CRYSTALLIZATION.

A is a rotating machine;ais a skein of thread;áis the skein rotated;bis a chain;cis an onion;dis an apple;eis a glass fish aquarium, one tenth full of water, and rotated. A stick, hoop, shingle, or any such body suspended by a cord, when rapidly rotated will rise and revolve around its shortest axis.

Terrestrial gravity is constantly affecting the motion of bodies. Motion is the act of changing place, and always indicates the presence of some force; force or energy being that which tends to produce motion or rest. Motion in curved lines is produced by two or more forces acting upon a body, one of which must be constant. Example: A cannon ball is acted upon by the sudden explosion of the powder, the resistance of air, and the constant downward attraction of gravity. Nature seems to delight in curved motion; the waves, the flight of birds, the running brooks, the clouds, even the waving trees and grasses, all furnish illustrations of this. A little reflection upon any such instances will show that they areusuallyproduced by the united action of an instantaneous and constant force.

The center of gravity is that point around which the opposite particles of a body balance each other. This point does not necessarily coincide with center of figure or center of motion, the former of which is a point equally distant from opposite parts of a regular body, while the latter is a point in a substance around which it revolves.

Ex.—A lead pencil poised on the finger. This experiment can be varied in many ways, showing the nature of stable and unstable equilibrium.

If the sun and all the planets could be strung on a rod passing through their centers, with the planets to the east, the center of gravity of the solar system would be somewhere in the sun, east of its center. As the planets assume various positions with reference to the sun, it must follow that the center of gravity in our system must vary accordingly.

The same is true of objects on the earth. The center of gravity may be elevated or depressed, moved to the right or left. We instinctively adjust our bodies so that a perpendicular let fall from the center of gravity will constantly fall within the base. The most surprising exhibition of this power of automatic adjustment was seen in Blondin, in his performances on the tight rope.

Stability in structures is usually secured by lowering the center of gravity in one of two ways: either by broadening the base or by making it of heavy materials.

Specificgravity is the weight of a substance as compared with an equal bulk of something taken as a standard;waterhaving been selected as the standard for solids and liquids, andairfor gases.

Gravity furnishes moreunits of measureof various kinds—weight, work, heat, tenacity—than any other force of nature.

It will be remembered thatPhysicsis that branch of science that considers the general properties of matter, and the character of those forces which affect matter without destroying its molecule. It includes many subdivisions. In addition to those already mentioned, we find Molecular Attraction, or the operation of forces that act at insensible distances; Hydrostatics, which treats of liquids at rest; Hydraulics, of liquids in motion; Pneumatics, of gases; Machines, of means for applying force; Acoustics, of the laws of sound; Heat; Light; and Electricity.

As many physical properties have been mentioned in the articles on Air, Water, and Fire, they will not now be considered. Our discussion here applies more especially to those substances which, at ordinary temperatures, are solid.

Ex.—A body buoyed up in water displaces its own weight of the liquid. The glass is nicely graded, and as the water rises in the vessel, the registration at once indicates the amount of water displaced. This proves the truth of the “Law of Archimedes”[9], ascertained while he was investigating the problem of the golden crown.

The most characteristic properties of solid bodies are the following: Hardness, tenacity, malleability, ductility, and crystalline form. Hardness is the resistance which a body offers to being scratched. Tenacity is the resistance offered by a body to a separation of its parts. Malleability is that property of a body which makes it capable of being rolled into sheets. Ductility is capacity for being drawn into wire, and crystalline form is the property which causes it to assume regular shapes.

As will be observed, these peculiarities are closely dependent upon cohesion and adhesion. By the former we understand the force which holds together the similar molecules of a substance; and by the latter, the force which unites the surfaces of different materials. Familiar as we are with these two agencies, their nature is not yet understood. We can easily discover that they are very dependent upon heat, by the application of which most solids pass from the stable form, to one in which, instead of cohesive force between the molecules, there is repulsion; as in the conversion of ice into water, and then into steam.

This movement of molecules is also dependent upon pressure. The most interesting illustration of this is seen in the action of glaciers. It has been ascertained that the melting temperature of ice lowers one two hundred and fiftieth of a degree for every fifteen pounds of pressure to the square inch.

The immense superincumbent mass of ice must, in many places, set free so much latent heat that a portion of the ice melts, so that here and there cells and liquid veins would be opened in the interior of the glacier. But the particles whichseparate these thin layers of water would almost immediately close up. This is the brilliant demonstration of Prof. Tyndall, who has given the operation the name of “regelation.” It has been thus described: “This phenomenon takes place at every point in the thickness of the glacier. Particles of ice approach one another, and unite across little veins of water, which permeate it in every direction; fresh liquid films are formed under the pressure from above; fresh unions take place between the divided morsels of ice; and, by this continual process of change, the air contained in the mass of that which once was snow, is gradually expelled. Thus it happens that the whole mass ultimately assumes an almost perfect transparency and a beautiful azure color.”

One of the most beautiful illustrations of cohesive attraction is seen in crystallization. In every instance in which substances pass into the form of a solid, they tend to assume regular shapes called crystals. Each material has its own characteristic form, so that a crystal is a type of a species in the mineral world, even as a plant or an animal is in the organic kingdom. A crystal is a substance bounded by plain surfaces and symmetrically arranged about imaginary lines called axes. The final form depends upon certain smaller forms in its interior structure. They possess lines of division, often in three directions, called “cleavage.”

While there are millions of crystals, they have all been classified under six systems, as follows: 1. Monometric, where the three axes are equal. 2. Dimetric, having one axis unequal to the other two, which are equal to each other. 3. Trimetric, having no two axes equal. 4. Monoclinic, having one axis inclined. 5. Triclinic, in which all the three intersections are oblique and the axes unequal. 6. Hexagonal, which has the form of a regular hexagonal prism.

Ex.—Showing change of volume. The upper part of the figure represents a substance expanded. There are no more molecules here than below, but they are pushed further apart. This is supposed to be the way in which all bodies are enlarged by heat.

While contemplating the thousand beautiful forms in which molecules are arranged into crystals, whereby many economic purposes are served, as well as taste manifested, one can not resist the conviction that such displays of wisdom, benevolence and love of beauty can alone emanate from the eternal Mind.

Another wide-spread effect of cohesion is seen in

Everywhere in fossiliferous rock may be found organic remains in which the material of which they were originally composed has been replaced by some mineral substance. Some have supposed that these plants and animals have actually been converted into stone by a change of their elements. This is of course absurd. Carbon can never be anything but carbon, nor indeed, can any element ever become anything other than itself. This dream of the alchemist was long since dissipated. No, strange as it may seem, the molecules of these fossilized organisms must actually pass out, and silica, lime, clay, or some such matter pass in and take their places. Beautiful specimens of petrified wood, found especially on the Pacific coast, are often hard as glass. One very handsome variety, called “opalized” wood, clearly indicates that petrifaction was either accompanied or followed by crystallization.

Myriads of shells, bones and plants scattered through the earth’s strata have been transformed in the manner indicated. Although petrifaction is usually a long process, there is reason to believe that it sometimes takes place rapidly. This operation must not be confounded with incrustation, which is often mistaken for it, and takes place where substances, like bending twigs, have deposited upon them layer after layer of lime, salt, sulphur or ice.

The molecules ofsolids, even, are in intense and ceaseless motion. As has been said, “A continuous and restless, nay, a very complicated activity is the order of Nature throughout all her individuals, whether these be living beings or inanimate particles of matter. Existence is, in truth, one continued fight, and a great battle is always and everywhere raging, although the field in which it is fought is often completely shrouded from our view.”

Ex.—A simple illustration of the convenience of machinery in applying force and changing direction.

The motto of the brave Huguenots in the time of Louis XIV. was “Ever burning, but never consumed.”

Nature’s motto, both for matter and energy is, “Ever changing, but never destroyed.” Let us next notice some instances of the

Energy is the power to do work or overcome resistance. It is of two kinds—potential and kinetic. The former is the energy or force due to position, but it is latent or inactive. The latter is the energy of a body which is in motion. A stone resting on a mountain top, the water in a quiet mill pond, a coiled spring, are all examples of potential energy.

The stone, crushing through the cottage of a peasant, the water turning a factory wheel, the spring turning the wheels of a clock, are examples of actual or kinetic energy.

Ex.—Lay a magnet down on iron filings. They will gather in greatest abundance about the poles, and diminish toward the center, where there are none; thus showing the nature of polarity.

Energy often disappears to reappear under a different name. If we lift our hand to strike the palm of another, our vital energy becomes motion, and that in turn is changed into heat.

In the Bell telephone the sound-waves in the mouthpiece are converted into electric vibrations in the wire, and these, in turn, induce sound-waves in the receiving instrument at the other end of the line.

In dynamo-electric machines we have a chain of transmutations of force—chemical affinity in the fire-box, expansion in the boiler, becoming in turn, motion, magnetism, electric currents, until it appears as resplendent light and intense heat between the carbon points.

Potential energy slumbers in the raindrop, and, anon, as kinetic energy, flashes in the lightning.

In short, the sum of all the energies of nature is a constant quantity, although it manifests itself in a thousand different ways. The foregoing reflections indicate that the researches of modern science all point to a grand unity in God’s universe. Let us conclude by briefly referring to some instances of plan or design in the

The most characteristic feature of all science is that it arranges facts in an orderly manner, under principles or laws.

Nature seems to delight, likewise, in doing a variety of things under one general principle. Note a curioustrinityin her method: We have three great departments of nature—animal, vegetable and mineral; three parts to our being—physical, mental and moral; three divisions of the mind—intellect, sensibilities and will; three parts to all plants—root, stem and foliage; there is earth, sea and sky; three great classes in all mechanism—lever, cord, and inclined plane—and many others that might be mentioned.

Observe another group of laws in physics: Variation, in accordance with an exact proportion.

Gravity varies inversely as the square of the distance; heat varies inversely as the square of the distance; light varies inversely as the square of the distance, and sound varies also in exactly the same ratio.

Who can contemplate this exact mathematical arrangement, extending through many departments of matter, without concluding that “Nature is but the name for an effect whose cause is God?”

BY JOSEPHINE POLLARD.

A distinguished writer has said: “The eyes are of no use without the observing power,” and surely no faculty we possess is capable of so much cultivation as the sight. The facility with which the eye can express the emotions of the soul has been the theme of poets of all ages, who have not hesitated to confess which style of eyes pleased them the most. Says one:

“I everywhere am thinkingOf thy blue eye’s sweet smile;A sea of thoughts is spreadingOver my heart the while.”

“I everywhere am thinking

Of thy blue eye’s sweet smile;

A sea of thoughts is spreading

Over my heart the while.”

And others:

“His eyes are songs without words.”

“A suppressed resolve will betray itself in the eyes.”

“An eye can threaten like a loaded and leveled gun, or can insult like hissing or kicking; or, in its altered mood, by beams of kindness, it can make the heart dance with joy.”

“Eyes are bold as lions, roving, running, leaping, here and there, far and near. They speak all languages; wait for no introduction; ask no leave of age or rank; respect neither poverty nor riches, neither learning nor power, nor virtue nor sex, but intrude, and come again, and go through and through you in a moment of time. What inundation of life and thought is discharged from one soul into another through them!”

There are

“True eyesToo pure and too honest in aught to disguiseThe sweet soul shining through them;”

“True eyes

Too pure and too honest in aught to disguise

The sweet soul shining through them;”

and “eyes that have murder in them, whose flash is the forerunner of thunder.” One has “an eye like Mars, to threaten and command,” and other eyes are “the homes of silent prayer.”

But the variety in color and expression of the eye is as nothing compared to difference in the power of observation. Those ancient companions, “Eyes and No-Eyes,” the story of whose wanderings conveyed a valuable lesson to young and old, were but prototypes of people who go through the world to-day, some of whom see everything, while others see nothing at all. Poets, who could write so beautifully of the eyes, must first have trained their own vision to perceive the beauty or baseness they described, and it is the exercise of this far-seeing, penetrating, analytical power that is the prerogative of genius.

The specialist devotes himself to the closest examination of details. The naturalist does not let the smallest insect escape him, and his trained eye perceives the least peculiarity that denotes the varieties of species.

A person with ordinary eyesight takes up a rose, a lily, or a daisy, and only admires color, shape, or perfume; while the botanist examines the flower in every part, and tells who was its grandfather or grandmother, and feels as tender an interest in it as if it were a human being.

The artist has to train his eye to look for beauty where apparently none appears. He must have an eye for color, for form, for expression, for whatever line he proposes to follow, and he will never rise to eminence if he is satisfied with a hasty, careless, superficial glance.

Turner[1]was one day painting a landscape with the richness of color that was his specialty, when an English girl who was painting near him left her easel and came to look over his shoulder. “Why, Mr. Turner,” said she, “I don’t see any of those colors in the grass or the trees.”

“No?” said Turner. “Don’t you wish you could?”

It is astonishing that with so much of beauty as there is around us, so many people are found who travel through the world without having used their eyes to any profit whatever. The training needs to be begun in early life; children should be taught how to observe; and as some are duller than others they need to have things pointed out to them, until the habit of examining closely becomes fixed, and like second nature.

What a wonderful field for study there is in the sky above us! Look at the clouds; here, in great, heavy masses; there assuming strange shapes, and taking on an infinite variety of coloring. See the setting sun; never twice alike; a marvel of beauty and grandeur; a feast for even young eyes.

Let us go down by the seashore and watch the great waves come in. The sea is broad, and grand, and deep; but is that all? Note how it reflects the color of the sky; mark the waves that rise afar, and show their white manes like wild horses of the sea, and dash on the shore like a charge of cavalry. How they come galloping, galloping on! Watch for the ninth wave, and look out for yourself! Observe the height that each succeeding wave obtains when the tide is on the rise, and how the character of the beach is changed after a severe storm of wind or rain. There is a volume of interesting study in a handful of sand, a tuft of moss, a small patch of grass, or a bunch of seaweed.

Ruskin,[2]that exceedingly close observer of art and nature, and eminently sharp critic of men and things, gives us some excellent instruction in the art of looking below the surface. “There is no bush,” he says, “on the face of the globe exactly like another bush; there are no two trees in the forest whoseboughs bend into the same network, nor two leaves on the same tree which could not be told one from the other, nor two waves in the sea exactly alike. And out of this mass of various yet agreeing beauty, it is by long attention only that the conception of the constant character—the ideal form—hinted at by all, yet assumed by none, is fixed upon the imagination for its standard of truth. Ask the connoisseur, who has scampered over all Europe, the shape of the leaf of an elm, and the chances are ninety to one that he can not tell you, and yet he will be voluble of criticism on every painted landscape from Dresden to Madrid, and pretend to tell you whether they are like nature or not. A man may recognize the portrait of his friend, though he can not, if you ask him apart, tell you the shape of his nose or the height of his forehead.

“The color of plants is constantly changing with the season, and that of everything with the quality of light falling upon it; but the nature and essence of the thing are independent of these changes. An oak is an oak, whether green with spring or red with winter; a dahlia is a dahlia, whether it be red or crimson; but let one curve of the petals, one groove of the stamens be wanting, and the flower ceases to be the same. Two trees of the same kind, at the same season, and of the same age, are of absolutely the same color; but they are not of the same form, nor anything like it.”

How few of us observe these things! and how much we miss daily and hourly through lack of this special training of the eye!

A geologist was with a party of friends in the Yosemite valley and called their attention to the play of the light from a campfire on the underside of the leaves of the trees above them. It was a beautiful revelation, and all wondered that they had never noticed it before.

If you are living in the country you should educate the eye to study nature in all its phases, and every day add something to your store of knowledge. Observe the habits of birds, and their haunts; watch the ants and other insects; familiarize yourself with plant life so that you can tell a weed from a flower, and a medicinal herb from a poisonous plant.

If a dweller in the town, observe varieties of architecture, the materials used in the manufacture of houses; compare modern with ancient styles; and lose no opportunity of obtaining information in regard to all that is new and strange. Wherever you are, be less intent on reading novels than in observing wherein you can improve your surroundings. The slattern, with her nose in a book, is blind to the cobwebs that hang from the ceiling, and the rags and dirt visible to every one else. She is cultivating the eyes of her imagination, and reveling in scenes of fairy-like splendor, and has no eyes for the common things of every day life. Her powers of observation are exceedingly limited, and her home is no better for her being in it. She is content to lead an idle life, and does not see in how many ways she might amuse and improve herself.

The trained housekeeper has made good use of her eyes, and by noticing trifles has brought her department to a high state of perfection. It is not enough that she has a natural taste for it; she must be continually looking after things with the searching gaze of an inspector-general. Her practised eyes see when the table-cloth is awry, or the dishes not in their places; when the furniture needs renovating, or the dust has accumulated, and she feels that her reputation is at stake if the defects are not speedily remedied.

An expert in precious stones can tell almost at a glance the value and weight of each gem, and is not easily deceived by counterfeits.

The physician can so train his eye that he has merely to look closely at the patient to determine the nature of his disease; while the microscopist, the geologist, and the astronomer acquire such accuracy from their close and long continued investigations that they can detect the least change in the appearance of the heavens above or the earth beneath.

But the astronomer may have his eyes so fixed on the stars that he can not observe what is going on below; the geologist may be able to analyze a stone and tell to which stratum it belongs, and yet take no interest in anything that is above ground; and the devoted student of the microscope may be so entranced by the wonders continually opening before him, that he is utterly oblivious to all else surrounding him. Without this habit of observation, the world would have had no Galileo, no Humboldt, no Newton, no Agassiz, no Hugh Miller, no Edison,[3]and no progress. But all are not gifted in the same way; and often the sphere we move in or the place in which we are born, determines and decides our calling, and controls our habits to a very great extent. It is natural that one accustomed to an open country should have his eyes attracted toward the heavens, which are constantly revealing new wonders; and that one brought up among the rocks should take to hammering them to bits, boy-like, to see of what they are made, or how they look inside.

The differences between men consist in a great measure in the intelligence of their observation. The Russian proverb says: “He goes through the forest and sees no firewood.” “The wise man’s eyes are in his head,” says Solomon, “but the fool walketh in darkness.” It is the mind that sees as well as the eye. Where unthinking gazers observe nothing, men of intelligent vision penetrate into the very fiber of the phenomena presented to them, attentively noting differences, making comparisons and recognizing their underlying idea. Many before Galileo had seen a suspended weight swing before their eyes with a measured beat; but he was the first to detect the value of the fact.

One of the vergers[4]in the cathedral at Pisa,[5]after replenishing with oil a lamp which hung from the roof, left it swinging to and fro; and Galileo, then a youth of only eighteen, noting it attentively, conceived the idea of applying to it the measurement of time. Fifty years of study and labor elapsed before he completed the invention of his pendulum—the importance of which, in the measurement of time and in astronomical calculations, can scarcely be overrated. In like manner, Galileo having heard that a Dutch spectacle-maker had presented to Count Maurice, of Nassau,[6]an instrument by means of which distant objects appeared nearer to the beholder, began to inquire into the cause of such a phenomena, and this led to the invention of the telescope, and proved the beginning of the modern science of astronomy.

While Captain (afterward Sir Samuel) Brown[7]was occupied in studying the construction of bridges, with the view of contriving one of a cheap description to be thrown across the Tweed, near which he lived, he was walking in his garden one morning when he saw a tiny spider’s web suspended across his path. The idea immediately occurred to him that a bridge of iron ropes or chains might be constructed in like manner, and the result was the invention of his suspension bridge.

So James Watt,[8]when consulted about the mode of carrying water by pipes under the Clyde, along the unequal bed of the river, turned his attention one day to the shell of a lobster presented at table, and from that model he invented an iron tube, which, when laid down, was found effectually to answer the purpose.

Sir Isambard Brunel[9]took his first lessons in forming the Thames tunnel from the tiny ship-worm; he saw how the little creature perforated the wood with its well-armed head, first in one direction and then in another, till the archway was complete, and then daubed over the roof and sides with a kind of varnish, and by copying this work on a large scale, Brunel was at length enabled to construct his shield and accomplish his great engineering work.

It is the intelligent eye of the careful observer which gives these apparently trivial phenomena their value. So trifling a matter as the sight of seaweed floating past his ship enabled Columbus to quell the mutiny which arose amongst his sailorsat not discovering land, and to assure them that the eagerly sought New World was not far off.

It is the close observation of little things which is the secret of success in business, in art, in science, and in every pursuit in life. When Franklin made his discovery of the identity of lightning and electricity, it was sneered at, and people asked, “Of what use is it?” To which his reply was, “What is the use of a child? It may become a man!” The great Cuvier[10]was a singularly accurate, careful, and industrious observer. When a boy he was attracted to the subject of natural history by the sight of a volume of Buffon,[11]which accidentally fell in his way. He at once proceeded to copy the drawings, and to color them after the descriptions given in the text. At eighteen he was offered the situation of tutor in a family residing near Fécamp, in Normandy. Living close to the seashore, he was brought face to face with the wonders of marine life. Strolling along the sands one day he observed a stranded cuttle-fish.[12]He was attracted by the curious object, took it home to dissect, and thus began the study of the molluscæ, in the pursuit of which he achieved so distinguished a reputation. He had no books to refer to excepting only the great book of nature which lay open before him. The study of the novel and interesting objects which it daily presented to his eyes made a much deeper impression on his mind than any written or engraved descriptions could possibly have done. Three years thus passed, during which he compared the living specimens of marine animals with the fossil remains found in the neighborhood, dissected the specimens of marine life that came under his notice, and, by careful observation, prepared the way for a complete reform in the classification of the animal kingdom.

The life of Hugh Miller furnishes another illustration of the advantage of making a good use of the eyes. While Hugh was but a child, his father, who was a sailor, was drowned at sea, and he was brought up by his widowed mother. He had a school training after a sort, but his best teachers were the boys with whom he played, the men among whom he worked, the friends and relatives with whom he lived. With a big hammer which had belonged to his great-grandfather, an old buccaneer, the boy went about chipping the stones and accumulating specimens of mica, porphyry, garnet, and other stones. Sometimes he had a day in the woods, and there, too, his attention was excited by the peculiar geological curiosities which came in his way. While searching among the rocks on the beach, he was sometimes asked, in irony, by the farm-servants who came to load their carts with seaweed, whether he was getting “siller in the stanes,” but was so unlucky as never to be able to answer in the affirmative. When of a suitable age he was apprenticed to the trade of his choice—that of a working stone cutter—and he began his laboring career in a quarry looking out upon the Cromarty Firth.[13]This quarry proved one of his best schools. The remarkable geological formations which it displayed awakened his curiosity. The bar of deep-red stone beneath, and the bar of pale-red clay above, were noted by the young quarryman, who even in such unpromising subjects found matter for observation and reflection. Where other men saw nothing, he detected analogies, differences, and peculiarities which set him thinking. He simply kept his eyes and his mind open; was sober, diligent and persevering, and this was the secret of his intellectual growth.

His curiosity was excited and kept alive by the curious organic remains, principally of old and extinct species of fishes, ferns, and ammonites,[14]which were revealed along the coast by the washings of the waves, or were exposed by the stroke of his mason’s hammer. He never lost sight of the subject, but went on accumulating observations and comparing formations, until at length, many years afterward, when no longer a working mason, he gave to the world his highly interesting work on the “Old Red Sandstone,” which at once established his reputation as a scientific geologist. But this work was the fruit of long years of patient observation and research.

We learn from these interesting records that, no matter how or where one is situated, he will always find opportunities for observation if he will only keep his eyes open and his mind open at the same time. It is the brain behind the eyes that makes seeing of any value. Every gift may be perfected by self-culture, and by keeping our eyes busy on things about us, by observing and comparing, we color our future lives, increase our intelligence, and are never at a loss for new worlds to conquer.

What the world needs to-day is lessoutlookand moreinsight; more careful observance of what is needed in our homes by those we love and those who love us. We need eyes to see our own duty in every department of life, to note our own faults, and to observe the beauty rather than the blemishes of others; to see wherein we can be of service, and in what way we may enlarge our opportunities, and in order to acquire any skill or proficiency we need continually to pray, “Lord, open thou the eyes of our understanding.”

This subdivision of the animal kingdom, containing articulated or jointed animals and insects, exceeds every other in the number and diversity of the species. The articulation may belong to their bodies, limbs, or outer covering. The tough shells of some, formed by a secretion of a hard, horn-like substance, have numerous segments, or rings, either closely joined and firmly cemented, as those about the head and thorax, or loosely cemented, as those which encompass the abdomen. The skeleton of some is external, and consists of these articulated segments, which serve the double purpose of framework and covering. The muscles, or elastic cartilages holding them together, are striated, or furnished with small grooves in the sheath or shell. If the animal has limbs, they also are jointed, and hollow.

Class I.—Crustacea, so called from the crust in which their soft bodies are encased. They are a very large family, mostly of air breathing animals, with enough in common to indicate their relationship, yet distinguished by a great diversity in their forms and modes of life. Some are very small, and are as numberless as the sands on the shore. Others, when their members are all extended, can stretch themselves over a circle several feet in diameter.

The chief orders of the Crustacea are the Barnacles,[1]the Water-flea, the Fourteen-footed Crustacea,[2]and Ten-footed Crustacea.[3]

The Crayfish may be taken as a type of the structure of the Crustacea. The body has two principal sections. The anterior, called the cephalo-thorax,[4]extends to the first distinctly marked ring, and the shield, thus far, is comparatively smooth, the segments fitting so closely as to be practically one. In front and between the two pairs of antennæ, or feelers, is a small pointed process in the place of the nasal organ, but serving some other purpose. At the base of each of the smaller antennæ, on the under side, is a minute sac, the mouth of which is protected with delicate hairs. These are the organs of hearing, and near them, on the outer side, are the organsof smell. The sense of touch is in the fine cilia that fringe the mouth and the antennæ.

There are numerous appendages. Of the five pairs of legs, the first two are provided with claws, or nippers. The fore-legs, or arms, have, in the place of hands, strong pincers, similar, but not entirely alike; the one with sharp edge and smaller teeth is used for cutting, the other for mashing, or grinding the food. The other legs terminate in feathery points, and are used, in part, for locomotion, and by the female for carrying her eggs. The posterior pair, called swimmerets, together with the expansion of the last segment of the abdomen into a kind of caudal fin, are the main dependence for swimming. The segments are so loosely jointed that the “tail” can be moved freely, and by flapping it the animal moves easily. As there is no neck, in order to see objects in different directions, the eyes are not sunk in the head, but placed at the extremities of little muscular processes, or “eye stalks,” which are movable, making even hind-sight practicable when backward motion is desired.

THE CRAYFISH.

The crayfish breathes through branchiæ, or gills, situated at the sides of the thorax, protected by the carapace,[5]or horny covering, under the edges of which the water and air reach the gills. Here a very curious appendage is attached, called the “gill bailer,” which moves back and forth, creating a current of water through the gills that finds its way out through an opening near the mouth.

Under the welded sheath or cover of the head are the mandibles, or jaws, between which the mouth opens; a short passage, leading to the capacious, gizzard-like stomach, is provided with grinders, to still further masticate the food before it passes into the intestine. The eggs are small, and attached by glutinous threads to the appendages until they are hatched; the young are also attached, until sufficiently developed to live apart from the parent.

This class of animals undergoes periodic changes which are attended with some degree of violence. The crustaceous covering is a kind of epidermis,[6]having beneath it the true skin. It is formed by some process of exudation from the growing body. This sheath, while soft, expands slowly, but when hardened, the growth is retarded, and in time it is found too small for convenience, so it is cast off, and a new and larger one supplied to take its place. In this process of moulting the animal attempts to put off its outer covering, not in fragments or parts, but in one piece, though many delicate attachments have to be sundered, membranes rent, and sometimes even a limb torn off in the resolute effort to undress. This can not be done at all times, or at any time, without special preparation. A period of apparent sickness precedes, and the muscular parts of the limbs become shrunken, so that they are more easily extricated. The loss of a leg is not so serious a matter, since the damage is repaired by a new one with the same form and articulations. As the work of repairing the limb begins at the joint nearest the body, if the member is torn between that and the extremity, the partially mutilated animal has the strange power of throwing off all that remains beyond that joint.

Of other crustaceans, the common lobster is in most respects so similar to that shown in the first diagram as to need no further description than to say the cephalo-thorax is comparatively smaller, while the forearms and claws are larger.

There are also marine crayfish that are very numerous about the coral reefs off the Florida coasts, and have substantially the same characteristics, only their claws are considerably less, and their ciliated antennæ larger.

Crabsare closely allied to lobsters, and belong to the highest orders of the crustaceans. The lengthened, loose-jointed abdomen of the typical crayfish is wanting, and there is a general concentration of the parts; all the most important viscera being included in the thorax, and covered by a single, closely compacted shield. There are many species of crabs, differing in other respects as well as in the form of the shell or back, which in some is nearly orbicular, in others it is oblong, longer than it is broad, or broader than it is long. They differ in the smoothness of their shells, and in the length of their legs, which they stretch out from under their horny covering. Their first pair of limbs is not fitted for locomotion, but shows a vigorous development of the strong claws and pincers of other decapod crustaceans. Though found in almost all seas, they are poor swimmers, their legs being formed for walking or creeping, rather than as oars to propel them through the water. They are found in pools, among seaweeds, and particularly in marshy places left by the receding tides. Most species live in water, some in moist places on land. Many kinds of crabs are used for food. Its black claws and broad carapace readily distinguish it from other species. From activity in seizing, tearing, and devouring their food, and from their pugnacity, crabs are interesting inmates of the aquarium. They also moult, or cast off their shells; not at regular seasons, but when the demand for more room requires it.

Class II.—Arachnidaare closely related to the crustaceans, having, like them, the body divided into two sections—cephalo-thorax and abdomen. To the former are attached four pairs of legs, but the abdomen has no appendages for locomotion. There are about 5,000 species, produced from eggs, and undergoing no metamorphoses in their development.

The lowest forms, under the common name ofAcarina,[7]have the anterior part in a mass with the abdomen, and short legs near the head, terminated in little claws suitable for taking hold of hairs and feathers. They are mostly parasitic, and all birds and animals, even parasites themselves, are liable to suffer from acarina peculiar to their own species.Pedipalpi[8](scorpions), andAraneina[9](spiders), though much larger, belong to this class. The body of the scorpion is divided into segments, though the anterior of the abdominal part seems but a continuance of the thorax, and is as large. It, however, soon tapers off into a long, jointed, tail-like process, in the terminus of which is its hooked sting, perforated and connected with the poison sac. In striking, the tail is raised over the back and struck down. Its other weapons are the crab-like claws on the strong forearms. TheAraneina, at least some classes of them, are well known. The soft, unjointed body is separatedfrom the thorax by a narrow constriction or tie, and at the posterior end there are little appendages called spinnerets, through which the silken lines issue that form the web. The hinder feet are skillfully employed in arranging the gossamer threads after patterns that are instinctively followed.

Class III.—Myriapoda, Centipedes,[10]have the thorax merged with the elongated abdomen, while the head is free. They resemble worms in form, but the skin is stiffened with chitine,[11]and the many legs are articulated. There are two orders: theChilognatha,[12]which move slowly, and are harmless, the “thousand legged worm” is a representative, and theChilopoda,[13]more active, and having a flattened body of about twenty segments, each carrying one pair of legs. Their mouths are armed with formidable fangs connected with poison glands. They are carnivorous, and may be distinguished by their general appearance, quicker movements, and by having longer antennæ than the innocent vegetarians.

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