Part 6

Many of the sea cucumbers, or holothurians, are very sensitive, and when captured will often cast off their various organs. This does not indicate the death of the animal, as they are soon replaced. A singular trait of the long glasslike Synapta is that of cutting itself in two when starved. At first an end of the animal is dropped, then another piece, and this is continued until nothing but the mouth remains, everything having seemingly been sacrificed to save this portion. If food is now supplied, this animal will soon recover and assume its normal condition.

Synapta has no feet, their place seemingly being taken by peculiar limy spicules, shaped like anchors (Fig. 59), which are deeply buried in the skin. In its structure the sea cucumber resembles its cousins the starfishes and sea urchins, and standing on end, may be compared to an elongated sea urchin. Nothing can be more uninviting than these animals, and when dried the flesh has the consistency of leather. Yet the sea cucumber is highly regarded by the Chinese as a delicacy, and the Malays have a large fleet engaged in the business of gathering and preparing them. The animals are collected and dried, then smoked and packed in bales and sent to China. They may be found in any of the markets of these people, in San Francisco and New York. About the Pacific island of Santa Catalina they are very commonly seen through the bottom of the glass-bottomed boats, lying in the seaweed and imitating it in color.

X. THE WORMS

Few groups of animals differ so much in general appearance as the worms. Some resemble miniature snakes; others are flat, some are like needles, one lives in a cell; another stays in the tissue of some animal, while certain others infest the soil. Almost everywhere, on land and in the sea, under nearly all conditions, we shall find these remarkable creatures, which may be briefly described as animals having a head, tail, and upper and lower surfaces, and made up of a great many rings, or segments. In them we find an approach to the higher animals. Thus they have a heart, with red or green blood, breathing organs, though many breathe through the body walls, and a nervous system consisting of a minute brain in the upper portion of the small head.

All the worms deposit eggs, and nearly all are remarkable for the wonderful changes through which they pass before they attain maturity. This is well illustrated in a planarian worm (Fig. 60), which seems to require the presence of another animal to enable it to complete its development. The little creature which breaks from the egg (A) is a free-swimming creature surrounded by cilia or hairlike swimming organs. By these it moves through the water, and with strange instinct searches for some animal, generally a snail, which it enters. There it becomes surrounded by a sack and produces a little creature called the nurse (b), which soon grows to resemble the tadpolelike creature (C), which is filled with small egglike or germlike objects (a). It now changes into a wormlike creature (D), in which the germs have assumed the shape of worms (a), and soon breaks forth as a little form with a tadpolelike tail (E)--a remarkable performance. But the end is not yet; another animal is necessary to complete the change. Swimming about, the little creature is swallowed by some animal in drinking, and finds its way to the liver, where it lives, the tail being lost. The animal now changes into a perfect flukeworm (F), which finally leaves the animal or host and lays eggs in the water; these pass through the same wonderful transformation. The flukeworms (Fig. 61) are disagreeable flat creatures, not often seen, the marine forms attaining large size.

Many of the worms are parasites living upon other animals. The thorn-headed worm (Fig. 62) is an example. Who has not heard the story of the living horsehair? Almost every country newspaper has told the story, that some farmer after washing his horses had found several hairs taken from the horse's tail which "were alive," and to prove the story the farmer produces the "living horsehair" which is a remarkable imitation of the long hair of a horse's tail. But the hair is a well-known worm (Fig. 63) called Gordius aquaticus. It is almost exactly like a horse's hair, two or three feet in length, and found coiled up in ponds or snugly tucked away in the interior of a beetle or grasshopper which it has seized upon as a host. The deadly Trichina spiralis belongs to this group (Fig. 64). If the vinegar bottle is examined, in what is popularly called the "mother" at the bottom, still another member of the family will be found. This is a minute round worm almost invisible to the naked eye. It is very active and disagreeable to contemplate, living in the sour, fiery liquid.

In this group are many dangerous worms, as the guinea worm of remarkable length. While nearly all worms are disagreeable creatures, a few are very beautiful. Such are the rotifers or wheel animalcules (Fig. 65). These are the smallest and most active of the tribe of worms. To be found they must be sought in a drop of standing water, and as they are rarely ever over one thirty-sixth of an inch in length, a microscope is necessary. Among the throng of wonderful creatures one will be seen seemingly rolling over and over like a barrel, a minute whirling Dervish of the water. The rotifers assume a variety of shapes. One is a typical worm, another darts along by the aid of two circlets of cilia which vibrate so rapidly that the illusion of rolling is produced. No more wonderful creatures than these little worms are known, and they well repay the study required to know them well. Some of them are fixed and unable to swim, and many of the stories of spontaneous generation are due to the faculty these minute rotifers (often but three one hundredths of an inch in length) have of enduring almost any amount of drying. Thus if a pond is dried up by the sun, the rotifers seem to be able to lie dormant for a long time, and when a rain falls in the locality for the first time in years, the pool is at once peopled with rotifers which awaken from their long sleep. When it is known that Ehrenberg, the German naturalist, found that a certain species produced sixteen million young in less than two weeks, it is easy to understand how quickly a new pond might become rapidly equipped with a large population.

It is a singular fact that myriads of worms are seen daily, but are not known as such. These are the minute and beautiful Polyzoans (Fig. 66). They are marine animals, grow in colonies, and look like delicate seaweeds. They are often called moss animals. At the seashore we shall find the rocks and particularly the broad fronds of kelp near shore often encrusted with a delicate, beautiful tracery of pure white. In California I have found the kelp leaves at the surface covered with it, having the appearance of being coated with silver. Beneath the glass it develops into a beautiful tracery filled with cells. When magnified these cells are seen to resemble Figure 67, each one having its worm, which seems to blossom like a flower. These worms are minute imitators of corals, as they form a corallike structure, the worms having the faculty of secreting lime, as do the corals, yet they are much higher in the scale of life. One of the common seaweeds of the seashore is the sea mat or Flustra. No one would suspect it of being other than a beautiful marine plant with large leaves or branches, and many a collection of "seaweed," preserved through many years, contains the Flustra arranged among the real "plants" of the sea. But Flustra is merely a colony of worms. The minute spots seen upon it when enlarged beneath a microscope resemble so many cells of carbonate of lime secreted by the worms of the community or colony. Another species of Flustra is shown in Figure 68, and a part of the skeleton of the colony or of each cell is the peculiar bird's head which has a beak. This beak, even after the death of the worm, is seen to open and shut, snapping continually, much quicker than the little pincerlike objects we have seen in the sea urchins. The use of the so-called bird's head is not well understood.

This Flustra (Fig. 69) is very beautiful, forming a delicate little plantlike form about an inch and a half in size. But the crowning glory of these worms, as shown in the figure, is the circular crown of tentacles by which food is grasped as it passes by. Any one who has collected fossils in what is known as the Trenton limestone is familiar with the little fossil shell called Lingula, of which two thousand species are known. Curiously enough this little shell has come down to us to-day, and in Figure 70 we see the living Lingula of our waters with its long stem by which it fastens itself in the sand. Lingula resembles a small clam shell, has two perfect shells, and probably holds a place in many collections as a shell; yet Lingula is a worm which secretes a two-valved, unhinged shell, that is an almost perfect imitation of a bivalve mollusk. In the Santa Catalina Channel, California, from water six hundred feet in depth, I have dredged shells resembling those in Figure 71. They hung upon rocks in clusters, and were very striking in their rich colors of yellow, red, etc. In shape these Terebratulas, also common among the fossil shells, resemble ancient Roman lamps, and hence are called lamp shells. They too are worms, however, and many more shell makers called brachiopods. The "wick," a muscular stalk or byssus, becomes firmly attached to some object at the bottom. But in the instance of the little Lingula the stalk or anchor rope merely passes between the curious shells. If the latter are opened, we find a singular bridge or limy framework which is intended to support the soft parts of the bridge, a very conspicuous feature of which are what are called arms, long, ribbonlike, fringed processes (a) which are coiled up in the shell and serve as breathing organs and to obtain food as well. They can be extended some distance from the shells. The curious frame upon which they rest is well shown in Figure 72. It is on record that during the Sikh rebellion an entire English regiment was put to flight by a force of worms. The troops were marching through a forest when land leeches began to fall from every branch and leaf, dropping in such vast numbers that the men were almost crazed by the vicious bloodsuckers; hence they broke and ran for clear ground, where they could rid themselves of the terrible pests. Semper, the naturalist, states that he was driven from the forests of Luzon by these leeches, which fell upon him like dew. The ordinary leech of commerce (Fig. 73) belongs to this group. It has a sucking mouth, which bears three teeth. It was once much used by physicians for bleeding purposes, in fact, the animal derives its name from the fact that medical men in England were formerly called leeches. The leech had a high commercial value, over seven million being used in London in a single year, valued at ten dollars a thousand. Leech raising is a regular business in Russia, Bohemia, and Hungary.

The best known of all worms, perhaps, because all boys are fond of fishing, is the angleworm or earthworm (Fig. 74), which can be found where the earth is rich and moist. This worm is really a beautiful object, being highly iridescent, flashing a thousand hues in the sunlight to which it has a decided objection, as the heat soon dries it up. The ringed or segmented arrangement is easily observed as it moves along. By this marvelous arrangement a worm can either stretch itself out to an inordinate length, or telescope itself until it can hardly be recognized as a worm. There are several interesting features about earthworms which ordinarily escape the notice of even the angler. One is its feet, which differ from those of any other animal. They are very minute, and are bristles, each segment or ring being supplied with four. Another peculiarity of the earthworm is that instead of hunting out food in the earth it swallows the earth as it meets it, allowing the animal matter to be absorbed within. It then casts up the earth, which are the little mounds of mold found in the grass or turf every morning. This habit has made the worm a valuable aid to the farmer in preparing the soil, filling it with tunnels and constantly bringing new earth to the surface and turning it over. The amount of earth moved in this way was made a special study by Charles Darwin. In the year 1842 he spread a field with broken chalk, and after twenty-nine years examined it and found that the chalk in that time had been buried seven inches by the worms.

This gives us some idea of how important a factor these humble creatures are, working mainly at night, in burying the works of man. It is evident that in two or three centuries portions of buildings could be concealed. In England numbers of ancient Roman villas have been discovered, beautiful floors and foundations of ancient buildings which have been lost to sight by being covered by these night workers. To give an adequate idea of the work they accomplish, Darwin says that the amount of vegetable mold brought to the surface in a single year amounts to ten tons to a single acre. They rarely descend below six feet, and Darwin estimated that in favorable localities there are 100,000 in every acre. In New Zealand 348,480 have been found in a very rich acre. The worms eat the earth, and drag leaves and soft twigs into their holes at night. They plant seeds and bury stones. Some of the casts of giant worms of India are a foot in length. They live entirely beneath the ground, lining their burrows with very soft fine earth, which appears to be powdered for the purpose. All their operations are carried on at night, when they come to the surface and eject the casts. They have a habit of lying near the surface at the entrance of their burrows, a fact which the birds have discovered, robins and mocking birds particularly being very clever in hunting them out.

One of the most remarkable features of these worms is their phosphorescence, which I have found is more brilliant than that emitted by any other animal. Crossing an orange grove in southern California one dark and rainy night in January, I stumbled over a clod of earth, and if I had kicked a mass of live coals, the result could not have been more marked, as flashes of vivid light darted in every direction with the earth, caused by several earthworms which had exuded so much phosphorescent matter that it had pervaded the entire mass of surrounding soil. The phenomenon on a small scale can often be seen in southern California, especially in winter, when the ground is moist and wet.

Probably the most beautiful of all worms are those of the sea, the marine forms found everywhere from the mud banks to the long fronds of kelp washed by the foaming sea. Perhaps the most gorgeous creature taken from deep water is Aphrodite, several inches long, an inch across, and about the size of a mouse. The worms are provided with an array of iridescent bristles, so beautiful as to appear artificial, blazing with golden lights. Some of these worms are covered with strange and brilliantly colored streamers, as Cirratulus (Fig. 75). Others are long and slender, as Nereis (Fig. 76), a very common form alongshore. It is sought after by fishes with good appetites, and often caught, despite the fact that it has four eyes, four hundred paddles, and fierce jaws for seizing prey. Nereis lives in the sand in a tunnel. It has a habit of coming out at night and swimming abroad, when, creating a blaze of light, it becomes a very conspicuous object and is quickly caught by some wandering fish. These worms are among the most brilliant of all light givers; not alone for the intensity of light, but for its variety in tint and color. The most remarkable light givers are Polynoë, Syllis, Chætopterus, and Polycirrus. The first-mentioned emits a green light at the attachment of each scale. In the second the feet are light givers and emit a blue light. In the third the light blazes on the back at the tenth joint alone. The last is a worm of fire, the strange, little understood light blazing over its entire surface, a vivid blue.

I was once sitting on the shore of Avalon Bay in southern California when, in the darkest corner in the shadow of a high cliff, I saw, two hundred feet away, what appeared like candle lights floating upon the surface. Rowing a boat to the lights, I found that each one came from a spot of phosphorescence floating on the surface. When it moved, as it often did, phosphorescence streamed away in its wake. When taken in my hand the latter became bathed with the light which ran from the invisible animal. I succeeded in capturing one entire light, but could not make out the animal. Soon I noticed lights upon the bottom in water five feet deep. They appeared to be as large as saucers, but grew rapidly in size until they were as large as dinner plates, then the yellow light gradually diminished until it was not larger than a hazelnut, and came wriggling upward in a zigzag of fire, finally reaching the surface and resting, as one of the peculiar lights I had seen so far away. I captured several, and in the morning found that my light giver was a minute sea worm not half an inch in length. When discovered, the little animal was leaving its burrow or cave in the sand for a nightly swim at the surface.

Many of the most beautiful of the marine worms are cell builders (Fig. 77). In some the worms secrete a tube of carbonate of lime. In others the den is made of bits of sand. I found on the Florida Reef many remarkable examples of the latter. The nest or tube was built among the seaweed, several inches above the bottom, and would naturally be a conspicuous object; but here the intelligence of the little creature is seen, for it covers the outside of the column with the plates of a lime-secreting seaweed, which look like shingles, and mounts upon the upper portion of the column a green bit of seaweed. This is glued to the tube and so arranged that it falls over the entrance and closes it, thus serving the purpose of a door and making the tube mimic a bit of sea grass. The worm lifts the grass door when it comes out.

One of the most familiar forms is Serpula (Fig. 78), whose tubes wind in and out in every direction. No garden of pansies gives a greater variety of tints than did a mass of these radiant creatures that I found on a floating spar in the Pacific at Avalon Bay. But touch these "flowers" or jar them and they disappear like magic, leaving a hole closed by a little door, which is formed by a part of the worm that thus defies all intruders.

XI. THE TWO-VALVED SHELLS

The beautiful objects which we know as shells, and which form ornaments in many a home far distant from the sea, are the coverings of a group of animals called mollusks. They are found in all seas, many upon land, and in fresh-water streams, and are among the most attractive of all natural objects, so much so that many persons devote their entire lives to their collection, and many others much time to the study of their habits. It is rare to find a new shell, so well have these shell hunters searched the waters of the world. Such collections, especially if complete, are very valuable, and many of the great museums have paid thousands of dollars for them.

The mollusks or shells present a striking contrast to the worms. They have no joints, are soft, seemingly without form, and are very helpless creatures. The body is enveloped in a muscular coat or mantle, as shown in the oyster (Fig. 79). They have a nervous system, and a heart (H) which pumps colorless blood. Some have a foot for locomotion and eyes more or less well developed. The oyster represents a large group which have two shells, called bivalves. In Figure 80 we see the animal portion of the common snail, which illustrates another group with but one shell. These are called univalves. To the bivalves belong the shells best known, the oysters, clams, scallops, pectens, pearl oyster, razor shell, and many others, of which the oyster is the most familiar. The mantle, the soft, delicate lining, is the shell maker, and not only forms it, but repairs damages to it, piling up layer after layer of pearly matter called nacre. As there is a mantle on each side, two values are secreted. The sharp portion of the oyster is called the beak. Here the growth of the shell begins, and here are the marvelous valves which fit with such accuracy. These complicated parts are easily seen in the clam (Fig. 81). The hinge is joined by teeth (c, d, d,) which fit into cavities on the opposite valve, while the valves are held together by a perfect hinge, a horny ligament (h) that tends to open the shell or throw the valves apart.

In the interior of all shells are seen certain scars; in others a purple mark. These marks (e, e,) indicate the location of a strong muscle by which the clam or oyster closes its shells and keeps them closed with such rigidity. In opening oysters the man severs this muscle and the shell opens, forced apart by its ligament. This explains why most shells found on the beach are wide open. The curious columnar objects in rows are the gills or breathing organs of the oyster, and are covered with little oars (Fig. 82), or cilia, which move to and fro, continually sweeping the currents of water along, bearing oxygen and food. The former is taken up by the gills to purify the blood, and the latter is swept into the mouth located near the lungs.

There is great variety in the hearts of shells. In the oyster (Fig. 79) it is composed of one auricle and one ventricle; but in other shells the heart may be three-chambered, or there may be two distinct hearts, each having two chambers. The eyes of the shells are very minute and are situated along the mantle. Those of the pecten are very beautiful and are distinctly visible, resembling gems or emeralds.

The clams (Fig. 83) differ from the oysters in having a pronounced foot (f) which protrudes from the large end of the shell; and with it the animal digs its burrow. It also hears indirectly by its foot, as its ears are in this organ, little transparent sacs containing a clear fluid in which floats a glassy globule. The clam also has a siphon (s), which in the common clam is very long. It has a black head or tip and the clam may rest some distance down in its hole and take in water through its siphon, which is double-barreled. One opening (in.) receives water containing food and oxygen; the other (ex.) expels the water. In strolling along the sands at low tide one often sees a spurt of water shoot out of a hole, and may assume that a clam has been alarmed and has retracted its siphon so suddenly that it has shot a stream of water above the surface. The shells increase by eggs, the oyster depositing a vast number, which at first are curious little free-swimming objects (Fig. 84) paddling by the aid of cilia or whips, but soon attaching themselves to the bottom and taking the oyster form.

The oyster is perhaps the most valuable bivalve to man, being a favorite article of food, for which $1,500,000 is paid annually in New York alone. Thousands of men find employment collecting them in various parts of the world. In this country the most valuable oyster beds are in the vicinity of New York, at the mouth of the Shrewsbury River, in the Chesapeake Bay, and at various points alongshore to Florida, where there are large banks at the mouths of the rivers. In watching the excavation of a cellar at the town of Mayport at the mouth of the St. John River I saw oyster shells thrown up as deep as the men went. The town is built on an ancient oyster bed. Among the old shells numerous pieces of pottery have been found, showing that the early natives frequented the spot. The living oyster bed here to-day is some distance out in the stream. When sailing up a small river in Maine some years ago, I found, about ten miles from its mouth, a mound of oyster shells thirty or forty feet high. The river appeared to have cut the bed in two, and out of the top of the mound, which was of solid shells, grew a tree which must have been a century old. I believe there are no oysters on the Maine coast to-day, and the great pile was accumulated ages ago when Maine had oyster beds and the Indians carried the oysters ten miles up the river to this spot which must have been the site of an ancient Indian town or city. The pearl oyster is another valuable shell (Fig. 85). It is common in warmer waters. Near La Paz in the Gulf of California is a famous fishery, which is owned by the government and farmed out. In Ceylon it is estimated that 17,000,000 oysters are destroyed to obtain $80,000 worth of pearls. The shells are also very valuable, being made into buttons and various other objects. Liverpool is the great receiving port for these, and many tons are used annually. In diving for pearls the Ceylonese, who are able to remain beneath the water several minutes, place as many shells as possible in a basket and then ascend, leaving the crew to haul the basket up. In Lower California many divers of to-day go down in armor.

Pearls are generally valued according to their symmetry and color. Some are perfect, and when of large size bring vast sums. One of the shahs of Persia owned a necklace in which the pearls were perfect and as large as hazelnuts. The pearl is the result of the oyster's attempts to protect itself from injury. If we should take one of these beautiful pearl oysters and with a gimlet bore a hole through the shell from the outside and replace it in the water, we should find, months after, if the oyster was examined, that it had, by using its mantle, secreted a large amount of pearly nacre over the wound, not only filling up the hole, but heaping the pearly secretion over it until a projection a quarter of an inch high was the result, resembling a pearl attached to the shell. This is the way imperfect pearls are formed; they are the attempts on the part of the oyster to prevent injury to itself. Occasionally some foreign body, like a grain of sand, will enter the shell. Its sharp edges will cut the soft flesh of the delicate creature, which immediately covers it with pearly nacre. The larger it grows the more the oyster notices it among its folds, and the more it instinctively covers it with pearl. In this way the pearls grow.

The seed pearls are those in which some impurity has been covered but a few times, while the very large pearls are those which have been bathed in nacre time and again. If a large pearl is cut in halves, the various layers can be counted, the sections recalling the interior of an onion. The skillful native fakirs of the East take advantage of this industry of the pearl oyster to introduce metal beads and figures of the Buddha into shells, which are then marked. The objects finally become covered, when they are removed from the shells and sold to the unsuspecting natives as "miracles."

One of the interesting shells of the seashore is the Pinna. I have found the shores of the outside islands of Texas scattered with them. They are also called fan shells, and are attached to the bottom by a peculiar cable, or byssus, formed of a silklike substance which has been woven. Gloves and hose of pinna silk may be seen in the British Museum.

The pectens are common forms famous for the beautiful gemlike eyes seen along the edge of their mantles. I once kept a number of these shells in an aquarium, and they were a source of much amusement, from their habit of dancing (Fig. 86). Generally they lay in the sand in the bottom of the tank with their valves open an inch or more, their bright eyes gleaming. Without any warning, one would open and close its valves with great rapidity, which would cause the shell to take convulsive and bounding hops. Then another shell would follow, and soon all the pectens were leaping up and down in a most extraordinary dance. The pecten changes its position or travels, not by pushing itself along, but by a sudden and spasmodic hop, clearing a foot or more.

The locomotion of shells itself is a fascinating subject. An interesting instance is observed in the common mussel. This shell has a remarkable foot, a pointed, fleshy organ which can be protruded. With this organ the mussel bores holes in the sand, jerks itself along, or clears the surface with a bound. But its most remarkable service is in aiding the mussel to climb. In the foot, near its base, is a gland which secretes a peculiar substance, which when exposed to the water hardens and resembles silk. The resemblance is so perfect that the "silk" has been woven into various articles, and an attempt was made in France to raise mussels for this purpose. When the animal desires to climb, it reaches out its foot as high as it can (Fig. 87), and presses it upon the pile or rock, whereupon a delicate cord, one of the cables of its byssus, is seen. Again the foot is extended, again a cable is attached, the entire operation calling to mind the action of a spider. Each step raises the mussel a little higher, and as it moves on, the cables that would hold it back are broken off, and the mussel at length reaches the position it desires.