CHAPTER III. Bivalve Molluscs

BIVALVE MOLLUSCS

PLATE XIII

THE OYSTER (1)

The “bivalve” molluscs are so called because they live in shells made of two parts, or “valves,” which are fastened together by means of a hinge. There are a great many of these, and the Oyster is one of the best known of them all.

This creature is only found in places where the bottom of the sea is muddy, because in sandy places the sand is very apt to get into the hinges of the shells and to prevent them from being closed; and in that case the animal very soon dies from suffocation. So oysters are generally found in the mouths of rivers, or in land-locked bays where there is no sand at all.

The history of these creatures is a very curious one indeed.

In the month of May the mother oyster produces a very large number of eggs--sometimes as many as eight or nine hundred thousand! These are called “oyster spat,” and for several weeks she keeps them in her gills. Then one day she suddenly opens her valves and squirts them out into the water, where they look like a little cloud of the finest possible dust. For a short time after these eggs hatch the baby oysters swim about, and travel backwards and forwards as the tide rises and falls. After a while, however, they sink down and fasten themselves to some object at the bottom of the sea; and when once they have done this they never move again. They always lie upon their left sides, with the smaller and flatter of the two valves uppermost; and there they remain for five years at least before they reach their full size.

Oysters feed, too, in a very odd way. You know, perhaps, that inside the shell of an oyster there is a tufted organ which we call the “beard.” This consists of the gills. Hidden away underneath these is the mouth; and the gills do not merely suck out the air which has been dissolved in the water, as those of other animals do, but sift out every little tiny scrap of decaying matter which the oyster can use for food as well. So an oyster’s gills enable it to breathe and to catch its dinner at the same time!

PLATE XIII

THE SADDLE OYSTER (2)

This is a very curious oyster; for in its flat lower valve, just below the hinge, is a large oval hole. Through this hole passes a strong band of muscle, to which is fastened a kind of shelly knob which looks just like a button. By means of this the animal fastens itself down to some object at the bottom of the sea; and very often indeed it is found attached to the shells of other molluscs, looking something like the saddle on the back of a horse. That is why it is called the “saddle oyster.”

Another curious fact about this creature is that very often its shape completely alters as it grows older. While it is quite small it looks very much like an ordinary oyster. But as time goes on it generally takes the form of the object on which it rests. So you might easily find half-a-dozen shells of the saddle oyster, not one of which would be shaped like any of the others.

1. THE OYSTER.

2. THE SADDLE OYSTER.

3. THE COCKLE.]

PLATE XIII

THE COCKLE (3)

This is one of the very commonest of all the creatures of the sea-shore, and you may find its heart-shaped shells lying about on the beach in hundreds and thousands. In many places, indeed, cockle-shells are found in such wonderful numbers that they are crushed up and used for covering pathways instead of gravel.

Yet you may wander about on the shore day after day for weeks together and never see a living cockle. How is this?

Well, the reason is that cockles live buried underneath the sand. If you go down near the edge of the waves when the tide is quite low, and just stand still for a minute or two and watch, you are almost sure to see first one little jet of water, and then another, and then another, come squirting up out of the sand into the air. Now these little jets of water are thrown up by cockles which are lying buried in the wet sandy mud below. For every now and then these creatures draw down a little water into their gills, through one of their siphon tubes, and when they have sucked all the air out of it they squirt it up again through the other.

Would you like to dig one of them up and look at it? Well, just take a wooden spade and try. You will find that you cannot do it, for the cockle can dig a good deal faster than you can. The fact is that he has a very strong, fleshy organ which we call the “foot,” and with this he can burrow down into the sandy mud so quickly that by the time you have dug to a depth of six inches, he will have gone down to the depth of ten or twelve.

The cockle uses this “foot” for another purpose as well, for he can jump with it. And if you did succeed in digging him out of the ground, you would very likely see him skipping about in the most active way, almost like a sandhopper!

Upon some parts of the coast another kind of cockle is found, which has its “foot” of a bright red colour. For this reason it is generally known as the “red-nosed cockle.”

PLATE XIV

THE MUSSEL (1 and 2)

Mussels are almost, if not quite, as plentiful as cockles. If you walk down underneath a pier or a jetty when the tide is out, you will often find that the pillars which support it are covered with great clusters of these creatures; and very often the rocks which are left dry at low-water are covered with them in just the same way. They fasten themselves down by means of a bundle of very strong threads, which we call the “byssus”; and these hold so firmly, that although the waves may beat upon a bed of mussels day after day all through the year, they never succeed in tearing them away.

Near the town of Bideford in Devonshire, indeed, there is a bridge which is only kept standing by means of mussels. This bridge, which is a very long one, with twenty-four arches, runs across the Towridge River, close to the place where it joins the Taw; and the tide runs so rapidly that if mortar is used to repair the bridge it is very soon washed away. So boat-loads of mussels are brought to the bridge from time to time, and these anchor themselves down so firmly by means of their byssus threads that they actually hold the stone-work together!

Sometimes, however, mussels do a great deal of harm, for they will get into an oyster-bed and fasten themselves down upon the shells of the oysters. Their byssus threads then form a kind of thick mat, which collects and holds the mud that is brought up by the tide every time that it rises; and this very soon covers the oysters entirely up, and smothers them to death.

Mussels do not remain fastened down in one place for the whole of their lives, however, as oysters do. They can crawl about quite easily whenever they like. And they do this, also, by means of their byssus threads. First they move a few of these threads forward, and take a fresh hold with them; then they draw the rest up after them; and then they move the front ones forward once more, and so on over and over again.

Mussels are very largely used for food, and also as bait for deep-sea fishing. In the Firth of Forth alone, indeed, nearly forty millions of these creatures are collected every year for this latter purpose alone, or one for every man, woman, and child in England and Scotland and Wales!

PLATE XIV

THE HORSE MUSSEL (3)

This is not a very handsome creature, for its shell is covered all over with a rather thick brown skin, which is very much wrinkled. It is quite common in many places, and yet one does not very often see it; for it is nearly always hidden underneath its byssus threads, which grow in thick masses. Besides this, it often burrows underneath the surface of the sand; so that unless you know just where to look for it, and how to look for it, you are not likely to find it.

1. INSIDE OF MUSSEL SHELL.

2. THE MUSSEL.

3. THE HORSE MUSSEL.]

But if you go down to the pools at the very edge of the water when the tide is quite low, and scrape away the sand which is heaped up against the bottom of the rocks, you may very likely come upon quite a large cluster of these curious creatures.

Horse mussels are not used for food as common mussels are, because they have a very strong and unpleasant taste.

PLATE XV

THE VARIABLE SCALLOP (1)

A good many different kinds of scallops are found on our shores. One of them--the Common Scallop--is as large as the palm of a man’s hand, and is used for food. You may often see it in fishmongers’ shops. But you are not at all likely to find its empty shells lying on the shore, for it lives in rather deep water. You may find those of the Variable Scallop, however, very often indeed in places where the shore is sandy. It is called the “variable” scallop because it varies so much in colour that one hardly ever sees two of its shells which are quite alike. Sometimes they are crimson, sometimes pink, sometimes mauve, sometimes dark yellow, sometimes golden yellow, and sometimes blotched and mottled with different colours. A number of ridges run down the shell from the hinge to the margin, and on each of these is a row of short spikes; so that the animal looks something like a tipsy-cake!

Scallops swim in a rather curious way, namely, by opening and shutting their valves over and over again. As often as they do this a jet of water is squirted out, and this acts on the surrounding water just like the jets which are squirted from the siphon tubes of the cuttle, and drives the animal along with some little speed. As it travels through the water it looks very pretty, for all round the edges of its shell it has a fringe of long feelers, which wave up and down in a most graceful way. By means of these it obtains its food. At the base of these feelers is a row of little black dots, which seem to be eyes.

PLATE XV

THE RADIATED SCALLOP (2)

This is rather a rare shell, and if you find it lying upon the shore you will be fortunate. You may know it at once if you do find it, for it only has six or seven ridges running down it, instead of about twice that number. It varies a good deal in colour, but is generally reddish brown, spotted and speckled with white.

1. THE VARIABLE SCALLOP.

2. THE RADIATED SCALLOP.

3. THE HUNCHBACK SCALLOP.]

PLATE XV

THE HUNCHBACK SCALLOP (3)

It is very easy to see why this creature is called the “hunchback,” for although when it is quite small it is shaped just like other scallops, it alters in form very much as it grows bigger; so that really it sometimes looks as if it had been crumpled up when it was quite soft, and had never recovered from the squeeze. Besides this, the two valves are not alike, as they are in other scallops, for while one is always very deep and rounded, the other is nearly flat. So when the animal is alive it really has a kind of “hunchbacked” appearance; and if you found its two valves lying apart from one another you would hardly believe that they could both have belonged to the same creature.

The colour of the hunchbacked scallop is white, mottled with brick-red.

PLATE XVI

THE SUNSET SHELL (1 and 2)

This is a very “local” shell. That is, it is very common indeed in some places, so that you might pick up hundreds and hundreds in a few minutes, while in other places it is never found at all. The best place in which to look for it is a part of the beach where sand and mud are mingled together, and there you will be almost sure to find it.

The name of “sunset” shell has been given to it because of the beautiful way in which the inside surface is coloured. Sometimes it is rosy pink all over; sometimes it is orange yellow; sometimes it has crimson streaks upon a whitish ground. But you can never look at it without being reminded of the evening sky after a very bright sunset. The outside of the shell, however, is always white and chalky-looking, and no one who saw the two valves fastened together as they are when the animal is alive would have the least idea how beautiful they really are.

This creature always lives buried in the sandy mud, just as the cockle does. It has a very powerful “foot,” by means of which it burrows, and two long and very slender siphon tubes.

PLATE XVI

THE GAPER (3)

This is another of the shell-bearing molluscs which live in burrows in the sandy mud, and it is called the “gaper” because the shells are always open at the top, just as if the animal were yawning, or gaping. Through this opening the siphon tubes project. These tubes are used in breathing, just like those of the cuttle, and are enclosed in a kind of leathery case, which the animal can stretch out or draw back at will; so that when it is lying at the bottom of its burrow it can keep the tips of the siphon tubes just above the surface of the mud, and so draw water down to its gills quite easily.

1. INSIDE OF SUNSET SHELL.

2. THE SUNSET SHELL.

3. THE GAPER.]

On some parts of the coast gapers are used as food. But if you want to buy some you must not call them “gapers.” You must call them “old maids”; for by that name they are always called by the fishermen. Some of the sea-birds are very fond of them too, and dig them out of their burrows with their long beaks. And in the far North millions and millions of them are devoured by walruses, and also by Arctic foxes, which prowl about the shore in search of them every day when the tide goes down.

PLATE XVII

THE PIDDOCK (1)

Now we come to one of the most wonderful of all the creatures which live in the sea; namely, the Piddock. You can find its empty shells lying about in numbers on almost any part of the shore where the cliffs are made of chalk or limestone. And if you look at the rocks which are left dry when the tide goes down you will see the entrances to its burrows--large, oval holes, several of which you may often find quite close together. For the piddock is a boring shell, which drives its tunnels through and through the rocks, until very often they are quite honeycombed by its tunnels. Sometimes you may meet with a big block of chalk which only weighs about half as much as it should, because all the rest has been cut away by piddocks. And if you could split it open you would find several of these creatures lying in their burrows.

But how they manage to cut their way through the hard chalk, or the still harder limestone, nobody quite knows. Most likely, however, they do so partly by means of the soft part of the body which we call the “foot,” and partly by means of the shell, which they turn first a little bit to one side, and then a little bit to the other side, just like a man who is using a bradawl. Every now and then, of course, the burrow gets choked up with the material which has been scraped away. But the piddock knows quite well what to do in order to clear it. It just squirts out a jet of water from the siphon tubes, by means of which it breathes, and so washes the burrow out!

Now let me tell you why I said that the piddock is one of the most wonderful of all the creatures which live in the sea.

First of all, then, remember that the sea, acting by itself, has very little power to wash away chalk. For as soon as the waves begin to beat upon the face of a chalk cliff, they leave on it the spores, or seeds, of sea-weeds. Very soon those spores begin to grow, and before long the surface of the cliff is covered with masses of weed, so that the sea hardly touches the chalk underneath them at all. The waves might beat upon the cliffs for hundreds and hundreds of years without breaking it down.

But the piddock comes and burrows into the chalk just below high-water mark. Backwards and forwards it goes boring on, till at last only thin dividing walls are left between its tunnels. Then the sea washes in, and breaks down these walls, so that the whole foundation of the cliff is cut away. The result is, of course, that before very long there is a landslip. Hundreds of tons of chalk come tumbling down into the sea. Then the piddocks begin work again a little farther back, and by-and-by there is another landslip.

You can see the effects of the piddock’s work upon any part of the coast where there are chalk cliffs. Just look at the beach when the tide is out. You will notice long spits of weed-covered rocks, which sometimes run far out into the sea. Well, those rocks were not always rocks. They were once the bottoms of cliffs. But the piddocks and the sea, working together, cut the cliffs down; so that the sea gained, yard by yard, upon the land.

Indeed, I think that it may be said, quite truly, that if it had not been for the work of the piddocks Great Britain would not be an island! At any rate we do know this, that once, a great many hundreds of thousands of years ago, Great Britain was not an island at all, but was joined to the mainland of the Continent of Europe. And we also know that the sea, acting by itself, could not possibly have cut a passage through what we now call the Straits of Dover. The piddocks helped it to do so! They kept on cutting away the foundation of the cliffs by boring backwards and forwards through the solid chalk, just below the level of the waves; and the sea finished the work which the piddocks had begun, by breaking down the thin dividing walls between their burrows.

PLATE XVII

THE LITTLE PIDDOCK (2 and 3)

The common piddock grows to a length of from three to five inches, and is almost always white in colour, though sometimes it is stained by the rocks in which it lives. But there is another kind of piddock which is very much smaller, for its shells hardly ever measure more than an inch and a half in length, and are a good deal narrower in proportion to their size. This creature is called the Little Piddock. It is generally of a brownish yellow colour, and you may often find its burrows in great numbers in limestone rocks.

1. THE PIDDOCK.

2. AND 3. THE LITTLE PIDDOCK.]

PLATE XVIII

THE SHIP-WORM (1 and 2)

This creature certainly does not look in the very least like a mollusc; and I do not think that anybody who had never seen it before would ever guess that it is really quite a near relation of the piddocks. It looks much more like a kind of worm, for it has a soft round body no larger than an ordinary drawing pencil, though it is often as much as ten or even twelve inches in length. But if you were to look at the head end of its body you would see its bivalve shells, though they are so very small that they might easily be mistaken for jaws. And these would show you that the animal is really a shell-bearing mollusc.

The shipworm is a most mischievous creature, for instead of burrowing into chalk or limestone rocks, like the piddocks, it bores into timber, such as the hulls of ships, and the posts which support jetties and piers. Very often it cuts away more than half the wood in a great beam, leaving only the thinnest walls between its tunnels. And as it works along it lines these tunnels with a curious shelly substance, which strengthens them and prevents them from breaking down.

By burrowing into timber in this way the shipworm often does most terrible damage. But it seems to dislike the taste of iron rust very much indeed. So when a beam of timber has to be protected from its attacks, a number of iron nails with very broad, flat heads are driven into the surface, with only the space of an inch or two between them. The salt-water acts upon these very quickly, and the result is that the whole of the beam is very soon covered over with a thin coating of rust, so that no shipworm will attempt to touch it.

1. THE SHIP-WORM.

2. WOOD BORED BY SHIP-WORMS.]

When the shipworm is quite small it is not in the least like the perfect animal. Indeed, if you were to see a baby shipworm, I do not think that you would ever guess what it was. It is really a kind of shipworm caterpillar. In shape it is nearly round, and is covered almost all over with tiny hair-like organs, by means of which it swims in the water. But the odd thing about it is that it keeps on changing its form. After about thirty-six hours it becomes oval. A few hours later, if you were to look at it again, you would find that it was almost triangular. A few hours later still it would be round again, just as it was when it first hatched out of the egg. And during this time of its life it has a strong fleshy “foot,” like that of a snail, so that if it becomes tired of swimming it can settle down and crawl about on the surface of the rocks.

Have you ever been through the Thames tunnel? If you have, you will be interested to know that it is made just like a shipworm’s burrow, for a kind of boring instrument, called a “shield,” was made, which enabled the workmen to line the walls with masonry as fast as the earth was cut away. In this way the walls were prevented from falling in, and water from the river above was kept from breaking through the roof and flooding the tunnel. And Brunel, the great engineer who constructed the tunnel, admitted that the idea had come to him one day when he was examining the burrow of this wonderful mollusc.

PLATE XIX

THE RAZOR (1 and 2)

If you walk about very quietly, when the tide is out, on the stretch of wet, sandy mud which lies just above low-water mark, you may often see a very curious object resting at the surface, and looking just like a little key-hole. And if you step heavily anywhere near it, it is almost sure to squirt up a little jet of water into the air and disappear. Then you may be quite sure that you have found the burrow of a Razor Shell.

This is a very long, narrow creature with bivalve shells, which are shaped almost exactly like the handle of a razor. It is generally about four or five inches in length and half-an-inch in width, and the object which looks so like a key-hole consists of its siphon tubes, the tips of which rest just above the surface of the sand when it is lying at the mouth of its burrow. It digs by means of its strong, fleshy “foot,” just as the cockle does, and its burrow, which goes straight downwards just like a well, is often as much as two feet deep. So it is not a very easy thing to get a razor out of its tunnel. But if you want to do so I can tell you how to manage it. Just take a good big pinch of salt, and drop it down into the hole. Now the razor does not like salt at all, even though most of its life is spent at the bottom of the salt-water, and it comes up to the mouth of its burrow in a great hurry to get rid of it. Then if you make a very quick stroke with a spade you can dig it out before it has time to get down to the bottom again. But if you should fail to get it up at the first attempt it is of no use to try again, for even if you pour down a whole handful of salt the animal will never come up a second time.

1. THE RAZOR.

2. TOP OF RAZOR FROM FRONT.

3. THE SABRE RAZOR.]

The razor is very good to eat, if its tough leathery skin is slipped off, and on some parts of the coast it is often used for food. The fishermen use it for bait, too, and catch it by means of a slender iron rod with a barbed tip, which they thrust into its body as it lies at the bottom of its burrow.

PLATE XIX

THE SABRE RAZOR (3)

There are several different kinds of Razors, and one of them is called the “sabre razor,” because its shells are curved, just like the scabbard of a sabre. It is fairly common, but you are never likely to find its burrows, unless you go to look for them just at low-water after a spring-tide, because it almost always lives below the ordinary low-water mark. But after spring-tides--which come twice in every month, once when the moon is new and once when it is full--the waves retreat much farther than they do at other times. Then, if you go right down to the water’s edge, you may often find creatures which you will never meet with higher up on the beach. And one of these is the sabre razor.

PLATE XX

THE PINNA

This is the largest of all the shell-bearing molluscs which live in our British seas, for it has been known to reach a length of nearly two feet. It is found chiefly on our southern coasts, and always lies upright, half buried in the mud at the bottom of the water, with its shells partly opened. And it always fastens itself down by a bunch of “byssus” threads, like those of the mussel, which are so strong that it takes a very hard pull indeed to tear them away from their hold.

In the British Museum you may see a pair of gloves which have been made out of the byssus threads of a pinna, and if these creatures were more plentiful their threads would no doubt be used in this way very largely indeed.

Now why do you think that the pinna always rests at the bottom of the water with its shells partly opened?

THE PINNA.]

Well, the reason is a very odd one. It is setting a trap for fishes! For fishes, as perhaps you know, are very inquisitive creatures. They always want to know all about everything, and whenever they see a hole they think that they must find out what is inside it. So when a little fish comes swimming past a pinna, and catches sight of its gaping shells, it is almost sure to venture in between them. Then the shells close tightly, and it finds itself in a prison from which there is no escape; and very soon it is killed and devoured.

In colour, the shells of the pinna are very pale brown, and a number of ridges run down it from the smaller end to the larger. When the animal is full-grown it is sometimes not at all easy to see its shells, for they are covered almost all over with barnacles and the tubes of sea-worms.