of shell-fish which inhabit this shore 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 jingleshells or goldshells, 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 shell underneath the bulging top one. In this undershell is a large hole, through which passes a stout stony stalk which anchors this creature as firmly as an oyster is fixed by the cementing of its undershell to whatever it has attached itself when young.

The jingleshells 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 shells 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-larvæ settle down on one of these shells, 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 shell 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 space between tide-marks is a fine place for crabs. We have seen some of these creatures already, elsewhere; and our book (see