The Animal World, A Book of Natural History Part 61

The sh.o.r.e of the eastern United States, at least south of New York, is formed of a line of long narrow islets whose outer beaches, and the sea-floor for miles out, are pure sand. They support very little life, as has been said. Behind them, however, are shallow bays and sounds, in which the water, though salt, is usually warm and still; mud gathers upon the sand, and eel-gra.s.s and other water-weeds grow in abundance.

Here is excellent ground for naturalists, old or young, and in a single walk you can discover enough to surprise you greatly. We must go when the tide is low, and it will be a good idea to take our rubber boots, so that we may not be afraid of the wet mud. We will also take a small spade or strong trowel, and some boxes and bottles.

What a lot of clam-sh.e.l.ls are lying about the sh.o.r.e! There are two kinds, the soft clam and the hard clam; but none of them are alive.

How is this? We have already learned, you will remember, that the clams are bivalves; that is, the sh.e.l.l is in two pieces, hinged together by an elastic ligament over the back, and covering each side of the animal.

The soft body is attached to each sh.e.l.l by a strong muscle, by which the creature can pull the sh.e.l.ls tight together, and so cover itself completely. When it wishes, however, it lets the sh.e.l.ls spring open somewhat, so that it may put out from between their lower edges its muscular "foot," and perhaps move about, while out of the front end it stretches a double-barreled tube, called its siphon. Down one of the tubes is sucked a stream of water which not only bathes the animal's gills, or breathing organs, but carries minute floating particles of food into its stomach, after which the waste water is forced out of the other tube.

Now you will understand what we shall see, and are ready for the answer to our question. You never find live clams crawling about the sand, because they live buried in the mud.

Now let us put on our boots and look about on the surface of the wet mud. Do you see ahead of us those little jets of water come spouting up into the air as if squirted out of tiny syringes? Every one of these little jets is thrown up by a soft clam, which lies perhaps several inches deep in the mud, with its siphon stretched up to the surface and held full of water, waiting for the tide to come in and refresh it. When it feels the jarring of our footsteps it squirts the water out; and you must dig deep and fast if you want to catch it. This is what those men are doing out there on the flat--digging out clams with long spades, and filling their baskets for market. Thousands of little ones lie in the mud, not yet big enough to eat.

The soft clam is a shapeless sort of mollusk, with a thin chalky sh.e.l.l, not at all pretty; but the hard clam, or quahog, is thick-sh.e.l.led and regular in outline; and in an end-on view takes the shape of an ace of hearts, like the Venus-sh.e.l.l, or the c.o.c.kle, which is so commonly eaten in Europe. This species likes much deeper water than the soft clam, and is gathered mostly from boats, by a kind of rake; but we shall no doubt find a few up here. Do you see that scratch in the mud? It looks like a trail, and there at the end is the traveler himself, standing upright in the mud like a half-buried wedge.

This shows another difference between the two clams; for while the soft clams and their relatives, such as the pretty razor-fish, and the "old maid" of English bays, never leave the burrow where they begin life, the quahogs slowly wander about all the time. As for the scallops, they fairly skip and jump.

What are scallops? Well, we shall hardly see much of them, for they live in deep water; but their half-sh.e.l.ls are to be seen cast up everywhere, for they also are bivalves. Our common ones are usually about the size of a silver dollar, and fan-shaped, the thin sh.e.l.l ribbed like the sticks of a fan, and the margin crinkled, and they are variously colored, but mostly in tints of reddish and yellow.

Several small bivalves and sea-snails may be added to our collection from this uncovered bay-bottom, and here and there s.p.a.ces are fairly sprinkled with little blackish fellows about the size of hazelnuts. When we have gathered a handful we shall find we can sort out three or four kinds.

A very curious denizen of the tide-flats of our Southern States is the pinna, a large bivalve with thin h.o.r.n.y sh.e.l.ls shaped like a slightly opened fan, which lies deeply buried, point down. The edges of its sh.e.l.l come just at the surface, and are exceedingly sharp, so that barefooted persons have to be very careful how they step where pinnas are common, as on the Gulf coast of Florida, and it is no wonder the people there call them razor-fish. Lying there in the mud, with its sh.e.l.ls parted, and a current of water always sucking down what we may call its throat, it forms a regular trap for little fishes and other small creatures. The instant one swims between the sh.e.l.ls, they close and the unfortunate curiosity-seeker finds himself in a prison from which there is no escape.

When a young pinna settles down in its place it at once anchors itself to some rock or fixed thing below it by throwing out from near its lower, narrow end a bunch of very strong threads, which hold it down so firmly that it takes a very hard pull to tear them away. This anchor-cable is called a byssus.

A short distance from us a narrow stream wriggles through the salt marsh, and we can get into a rough little boat and paddle down toward that old wharf whose weedy piles are covered with interesting things, which we may examine now that the ebbing tide has left them uncovered for a few hours. The peaty banks, with their growth of harsh salt-gra.s.s and algae, will keep our eyes busy as we float along the black and winding creek.

Now we shall get acquainted with some of the crabs. Look sharply down into the water and you will see the large "blue" crabs which we buy in the market, and eat, swimming near the bottom or crawling over the mud near the banks. There is one, now. He doesn't look very blue, nor very appetizing, does he? His back is brown and muddy, to be sure; but his big claws and lower plates have much more blue upon them than has any of the other large crabs, and so he gets the distinguishing name.

But, you say, you have heard of "hard-sh.e.l.l" and "soft-sh.e.l.l" crabs, and want to know the difference? It is simply a difference of condition. If you will turn to page 397, you will find described that extraordinary process by which crabs grow, by throwing off their stiff old skins and expanding to fill the elastic new one which has formed underneath.

Before this change, the creature is a "hard-sh.e.l.l" in fishermen's language, and just afterward, when he is large and tender, he is naturally a "soft-sh.e.l.l"; and then is the time to eat him.

Notice how the black ma.s.ses of peat along the banks are honeycombed with holes, as if somebody had been pushing down the point of his umbrella.

They are the homes of little fiddler-crabs, which scuttle into them by the hundred as we approach, and then creep up to peer out after we have pa.s.sed by, and make sure it is safe to go abroad again. In other holes live two other sorts of burrowing crabs. One is the little mud-crab (_Panopaeus_), which is a peaceful cousin of the fiddler; and the other is the sand-crab (_Ocypoda_) whose peculiarity it is to be perfectly sand-colored, so that it is almost impossible to see him until he moves; consequently he is commonly found only in the sandy places.

As we float nearer to the mouth of our winding creek, we begin to notice bunches of mussel-sh.e.l.ls, clinging closer to each other than grapes in a bunch; and when we try to pick one up we find it quite immovable. In fact, they are anch.o.r.ed to the roots of the gra.s.ses, and to each other, by a bunch of byssus threads from each mussel, like those of the pinna; and these threads are so strong that they can hold the mussels firm against the beating of the waves, so that a sh.o.r.e which is thickly covered with mussels is safe from wearing away. You may see an example of this in the tideway at the mouth of this very creek, and ma.s.ses of mussels strengthen the supports of that wharf we are approaching. If you were to go near the town of Bideford, England, you would see a bridge of twenty-four arches, which runs across the Torridge River close to the place where it joins the Taw. Now that bridge is held together by cl.u.s.ters of mussels! The force of the stream is so great, that if mortar is used to repair the bridge it is very soon washed away. So from time to time large boat-loads of mussels are taken to the spot and shot into the water, and they fasten themselves so firmly to the bridge by means of their byssus threads, that they actually hold together the stones of which it is built!

These binding mussels are mostly of the smooth, dark-blue sort which are found on both sides of the Atlantic, and in Europe are gathered and eaten. When our people become a little wiser and more economical, we also will take advantage of this great stock of excellent food right at our doors.

But in the bunches which are scratching the side of the boat as we glide along close to the bank are some which are much larger, though smooth, like the edible mussels. They are an American species. Then here and there in peat you may see a sort whose sh.e.l.l is rough, with ridges spreading out toward the large end, and these you may call horse-mussels.

Now we have got down to the boat-landing toward which we have been lazily drifting, and we will twist the chain around one of the piles that support it, and stop long enough to take a look at one of them.

Most of the time each pile is under water, and therefore is overgrown with a thick "fur" of plants and animals.

You will see that most of this fur consists of seaweeds, but their leaves are often the resting-place of several sorts of lowly animals.

Indeed, you must look sharp to make sure whether some of the feathery tufts that droop from a dank old post, or spray out so beautifully in the ripples at its foot, are plants or animals. We will not talk about that just now, but wait till we take our excursion to the rocky sh.o.r.e, where we shall find barnacles and corallines and sea-mats and polyps bigger and better than here.

But do you see between those green fronds that roundish yellow object about as big as a filbert? Touch it gently. Did you see tiny jets of water squirt out of two little nozzles on its surface? That gives it the name of sea-squirt. Into one of the nozzles, when the tide comes over it, is constantly sucked a current of sea-water which pa.s.ses into a stomach-like cavity, where the minute particles of food in the water are caught and digested; then the water pa.s.ses on through another cavity where the blood receives its oxygen, as in our lungs or a crab's or fish's gills, and then rushes out. So this little object is a real animal, with heart, blood, stomach, and something in the way of nerves--enough, at any rate, to feel your touch, shrink, and squirt out all the water in its bag-like body.

There are a good many kinds and forms of these ascidians, as naturalists call them, some larger, some waving about on the summit of stalks like lily-buds, and some cl.u.s.tered into colonies grown together, which form bands around the stems of plants, or make ma.s.ses called "sea-pork" by the fishermen, or float in chains, by millions, on the surface of the open sea.

Here, too, are small red and yellow sponges; some coa.r.s.e little sea-anemones, etc.; and wandering over the whole, feeding upon one or another of these, and cleaning the polyps and polyzoans off the algae, are a sort of marine daddy-long-legs, called no-body crabs, because they seem all legs and look crab-like.

It isn't very sweet-smelling under this damp old wharf, where the rising tide is beginning to bathe the piles, and one after another plants and animals are expanding as they feel the refreshment of the water around them; and we will move away as soon as we have dug a few things out of the mud, soon be hidden by the tide.

Let us run the bow of our boat up on that soft black slope, and see what we can find by leaning over the side. Just look at this hairy object, for instance, which has been left by the retreating waves. It seems like a big brown slug covered with bristles and is not very pleasant to handle; but you needn't be afraid of it, and you mustn't be squeamish.

Just dip up some water in that pail, and rinse it till you have washed every sc.r.a.p of mud from its bristly coat, and then look at it in the sunlight. Do you think it is dull and dingy now? Did you ever see a more beautiful creature? This animal is called the sea-mouse, although really it is a kind of sea-worm and if you will turn back to page 429 you will find it described. The reason why its coat is always so dirty is that the bristly hairs which cover it act as a sort of filter, and strain out the mud from the water which is pa.s.sing to the gills. But these hairs have another use as well. Each one is really a sort of slender spear, with a barbed tip, the edges being set with a number of sharp little points, all directed backward, forming a capital protection from such creatures as the fishes, a great many of which would be glad to feed upon sea-mice if it were not for their coating of spines.

Do you see those twisted little coils of muddy sand scattered about on the mud? Those are the casts of lugworms, which are made in the same way as the casts of earthworms seen in our garden-paths on damp mornings; in fact, these lugs are just marine earthworms (see page 427), and like them eat their way down into the mud, swallowing mouthful after mouthful for the sake of nourishing particles in it, and then voiding the useless remainder.

Perhaps you wonder how it is that the burrows of the lugworms remain open. Why doesn't the mud close in behind the animal? The fact is that the worm is always pouring out from its skin a sticky slime which quickly becomes quite hard and firm. And this binds the sandy mud together as the worm forces its way down, and forms a kind of lining to its burrow, just like the brickwork with which we line our railway tunnels.

You would scarcely suspect what interesting and often beautiful worms lie buried in the mud or muddy sand of sea-beaches and salt marshes.

They occur elsewhere, too, as upon weedy rocks, while a great many kinds dwell upon or within the bodies or coverings of other animals, from whales to periwinkles and crabs.

Most of the beach-worms belong to the highest cla.s.s of the tribe, called annelids because their bodies are made up of ring-like segments (a little ring in Latin is _annellus_), as you can easily see by examining one of the angleworms you dig in the garden for fish-bait. The red lugworm, or "red thread," as it is often called, is another plain example of this structure.

Digging down by low-water mark we are likely to unearth one or more of the ribbon-worms which, when they are large, seem rather terrible. Their bodies are flat, so that when they swim they move through the water like a floating ribbon, and they have been found five or ten feet long and as wide as your palm. Such big ones are rare, however, and we are more likely to have to deal with one two or three feet long and less than an inch broad. They are active creatures, burrowing into and through the mud in search of other worms upon which they feed, and which they seize by thrusting out a sticky proboscis. There is also a smaller one, pink in color, while the bigger species is yellowish.

Though we may not dig up a ribbon, we are pretty sure to turn out a nereis, or clam-worm, as the fishermen call it--a reddish creature a foot or two long, looking like a centipede, for there is a pair of minute feet on each ring, and every foot is feathered with a gill. This also is a ravenous enemy of all other worms or animals it can overcome; and young clams, limpets, starfish, and other protected creatures must be thankful for their armor when it comes crawling near them. Its rich green and salmon coat has no charm in their eyes, you may be sure. But the nereis itself must have its fears, for it is not only hunted by ribbon-worms, by a big active annelid called "four-jawed," and by winkles and dog-whelks, but is well liked by various fishes; and, last misfortune of all, it is constantly sought by fishermen for bait. In spite of all this, clam-worms of all kinds remain immensely numerous all along the coast. On calm summer nights they leave their burrows, swim up to the surface at high tide, and cast out vast numbers of eggs, from which presently hatch little pear-shaped larvae, which swim about a short time, when the few that have survived settle down, change to the worm-like form, and burrow into the mud.

When we come to explore the rocky places, and peer into the still pools left by the ebb-tide among the reefs and boulders, we shall make the acquaintance of some other worms that display themselves in such places as in a natural aquarium.

III

ON THE ROCKY LEDGES

There are practically no rocks on our Southeastern coast, so that we must imagine ourselves now somewhere in New England--let us say on the southern sh.o.r.e of Rhode Island. All along the north side of Long Island Sound, about Buzzards and Narragansett bays, and then from Boston Harbor right up to Labrador, the sh.o.r.e is rock, with many headlands, reefs, and islets, separated by shallow coves or by swift tidal runways. This is good hunting-ground for the seaside naturalist, and one visit to the s.p.a.ce left uncovered at low tide will be no more than a glance at what might easily keep us busy and interested a whole summer through.

As the water ebbs away, the tops of the ledges and boulders emerge like the hairy heads of some sea-monsters, for they are mostly overgrown with long tresses of olive-brown rockweed and green ribbons of sea-cabbage, (_Ulva_), which trail, wet and shining, down their sides. Step carefully, for it is all extremely slippery. Do you hear that continual popping under your feet? That means that you are crushing the little bladder-like swellings strung like big beads on the stems of the rockweed. They are filled with air, and keep the long and heavy stems and leaves of the weed afloat, as you may see if you look down where it is swishing back and forth in the lapping waves. These plants must be exceedingly strong to resist the pulling and pounding of the surf in a storm; and their power to keep afloat by means of these gas-filled "bladders" is of a.s.sistance, not only in enabling them to hold together, but to form a breakwater which protects the rocks and ledges they cover from being beaten to pieces by the surf.

Underneath and upon these ma.s.ses of seaweed hide a great quant.i.ty and variety of small plant and animal life, some of which we shall be able to find and study, though a large part of it requires more thorough work than we have time for, and the aid of a microscope.

But first let us look at some of the bare places, where there is no seaweed. Here is a black rock with white patches of rough little things growing upon it by the hundred. They are not mollusks, however, but rock-barnacles (see page 407), which English boys call acorn-sh.e.l.ls.

They are small and distant cousins of the crabs.

The story of these barnacles is a very curious one. When first they hatch from the eggs which older barnacles have cast out into the sea, they are not in the least like their parents, but are queer little round-bodied creatures, smaller than pin-heads, with six feathery legs by which they paddle about, one round black eye, and two feelers. Every two or three days they throw off their skins, as caterpillars do, and appear in the new ones which have formed underneath; and every time they do this they change their shape, so that sometimes they are round, and sometimes oblong, and sometimes almost triangular!

At last they reach their full size. Then they cling with their feelers to the first rock, log, or other hard thing they come to, and pour out a drop or two of a very strong cement, which hardens around them and fastens them firmly down. After this they never move again; but a day or two later they change their skins once more, and appear as perfect acorn-sh.e.l.ls.

Now look at one of them carefully through this magnifying-gla.s.s. Do you see that there is a little hole in the top of the sh.e.l.l, which is made of several pieces? That is the hole through which the animal inside fishes for food. If you were to watch it when the rocks are thinly covered with water, you would see that it kept poking out a net-like scoop, and then drawing it in again. This net really consists of the hairy legs; and as they wave to and fro in the water they collect the tiny sc.r.a.ps of decaying matter on which the little creature feeds. They also bear the gills by which the barnacle refreshes its blood.

You must be very careful not to knock your hand against these sh.e.l.ls when you are hunting about among the rocks, for their edges are so sharp that they cut almost like knives.

"Another sort of barnacle," you say you have found? No: there _are_ other sorts--the strange goose-barnacle, for instance, which attaches itself to the bottoms of ships--but what you have found is one of the limpets, and that is not a crustacean, but a gastropod mollusk. It is shaped like a tiny rough mountain, or rather like a volcano, for you see there is a hole in its summit; and we call it the keyhole limpet on account of the shape of that hole. Pick it up. Oh! you can't, eh? Of course not. Pull and push as hard as you like, you won't be able to move it, nor can the heaviest waves wash it off.

Would you like to know why?

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.