Popular Science Monthly/Volume 50/November 1896/Shells
By MARGARET WENTWORTH LEIGHTON.
See what a lovely shell,
Small and pure as a pearl,
AS we watch the little pools of water left among the rocks by the retreating tide the pearly luster or the violet or golden tint of some tiny shell catches our eye. How exquisite its form and coloring!
Shells have always, from the most ancient times, been greatly prized. Prehistoric men discovered in the burial caves of Auvergne have chaplets of shells which scientific men tell us they must have traveled long distances to gather. It is only of late years that their curious little occupants have been interviewed and some ideas obtained with regard to their characteristics and mode of living.
All shells with their inhabitants belong to the immense class known as Mollusca, or soft-footed animals. Shells are divided into two groups—univalve, those having but one valve, as the snail, whelks, cowries, etc.; and bivalve, as the oyster, clam, and mussel.
If we take the clam as a typical mollusk we shall see that each little line on the inside or outside of the shell reveals an interesting fact. On the outer surface of each valve are a number of concentric lines parallel to the edge and growing fainter toward the hinge part. These are called the lines of growth, and are made by the mantle. The clam's mantle is quite as useful to him as are our hands to us, and he uses it for similar purposes. The mantle surrounds the clam's body inside the shell, its edge protruding and looking like a row of little frills. This edge secretes carbonate of lime from the water and adds it to the shell all along the outer edge, forming a new line of growth. Thus, as the clam increases in size his house grows proportionately, so that it always exactly fits him. The two halves of the shell are joined by a curious hinge. In some kinds of shells the hinge is external and in some internal. It consists of teeth (two or more) with spaces between on either half. These lock together, and are held by a strong, elastic muscle. On the inside of the clam shell are two slight depressions, where the powerful adductor muscles of the body were fastened.
The clam's body is completely enshrouded in the mantle, except for two openings, through one of which the foot can be
Clam Shell. Exterior showing lines of growth; interior showing muscle scars.
pushed out. The other is for the siphon, or what is commonly known as the "neck" of the clam. In some respects the clam may be better off than we are, for he has a little brain in his foot and also a gland for secreting strong fibers. With this he spins a byssus by which he can attach himself to whatever he likes. He does not even have to search for his food, but waits for it to come to him. He makes a burrow in the mud or sand, attaching himself to the bottom by the byssus. Then he thrusts his siphon up through the mud and water until it reaches the surface. The siphon is made up of two tubes, the water flowing in through one and out through the other.
When the inflowing current, laden with minute plants and animals, reaches the gill chamber, some of these are sifted out and retained for food, while the water and waste matter flow out through the other tube.
The clam's eggs are carried by the mother on her gills. When there are fish in the water with them the mother clams discharge the eggs, which soon hatch, but if there are no fish they carry the eggs until they decay. The reason of this strange behavior is this: When the eggs are set free in the water they soon hatch, and the little ones swim about until they find some fish to which to attach themselves. They live for a time on the mucus of the fish and then drop off, sink to the bottom, and form burrows for themselves. This curious semiparasitic life is no doubt a reversion to the habit of some ancient ancestor.
The white-shelled clams live in sand, the black-shelled in mud. Besides living on the seacoast, clams inhabit all United States fresh waters, and in some New York and Western rivers clams have been found which contained pearls of great beauty and considerable value. I have never seen anything more exquisite than the pink pearl lining of some river clam shells.
The razor shell, familiar to all on account of its universal distribution, belongs to the clam family. It has a powerful foot, with which it can scoop out a passage through the sand faster than a man can dig with his spade. One of the clams inhabiting warm inlets south of Boston is the quahog. The shells have a finely beaded edge and are partly lined with deep violet. It was from this that the New England Indians made their purple "wampum" (money), which was considered twice as valuable as the white "wampum." The old-time spelling of clam, clamp, was characteristic of one of its chief features, the two halves being so tightly clamped together.
The oyster, a close relation of the clam, is perhaps the most useful and valuable member of all the molluscan group to mankind. The left half of the shell is generally attached to some submerged object and is quite hollow, for it is in this half that the body lies, the upper or right half being almost flat. The oyster readily adapts its shell to surrounding objects, growing about them in most fantastic ways.
When a grain of sand or any minute particle gets in between the oyster's mantle and the shell it is very irritating, and causes a great excretion of matter to take place. This collects around the nucleus in concentric coats like those of an onion. If the lining of the shell be mother-of-pearl, these coats of matter which cover the little grain of sand will also be pearly, and perhaps form a gem of priceless value. Sometimes one of the oyster's own eggs lodges between mantle and shell and is transformed into a wonderful tear of rainbow hues. It is only those shells having a pearly or nacreous lining which can form these gems. For hundreds of years pearl fishing has been a lucrative industry. The most renowned fisheries are at Panama, Ceylon, and in the Red Sea. The pearl oysters are very large, live in from six to twelve fathoms of water, and are gathered by diving.
Pliny calls the scallops (Pectinidœ) butterflies of the sea. They are very shy and live in the midst of the eel grass, where the water is warm. The shells range in color from pure white, through all shades of yellow, to bright orange, and some are exquisitely banded and shaded with light and dark brown. The edge of the mantle is fringed with long and short tentacles, among which are thirty silver-blue eyes. As they are not as highly organized as our eyes, Pecten needs a much larger number of them. The scallops, unlike most of the mollusks, can swim through the water
Razor Shell (Ensis americana).
by rapidly opening and shutting the valves. Closing them suddenly drives out the water in a powerful jet, which by reaction sends the shell forward. It must be a strange and beautiful sight to see a flock of these "butterflies" flying through the blue water on a fair summer day. Scallops used to be known in Europe as pilgrim shells, because they were used by the pilgrims of the middle ages as a badge.
A most remarkable family of shellfish are the piddocks, living in England, America, and Borneo. They are all borers. The shells of those which inhabit the English chalk cliffs are snow-white, to match their home. Some bore in rock, some in the red chalk, and the most wonderful of all, the East Indian species, lives in the trunks of dead trees. Their shells are covered with deep grooves crossing each other and forming a sort of rasp. The foot, which is covered with a hard dermal armor, is pressed against the sides and the shell turned about, thus easily scooping out a cavity in the soft chalk. The piddock continually floods his burrow with water to wash out the particles of chalk that collect as he works. The piddock has a little light of its own, so that it could travel safely about after dark were it necessary. This is a peculiarity of many of the creatures of the sea, and often on a summer night in the tropics the water is ablaze with their phosphorescence.
Mussels, living in both salt and fresh water, form a large class of mollusks. Some of them can climb about on the rocks by throwing out a byssus thread, pulling themselves up, then fastening another above that, and so on.
The horse mussel is one of the largest, and very interesting on account of a boarder which it often entertains. A tiny crustacean, the pea crab, lives inside its shell in peace and happiness. The crab is not a parasite, as it does not live on the mussel itself, but merely a messmate eating the refuse of its food.
The Noah's ark is a most oddly shaped shell, and was named by Linnæus from its resemblance to that primitive craft. One ark found in the Mediterranean sometimes contains violet-colored pearls, and one on our coasts is called the "bloody clam," from its fiery gills and the crimson fluid in its tissues. Some of the arks live in submerged clefts of the rocks, and are so busy eating and growing that before they know it they have grown too large to get out, and must remain prisoners for the rest of their lives.
The teredo, or ship worm, would hardly seem to belong with this group of animals, but it is a true bivalve, having a pair of tiny shells at one end of its wormlike body. It has been a most terrible pest ever since men began to traverse the ocean, for its favorite home is the bottom of a wooden ship. It belongs to a family of borers. Some bore in coral, some in rock, and others in wood. The baby teredo, when floating about in the water, comes across a vessel or piece of wood, and immediately begins to bore into it with the edges of a pair of pallets which it has for the purpose. As it proceeds, a calcareous lining is formed to the burrow, which increases in size as the teredo grows. It never leaves its hole again during life.
One very curious fact connected with the teredo is that the burrow of one never runs into nor crosses the burrow of another, even though the wood between is no thicker than a sheet of paper. These little fellows work very rapidly, as the following item from Quatrefages will show: A ship was sunk near St. Sebastian, Spain, and in four months, when it was raised, all theSunset Shell.timbers and planks were so riddled with teredo burrows that they were entirely worthless.
The most brilliant and withal attractive shells in my collection are from the West Indies. I call them sunset shells, because they look as the sky often does on a beautiful summer evening. They are somewhat like clam shells in shape, but narrower and flatter, and most delicately finished. Some are flushed with delicate pink, with rays of pale yellow, others are violet and white, still others green. All the colors of the rainbow are here blended and harmonized with the matchless perfection with which the Great Artist works.
The univalves are more highly developed than the bivalves. They are called Gasteropods, which means stomach-footed, because they have a long foot lying the whole length of the body. Unlike the bivalves, they have a distinct head, in which the brain is situated. Often there are tentacles or feelers, as in the snail, on the ends of which the eyes are placed. Gasteropods have a wonderful eating apparatus called the odontophore or tooth ribbon. It is covered with hooked teeth, pointing backward, and is in the lower side of the mouth, situated about the same as our tongues. On the upper side of the mouth is a hard plate or jaw, and the food is ground up by the toothed ribbon against this plate. The
Violet Snail and Egg Float (Ianthina fragilis). Copied from the Riverside Natural History by kind permission of Houghton, Mifflin & Co.
odontophore wears out rapidly, but as the front part is used up it grows from behind, and these animals are so fortunate as to have a new set of teeth every little while.
There is only one shell to take the place of two in the bivalves, so most of the univalves have an operculum. This is a little lid (either horny or calcareous) on the upper side of the foot which exactly fits the aperture in the shell. If a Gasteropod wishes for any reason to be alone and rest for a time, he only has to draw in his foot, pull to the door, and he is in complete seclusion from all the world.
The shells of the Gasteropods, like those of the bivalves, are often covered with a sort of horny membrane or epidermis which protects them from the eroding power of the water and other external injuries.
At the bottom of the Gasteropod group is a wonderful creature which we may call a multivalve, as its shell is made up of a number of plates (usually eight) which look like ancient armor. It is called the mail shell, or chiton, and is the only example in the world of a shell composed of more than two parts. It is common on the Atlantic coast, in some of the bays and inlets south of Boston, on the Pacific shores, in England, and other places. Chitons sometimes have as many as eight thousand eyes, their backs being covered with them.
The limpets range in shape from those which are almost flat to a perfect cone. Some of my prettiest from Sitka are snow-white, and look like little peaked nightcaps. One is the cup-and-saucer limpet, and indeed it might easily serve as such, on the table of some water sprite. It is glistening brown in color and looks like porcelain. The slipper limpets or boat shells are very-pretty, being shaped like little rowboats with one seat. The shallow-water boats are flat-bottomed and thin, while the deepwater ones are much stouter and round-bottomed. Limpets each have a particular spot on the rock to which they attach themselves, and when they wander off between the tides for their dinners of seaweed they always return to the same spot. If you should try to pull a limpet off of his stone you would find it very hard work, for his strong foot sucks the rock with great force, and as soon as he felt you pulling or prying he would redouble his energies to cling to his home and would probably succeed.
A king among shells is the Haliotis, or, as the Spaniards call it, abalone. It is found in all collections, and is extensively used for its pearly lining in the manufacture of buttons, buckles, and other ornaments. It is sometimes called the ear shell, on account
Natica heros (1) and egg mass (2), known as "sand-saucer."
of its resemblance to the outline of the human ear. In life the animal thrusts his tentacles out through the row of holes along the edge. On the outside the shell is rough, often closely resembling the rocks on which it lives. The animals are eaten in Europe and by the Chinese in California. While I was living in San Francisco a Chinaman went out on to the rocks at low tide to gather some. As he attempted to wrench one from its home his hand was caught between shell and rock, and so firmly-held by the animal that he could not escape the rising tide, and was drowned.
The pearly lining of the abalone is richly shaded with all colors of the rainbow, an opalescent green often predominating. TheHarp Shell (Harpa ventricosa).mother-of-pearl is composed of undulating layers. The iridescence is caused by minute lines reflecting different spectra.
Some members of the snail family, with their world-wide reputation for slowness, have made amazing progress in the ascending scale. They have gone so far as tofrom the gills, with which they breathed in the water, lungs suitable for air-breathing, and have come to enjoy the pleasures which a life on terra firma affords. You can find them in the woods or in your garden, thrusting out their inquisitive little heads and investigating everything with their eye-tipped feelers. Some snails, after trying the experiment of a land life, have decided that on the whole a water life is preferable, and gone back to live there, where they have developed gills again, but of a different kind from the original ones.
There are sea snails, pond and river snails, as well as land snails. Many of them are carnivorous and can bore into other shells with their lingual ribbons. The hole usually strikes a muscle, when the shell gapes open, and his snailship enters and devours his prey.
Some kinds of snails, especially the land group, can live for a great length of time without food. A snail was fastened to a card and put in the British Museum in 1846. Four years afterward a discoloration appeared on the card, showing that he had been moving about. He was taken out, immersed in warm water, and was soon quite lively.
In creeping about, the snails always leave a track of mucus, which glistens when it is dry. It is in this mucus that they immure themselves for their long winter's nap, sometimes making several layers or partitions over the opening to the shell.
In the middle ages snail shells were worn as amulets, protecting the wearer against certain diseases as well as witchcraft.
Prof. J. S. Kingsley says of one North American species (Helix harpa): "In motion it is exceedingly graceful, at times poising its beautiful shell above its body and twirling it around, . . . again hugging its pretty harp close to its body."
The shells of the common wood snails are quite transparent and pale brown, but some of the land snails have splendid houses. One in my cabinet is in broad bands of white and brown, lined with rose color. The violet sea snail is one of the most fascinating of all the group. Great herds of these bright, purple creatures are sometimes seen on the surface of the ocean feeding upon Medusæ (jellyfish). Each carries an enormous float, from the under side of which the eggs hang down. The float is formed
by a secretion from the foot and is made up of a great many little bubbles. When storms occur the floats often become separated from the creatures to which they were attached, but the eggs develop just as well. The violet snail is never found on shore, except when cast up by violent storms, being a lover of the high seas.
The largest littoral (shore-inhabiting) univalve on the Massachusetts coast is a common globular snail (Natica heros). It lives on clams and other bivalves, and is interesting on account of its curious egg masses, known to the children who gather them on the beaches as "sand-saucers."
The olive shells are so called from their resemblance to that fruit. They are all pretty, being curiously marked with different shades of brown, but the most striking of the family is the East Indian harp shell, which is very beautiful, with its longitudinal ribs, representing the strings of the harp. The animal which lives in it is exceedingly shy, and if it is captured it draws itself into the shell as far as possible. The whole of the foot will not go in, however, and this is quickly drawn across the sharp edge of the aperture and cut off. If the animal is set free again in its native element it will redevelop its foot.
The miter shells are varied and brilliant in coloring. Some of mine are white with orange spots, others brown and purple, and all showy. They are named from their resemblance to the bishop's miter, and are found in the Philippine Islands. If an enemy approaches, their occupants throw out a purple fluid and escape under cover of the stained water.
Notice in the miter shell how the color spots correspond to the whorls, in the scallops how the shades and bands follow the lines of growth. Have you ever thought why this is so? The mantle which builds the shell has spots or bands of color scattered through it, and as it works, the impressions of these same pigment spots are left on the shell. If the pigment cells of the mantle be yellow, red, or violet, these colors will be left on the shell and preserved forever.
Another marvelous accomplishment of the mantle is the ornamentation of shells with prongs, flutings, etc. We have a good example of this in the lovely Murex shells. The mantle sometimes works without cessation until the shell is finished, then turns up, forming the lip (the edge of the aperture). Often it works for a time, turns up, forming a frill or row of points, and rests. Then it begins its labors anew, building an addition and ornamenting it in like manner. Sometimes the mantle turns up at the end in a number of fingerlike radiations, as in the Pterocera lambis and curious pelican's foot.
The cowries form an immense group, some species of which inhabit almost every shore. They are called porcelain shells, on
account of their glossy, smooth texture. The little white cowries are used in some parts of Africa for money and to make girdles for the high chiefs. A stripe of a different shade from the body of the shell runs along the back, showing where the edges of the mantle met. In life it entirely covers the outside of the shell.
The helmet shells of warm seas are used for making exquisite cameos, the best being cut at Rome. The raised figure is chiseled out of the white layer and rests on a colored groundwork, blue, pale salmon, etc.
A very strange little fellow is Rhizochilus antipatharum. In his youth he has a well-formed shell, but as he grows older he cements about it bits of coral, other shells, and anything which he finds convenient, until the opening is entirely closed, and he can communicate with the outer world only by means of his siphon.
One of the largest shells found on the coasts of the northern and middle Atlantic States is Scycotypus canaliculatus. It is protected from injury by its coat of rough brown fur. The inhabitant comes ashore to lay her eggs, which are a great curiosity. There are hundreds of little leaf-like sacs which contain the eggs, all joined together, forming a long chain.
Æons ago the shells had very different forms from those of to-day, but we have left a few members of the group which existed in countless millions. The nautilus of the present time is not a very distant relative of the ammonites, which we find so marvelously preserved in the Silurian deposit, every line and penciling absolutely perfect.
Note.—I am greatly indebted to Prof. J. S. Kingsley, who was my teacher of biology at Tufts College, for his assistance to me when I was studying the shells, and for material in this sketch taken from his article on Mollusca in the Standard Natural History.