Popular Science Monthly/Volume 13/July 1878/Sea-Side Studies

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AMONG the thousands who visit the sea-shore in summer, there are many who, although not naturalists, are more or less interested in the various marine forms which there abound, and perhaps a brief notice of some of these forms will be acceptable to those who have not made a special study of life as it is revealed in the sea.

If an observer be on a rocky shore he will not fail to become interested in the "sea-anemones," those "flowers of the sea" which at first view appear to be on the border-land between plants and animals, so that one hardly knows whether to refer them to the vegetable or to the animal kingdom.

Fig. 1.—Actinia, or Sea-Anemone (Metridium marginatum, Milne-Edwards): c, closed; o, opening; e, expanded.

Although some sea-anemones live in the sand (Fig. 2), the home of the ordinary kinds is the pools and caverns among the rocks; here we may find and study them when the tide is out. Groups, sometimes in thousands, and standing so closely together that they cover the whole interior of the rocky cavern or grotto, are not uncommon. Some are expanded to their fullest extent, like a full-blown flower; others are only partly open; others are just opening; and others still are closed as tightly as the bud of a flower, which they more or less resemble.

Various are the colors which they exhibit, from pale or nearly white to the richest hues of pink, rose, red, and purple.

In the centre of the top there is an opening or mouth, which leads directly into a central sac or stomach, and around the mouth are rows of long and delicate hollow appendages, which the animal moves freely to and fro in the water, and with which it can bring the food to its mouth. As soon as a snail or other small animal falls among the fringes, these close around it and move it toward the mouth, where it is soon swallowed, the soft parts digested and the hard parts excluded. By means of its broad base or "foot," the sea-anemone attaches itself

PSM V13 D327 Sea anemones living in the sand.jpg

Fig. 2.—Actinia or Sea-Anemones which live in the Sand and are often unattached. 1. Peachia hastata, Gosse.—2. Edwardsia callimorphia, Gosse.—Halocampa chrysanthellum, Gosse—the last mostly buried in the sand.

firmly to the rock or shell on which it rests, and seldom moves from the spot which it has chosen, although it can effect locomotion by means of its "foot." By this it clings so firmly to the rock that it sometimes suffers itself to be torn in two rather than let go its hold.

PSM V13 D327 Cross section of a polyp.jpg

Fig. 3.—Cross-Section of a Polyp, or Sea-Anemone, showing the Septa.

The main cavity of the body in the sea-anemones is divided by septa or partitions, which run from the top to the bottom, and from the outer wall to the stomach. These partitions or septa are in pairs, and the number is some multiple of six. By what principle of selection this constant number was introduced we may be curious enough to inquire, but we must not expect to receive at once a perfectly satisfactory answer.

The partitions above mentioned are the infolding: of the body-wall of the animal, and are essentially the same as the wall itself. Toward the top of each partition there is a hole, permitting free passage for the water to flow from one chamber to another.

Fig. 4.—Cnidæ or Lasso-cells, some with the Lasso coiled within, and others with the Lasso extended; only a Part of the length is shown. 1. Lasso-cells of an Actinia, with the lasso coiled within—its actual length 1350 of an inch. 2. The same as 1, with the lasso extended. 3 and 4 are alike—the former with the lasso coiled within, and the latter with the lasso extended. 6, 7, 8, 9, 10, a lasso-cell in different stages of development.

On various parts of the sea-anemones, and of all other polyps, especially on their tentacles or fringes, there are very remarkable objects called cnidæ, or lasso-cells, like those on jellyfishes, each cell being less than one two-hundredths of an inch in length. In each there is a long, slender, coiled, and wonderfully-constructed thread, which can be instantly darted forth, paralyzing any little animal which it strikes; and thus the hungry polyp secures its food.

On the vertical partitions above mentioned the eggs are borne. These pass out into the water through the mouth. The newly-hatched anemone is oval in form, and swims freely about in the water by means of exceedingly delicate fringes called vibratile cilia. After a time it quits this roving life, attaches itself to the surface of the rocks, and grows into the form and size of the parent.

Sea-anemones have no proper nervous system. The sense of touch is distributed throughout the whole animal. It will, therefore, appear as a remarkable statement that some kinds have quite definitely-formed eyes. These are seen in some tropical species just outside of the tentacles, and according to Dana each of these eyes has a crystalline lens and an optic nerve!

Sea-anemones readily reproduce lost parts. If one is quickly torn from a rock, parts of the foot remain attached to the rock, and in many cases each portion thus left will become a perfect sea-anemone!

Sea-anemones vary greatly in size. Some species are only a fraction of an inch in diameter. Our most common kinds are from one to two inches, and expand from two to four inches. Some of the tropical kinds are a foot in diameter.

Fig. 5.—Cluster of Coral-Polyps (Asteroides calycularis, Milne-Edwards)—in various stages of expansion. Fig. 6.—Dead Coral (Asteroides calycularis, Milne-Edwards.)—The coral of Fig. 5.

Fig. 7.—Madrepore Coral (Madrepora aspera, Dana). Right-hand branches alive; the left, dead.

The sea-anemones, whether in their natural home or in the aquarium, are exceedingly interesting objects; and in the latter place we can study them to the best advantage. Carefully remove a dozen of them

Fig. 8.—Dana's Astrangia (Astrangia Danæ, Agassiz): c, a growing cluster; a, a single polyp enlarged; b, the dead coral.

from the rocks; lay them in your basket, with plenty of wet sea-weeds; break from the rock some fragments with the sea-weeds still growing upon them; on your return to your studio put the whole into your aquarium, well supplied with pure sea-water. At first, the sea-anemones will appear like so many mere lumps of soft flesh, without definite form. But leave them there for a night; and, when you look again, you will find each one has established itself, and has expanded into a thing of beauty.

Those polyps which form the beautiful clusters of coral that adorn our mantels and museums, and which build up the vast coral-reefs and islands, differ in only one important respect from the sea-anemones.

Fig. 9.Sarsia (Coryne) mirabilis (Agassiz). Cluster of Hydroids growing on sea-weeds. Fig. 11.Sarsia (Coryne) mirabilis (Agassiz). Adult, Massachusetts Bay.
Fig. 10.—Single individual of Fig. 9 enlarged, showing a b just ready to become free jelly-fishes or Medusæ; Fig. 11, c, young bud.

The sea-anemones are wholly soft; they secrete no skeleton, or only the merest particles of hard matter. On the other hand, the coral-producing polyps secrete a stony skeleton. The old notion that coral is something built by an insect is entirely erroneous. The coral-producing animal is in no sense an insect, nor does it toil to build the clusters and reefs of coral which it forms. The coral-polyp lives and eats, and the getting of its food is the only labor of its life.

Coral assumes a variety of shapes, imitating-almost all forms of vegetation on the land.

Most of the coral-producing polyps are confined to the tropical parts of the ocean. A few kinds live in temperate waters. Dana's Astrangia lives and flourishes in Long Island Sound, where it occurs in little clusters upon stones and shells.

The visitor to the sea-shore will hardly fail to find among the growing sea-weeds little plant-like clusters like the one represented by Fig. 9. This is a cluster of hydroids, and its life-history is very interesting. The beginner may well be pardoned if he mistake these little clusters for plants, but they are indeed animals. From each little branch there arise buds (Fig. 10), which enlarge, till at length they become detached, float away, and grow into the beautiful jelly-fishes known as Sarsia or Coryne; it has been called by both names, the latter name meaning a club, and the former coming from Sars, the distinguished Norwegian naturalist, who was one of their earliest investigators.

Other hydroids, called Campanularians, will be found among sea-weeds. Here the minute jelly-fishes are formed in little bell-shaped organs (Fig. 12). At length they drop out into the water and become free jelly-fishes similar to Tiaropisis (Fig. 14).

Fig. 12.—Campanularian (Obelia commissuralis, McCready). The hydro-medusæ in the cups drop out and become free Medusæ, or jelly-fishes, similar to Fig. 14. Fig. 13.—Tubularia Couthougi (Agassiz). m, Medusæ; ct, coronal tentacles; p, proboscis.
Fig. 14.—Campanularian (Tiaropsis diademata, Agassiz).

The visitor will find other hydroids, which appear like miniature trees with all their foliage crowded to the top, and from beneath which there hang bunches, as it were, of grapes or other fruit. Such is Tubularia, and the little fruit-like clusters are persistent jelly-fishes which develop and remain just beneath the row of tentacles (Fig. 13), instead of becoming free jelly-fishes as in Sarsia and in Campanularia.

PSM V13 D332 Portuegese man of war.jpg

Fig. 15.—Portuguese Man-of-War (Physalia arethusa).

In the Gulf of Mexico are communities of hydroids so organized that they seem to constitute but one animal. Such is the well-known "Portugese man-of-war" (Fig. 15). This community consists of an elegantly-crested air-sac floating upon the water, and giving off numerous long and variously-constructed appendages. According to Agassiz, the different parts are so many different kinds of members in the community, and fulfill widely-different offices, some catching and eating food for the whole, others producing buds, others being the locomotive or swimming members, and having tentacles that in some cases are twenty or thirty feet long. The air-sac itself is only a few inches in length.

But the most common jelly-fishes are those which are more or less disk-shaped, and hence are called the Discophoræ. The "sunfish" is one of these. This name, we hasten to say, is rather indefinite when used without modification, for it is not only applied to a jelly-fish, but it is also given to our fresh-water bream, and to one of the large marine fishes—orthogoriscus. The "sunfish" of which we now speak attains a diameter of six to twelve inches (Fig. 19). In the early spring it may be seen in large schools near the surface of the water, and at this time is only a small fraction of an inch in diameter. It becomes full-grown by the middle of summer, and great numbers may then be seen swimming slowly by a sort of motion that may be likened to that of partly shutting and opening an umbrella. The motion is indeed effected by the contraction and expansion of the whole umbrella-like disk.

In the study of the sunfish (Aurelia) we are able to see plainly the prominent differences between jelly-fishes as a group and polyps as a group.

The natural attitude of the latter is with the mouth upward, or at least not turned downward, and the body is divided into vertical chambers, by vertical partitions, and the substance of the animal is flesh-like. On the other hand, the typical adult jelly-fishes have their mouth on the under surface, or at least not turned upward, their substance is jelly-like, and their body is traversed by tubes which radiate from the centre to the circumference.

The sunfish, and all the disk-shaped jelly-fishes especially, are remarkable for their stinging properties. When they come in contact

Fig. 16.—Scyphistoma of Aurelia flavidula (Per. & LeS). Magnified about seven diameters. Fig. 17.—Strobila of Aurelia flavidula (Per. & LeS). Magnified about seven diameters. Fig. 18.—Strobila of Aurelia flavidula (Per. & LeS). Magnified fifteen diameters.

with the flesh of the bather, they cause a stinging sensation, similar to that produced by nettles. This is why they are called sea-nettles, and why in scientific books they are called Acalephæ.

Toward the close of summer the sunfish lays numerous eggs, and in the autumn it perishes. The eggs hatch into little oval bodies,

Fig. 19.—Sunfish (Aurelia flavidula, Per. & LeS). Offspring of Figs. 16-18.

which swim freely about by means of minute hair-like appendages. After a time each one of these free-moving bodies attaches itself to a rock, shell, or sea-weed, and takes the form of a plant, and is then called scyphistoma (Fig. 16). As this goes on growing, it soon begins to divide into segments by horizontal constructions. By this process our little scyphistoma becomes a strobila (Figs. 17, 18). By continued growth the segments become more and more marked, more and more separated, and at length the uppermost one drops off, then the next one drops, then the next, and so each in its turn breaks away from the parent stalk; and as each breaks away it assumes the natural attitude, mouth downward, and floats away to lead its independent life as a genuine sunfish. Here we have a good illustration of that strange mode of reproduction called "parthenogenesis" or "alternations of generations"—that is, the egg hatches into the planula, which soon becomes a scyphistoma; that little plant-like body becomes a strobila, and this breaks into segments, each of which becomes a perfect jelly-fish, which produces the eggs.

The species of jelly-fishes of the disk, or partially hemispherical form, are very numerous and varied in details of structure, in form, and in size. Some have the appendages around the mouth and the margin greatly prolonged as in Fig. 20. A few kinds attain a diameter

Fig. 20.—Jelly-fish (Pe'agia cyanella, Agassiz). Fig. 21.—Pleurobrachia (P. rhododactyla, Agassiz).

of two or three feet; and these largest kinds have in some cases tentacles a hundred feet long.

One of the most beautiful of all the jelly-fishes is the rose-colored idyia. It is often seen near the shore, and is so transparent that it reveals almost its whole structure as it floats in the water. It is sometimes so abundant that it gives a rosy hue to considerable areas of the sea. It attains a length of three or four inches, and in form is not very unlike an elongated melon with one end cut square off.

Closely related to idyia is pleurobrachia, one of the commonest of the "comb-bearers," or Ctenophoræ, on the northern coast of the United States. This jelly-fish, less than an inch in length, like all other Ctenophoræ, has eight rows of locomotive fringes dividing the surface of the body into regions as the ribs divide the surface of a musk-melon. Besides these eight rows of fringes, or locomotive organs, it has two most extraordinary tentacles; and no form of expansion, or contraction, or curve, or spiral, can be conceived of, which these wonderfully constructed tentacles do not assume as this transparent jelly-fish moves freely through the water.

If the visitor to the sea-shore will go down among the big rocks left bare by the retiring tide, and will lift up the long sea-weeds which hang from their sides, he will find the curious "starfishes," or "sea-stars," in some cases in great profusion, and clinging to the surface of the rock so firmly that they often leave some of their locomotive suckers attached when too quickly lifted from their places.

The starfishes have the body so gradually merging into the arms

Fig. 22.—Starfish (Asteracanthion).

or rays that one can hardly tell where the body ends and the arms begin; and this enables one to readily distinguish them at sight from the "serpent-stars," which are sometimes called star-fishes, and of which we will presently speak. The mouth is in the centre of the under side, and beneath each ray there are a large number of locomotive suckers. An eye is situated at the end of each ray; and on the back, near the junction of two arms, is a sort of water-filter called the madreporic body.

As in all similar cases, the dried specimens give us only a partial idea of the real starfishes, and those who have studied these animals in museums only have little idea of the readiness with which they make their way along the vertical and overhanging surfaces of rocks, and into holes and narrow fissures.

Starfishes are very voracious, and feed mainly on mollusks. They are exceedingly destructive to oysters in many places, and thus come in direct competition with man for the possession of this delicious bivalve. Instead of swallowing their food as other animals do, they turn the stomach out of the mouth and over the animal which they wish to devour!

Starfishes have a wonderful power of reproducing lost parts. If an arm is bitten off by a hungry fish, another grows in its place; and cases are known where all the arms but one have been detached, and the remaining arm and central portion of the body have lived on and reproduced all the destroyed parts. Examples of this may be seen wherever starfishes are abundant.

Starfishes are quite numerous in species, and vary greatly in form and size. The ordinary kinds are only three or four inches across, others a foot.

In the same localities where we find true starfishes, we may confidently expect to find the "serpent-stars" or serpent-tailed starfishes, Fig. 23.—Serpent-Star
(Ophiopholis bellis, Lyman).
so called because their arms taper like a snake's tail. They are also called "brittle-stars," because they break so easily.

Many visitors to the sea-shore come away without seeing a single living brittle-star, because the curious echinoderms which bear this name hide under the sea-weeds, and in the dark holes and crevices among the rocks, and are, therefore, found only by those who search carefully for them.

The long, gently-tapering arms, starting out abruptly from a well-defined disk, make the form of serpent-stars very distinct from that of the Asteroidæ or genuine sea-stars, already noticed. And, unlike the true starfishes, they have no interambulacral plates, but a series of large plates envelops the whole of each ray or arm, meeting in a ridge along its under surface. They have no genuine locomotive suckers like starfishes, but instead they have numerous tuberculated organs which pass out through holes in the sides of the arms. Their madreporic body is in one of the circular plates on the under side of the disk.

Allusion is made above to the brittleness of the serpent-stars. This sort of brittleness is not confined exclusively to these echinoderms: at least one species of starfish exhibits the same property. The lamented Prof. Edward Forbes tells a story of his experience with a starfish known as Luidia, which shows that it equals any serpent-star in brittleness. The professor went dredging for the Luidia. He brought up a fine specimen and laid it on one of the benches in the boat, and went on with his dredging. When he was ready to go home he found his Luidia in fragments. It had gone to pieces of its own accord—probably by contraction. Much disappointed by the loss of so desirable a specimen, he determined to take great precautions when he should capture another Luidia, that he might not again have to put up with pieces only. So, next time, he took the precaution to carry a bucketful of pure fresh-water, intending to plunge the Luidia into it, thus paralyzing and saving it before it should have time to break itself in pieces. Pulling up the dredge and seeing that he had a fine specimen,

Fig. 24.—"Basket-fish" (Astrophyton Agasaizii, Stimpson).

he let down his bucket of fresh-water so that its top was near the surface of the sea, intending, as soon as the dredge should reach the surface, to overturn it into the bucket; but, just when he would lift the dredge from the sea, the Luidia, as if apprehending the situation, went to pieces, and these began to disappear through the meshes; and, as one of the last was vanishing, the professor imagined that the eye looked back at him with a peculiar and suggestive wink of derision!

The most remarkable representative of the serpent-stars or Ophiurans, as they are called in scientific books, is the "basket-fish," or Astrophyton. The ordinary kinds, as we have seen, have the arms simple; but one genus has the arms extensively branched (Fig. 24). This kind inhabits the deeper waters, and will not be readily obtained except through the aid of dredgers or fishermen, who sometimes bring it up attached to their lines. It attains a diameter of ten or more inches, and the arms go on dividing and subdividing until the divisions are said to number more than 80,000!

If we imagine the Astrophyton with its mouth turned upward, and Fig. 25.—Crinoid
(Pentacrinus caput-medusæ),
West Indies.
its arms brought near together, and the ab-oral region furnished with a long, jointed, and flexible stem, we shall have a form not very unlike the Pentacrinus caput-medusæ (Fig. 25), of the West Indies, one of the few survivors of the order of Crinoids that was represented by a great number of species in the palæozoic ages of the earth's history.

Some kinds of crinoids, as the rosy feather-star of the European coast, have a stem in the young state, but at length become detached and live as free crinoids. They thus illustrate, in their embryonic stage, the permanent form of the living stemmed species and of those stemmed forms which fill the rocks in many regions, from the Silurian to the Triassic, inclusive.

It may be remarked here that in no place are fossil crinoids more abundant or varied, and beautiful, than in the sub-carboniferous rocks of this country, especially those in the Mississippi Valley; although larger species have been found in the Triassic rocks of Europe.

While the visitor to the sea-shore may hardly hope to secure a living crinoid, it is well to bear in mind that this form is a near ally of the starfishes, serpent-stars, and the Astrophyton, which he can secure.

It was the remark of one of the old students of Nature that there was nothing on the land that has not its counterpart in the sea. And, if we recall some of the names that have been given to marine forms, we shall see how men have been struck with the resemblances between animals of the land and those of the water. Among fishes we have "sea-vampires," "sea-eagles," "sea-wolves," "sea-hounds," "sea-robins," "sea-swallows," "sea-horses," etc. Among mammals we have "sea-elephants," "sea-lions," "sea-bears," "sea-cows," etc.

Among the lower forms we find sea-hedgehogs, that is, "sea-urchins," and "sea-cucumbers." It is of these I would now briefly speak, as they are among the interesting things which the visitor to the sea-side will be sure to find, if he search faithfully for them. But first as to the sea-urchins (Figs. 26, 27). If one would study these strange forms, he must seek for them in the deeper pools left by the tide. Nor is a casual glance into the pool sufficient to reveal the prize

Fig. 26.—Sea-Urchin (Toxopneustes drobachiensis, Agassiz). Fig. 27.—Top-View of Sea-Urchin, Spines removed. Shows ambulacral and interambulacral plates.

which he is in search of. The beginner may look into a clear pool where there are a hundred sea-urchins, and perhaps he will not see one until he has looked for some minutes; for sea-urchins not only resemble some of the sea-weeds in their color, but by means of their locomotive suckers they draw the sea-weeds closely about them, in many cases completely concealing themselves. If the collector reach down into the water the full length of his arm, and move his hand over the bottom among the sea-weeds, he will not be long in finding a sea-urchin. He will know when he touches one, as the sharp spines stand out on every side. Without moving from his position the collector will often secure a score or more of fine living specimens—some hardly exceeding a fraction of an inch in diameter, and others two inches or more; for he will find them of different ages, even if not of different species. Should he put them in a shallow pool while he goes on collecting, and then look for them again, he will at first think they have escaped into the sea or into some hole; for, true to their instincts, no sooner are they uncovered than they begin to conceal themselves again by drawing around them the sea-weeds by means of their long locomotive suckers. If we turn one over on his "back," that is, place the mouth upward, the urchin immediately begins to turn itself, and in a short time will regain its natural attitude, mouth downward.

The spines are very remarkable, not only in their appearance, but in their structure. A cross-section of one, under a microscope, reveals a structure so perfect and so beautiful that the richest mosaic is but rude masonry as compared with this natural mosaic.

Situated among the spines are curious three-pronged forceps, which have much puzzled the naturalists in days gone by; for it was doubtful what is their real function. But, at last, Agassiz—the younger Agassiz, I believe—discovered that these curious organs, called pedicillariæ, are for keeping the spines clean.

The mouth of the sea-urchin is provided with five pointed teeth, which shut together on a common centre; and these teeth can all be removed together, and, thus removed, they present quite a curious appearance, and are known among naturalists as "Aristotle's lantern."

The shell, which is composed of hundreds of pieces, presents a very beautiful sight when the spines are removed. It is made up of ten segments, radiating from the mouth, and converging to a central region on the top (Fig. 27). Every alternate segment is perforated for the numerous locomotive suckers to pass out, the intermediate segments being imperforate, and more prominently marked with tubercles, on which spines are borne. At the termination of the five perforated segments there is a triangular plate with a minute opening; here the eye is situated. Alternating with these five plates are five larger ones, each with a hole, through which the eggs are laid. The largest of these plates is the madreporic body, corresponding perfectly to that seen on the starfish, already spoken of, and which doubtless acts as a sieve or water-filter.

As sea-urchins do not shed the shell, as do crabs and lobsters, the inquiring mind will naturally ask how the animal can continue to enlarge when once it is invested with a hard shell. The answer is, that every piece of the shell grows at the same time, and in this way the whole shell enlarges together, and in a perfectly symmetrical manner.

As already indicated, the sea-urchin moves by means of its locomotive suckers. Extending these beyond the spines, it lays hold of the surface of the rock or sea-weed, and then, contracting the suckers, pulls itself along. And these suckers can be extended quite a distance beyond the spines. For example, a sea-urchin can extend a sucker from near the top of the shell, and bend it over, and lay hold of the surface upon which the animal is resting.

The sizes and forms of sea-urchins are very numerous. The ordinary kinds are two or three inches in diameter; some of the elongated kinds in the tropics have a diameter of five inches or more. Some are nearly hemispherical; others rise in the centre so as to be almost a cone; others are flat, and are known as cake-urchins (Fig. 28); others are more or less heart-shaped, etc. Some of the cake-urchins are curiously modified, as seen in Fig. 29.

On the coast of Maine especially, but also along both shores of the Atlantic, as well as in most other parts of the world, the sea-shore visitor will find the "sea-cucumber" appearing not very unlike a cucumber while it still retains the blossom upon its end. On the coast of Asia it is known as trepang, and is the animal which the Chinese use so extensively for food. Aristotle called it Holothuria, but for what reason he does not tell us, and we can only conjecture. The dead specimens give us but little idea of this animal. It must be seen and studied while in the sea or in the aquarium, in order to be appreciated. When dead, the suckers—which are like those of starfishes and sea-urchins—are retracted, and the tentacles are also in a mass, and the

Fig. 28.—Cake-Urchin (Echinarachnius parma, Gray). Fig. 29.—Key-Hole-Urchin (Mellita quinquetora, Agassiz).

whole form is shriveled. But in the water the form is full, and the fringed tentacles are extremely beautiful, and can properly be compared to the delicately-branched and beautifully-colored sea-weeds which we all so much admire.

The "sea-cucumber" has a wonderful power of changing its form. It elongates, contracts, enlarges at each end while it is small in the middle, and thus changes its appearance from time to time. In its power of going to pieces it almost excels the "brittle-star" and the

Fig. 30.—Holothurian, or "Sea-Cucumber" (Pentacta frondosa), North Atlantic.

starfish, Luidia, already noticed. It breaks off its tentacles, and yields up other parts, at will; and it has been known, when disturbed, to eject all its internal organs, thus leaving itself only an empty sac!

The "sea-cucumber" has no hard parts, excepting the merest calcareous particles imbedded in the thick, leather-like covering. These are of various and remarkable shapes, and are very interesting objects when seen under the microscope. They are the representatives of the calcareous plates which make up the hard shell of the sea-urchin.

Unlike as are the crinoid, brittle-stars, star-fishes, sea-urchins, and sea-cucumbers, in their form and general appearance, they are but different expressions of one and the same fundamental idea. They are all radiates, all possess calcareous plates—though these are at their minimum in the sea-cucumbers—and are covered with spines, tubercles, or a rough skin. They are all constructed according to a reigning number, the principal parts being in fives, or some multiple of five. If we imagine the sea-cucumber to be placed with its mouth downward and the tentacles to be replaced with teeth, the long body to be shortened upon itself so as to assume nearly the form of a hemisphere, and the microscopic calcareous particles to be enlarged so that they should touch one another, then we should have essentially the form and structure of the sea-urchin.

And if we imagine the sea-urchin with its segments spread out into a star-like form, instead of being brought near together, each perforated segment taking half of the imperforate one, and at the same time the spines to be reduced to tubercles and the plates to a network, then we should have essentially the form of a starfish.

Again, if the starfish had its body reduced to a well-defined disk, and its arms starting out abruptly from this disk, we should have all the most prominent features of the serpent-star.

And if the serpent-star had its mouth placed upward, its arms multiplied by branching, and its ab·oral region elongated into a stem, we should have the plant-like form of the crinoid.

And so it is in all parts of the material world. Nature has but comparatively few great types, but the forms included under these types are almost endlessly varied. Unity in diversity is a great law which prevails not only in the animal kingdom, but throughout the whole realm of Nature.