SEA-URCHINS—STAR-FISHES—FEATHERY STARS—SEA-CUCUMBERS—WORMS—LEECHES—ROTIFERS—TAPE WORMS—INSECTS.

The Echinodermata or spiny-skinned, are most commonly represented by the sea-urchins and star-fishes of our coasts. In some of the classes locomotion is performed by means of these spines or prickles, which serve as legs. In others, movement is carried on by suckers and tubes as in the star-fishes, these tubes being also the means whereby the animal obtains its food.

They have a digestive system, and possess a curiously horny skin even when spines are absent. The mouth is in the centre. We give an illustration of the sea-urchin, and of a section of a spine, which is a beautiful object when seen under the microscope, for these spines can be made quite transparent when cut across and ground. The shelly covering is porous, and as the animal grows the shell is added to at the edges. Underneath will be found the mouth, which has teeth fitted for devouring the small crustacea. These sea-urchins abound, and their porous shells may be picked up frequently after a storm.

The star-fishes are well known to all searchers amongst the rocks and those who study the shore, and are often taken home for an aquarium. They are very voracious, however, and when one is examined in a glass of sea-water, the observer will detect many suckers protruding from each of the rays. It is by means of these suckers, which are put forth from innumerable little holes called “ambulacral apertures,” that the star-fish makes his way up the rocks and along the ocean bed. The stomach of the star-fish is extensive, and situated in the centre of the rays wherein is a digestive apparatus. The rays are composed of detached but beautifully fitted pieces, so united as to be flexible, and around the mouth and in strong frame-work. The star-fish has no teeth, but manages to dispose of a vast quantity of matter, which if left alone would be injurious in decay.

Thus Nature has provided a shore scavenger to devour what would be harmful, just as the vulture on land eats the carrion. Besides this kind of refuse food, the star-fish eats small crustaceans, and oysters fall victims to him. By embracing the shell the star-fish manages to insert itself, and if it cannot bring the oyster out to its mouth, it will quietly turn out its mouth into the oyster-shell, and save the bivalve any trouble in the matter. Some writers declare that the star-fish stupefies or poisons its victim, and then the shell opens. These asteroidea can reproduce a ray that has been injured or cut off, or they can break themselves to pieces if caught.

The brittle stars and feather stars appertain to the order of the Ophiuroida or “serpent-armed,” because the rays are more flexible and thin than the common star-fish. But they differ very much from the star-fish in the arrangement, as well as in the shape of their arms. The former possesses rays which form an appendage of the stomach and enclose it. In the brittle stars the rays are limbs, and could be detached without taking the life of the animal, except in so far as to deprive it of means to obtain its food. The body is quite independent of the rays, the mouth occupying the centre, and is surrounded by minute suckers. The stars are much more flexible than the star-fishes, their rays are longer, and serve either as feet, fins, tentacles, or arms.

The crinoids also belong to the Echinodermata, and resemble plants more than star-fish. They are fixed upon a stalk like a flower, growing upright from the sea-bottom, and the body is called a calyx, which is composed of a ventral and dorsal surface. The arms branch out from the calyx, just as a small tree does, and if we can imagine one of the last planted trees on the Thames embankment reduced to half a finger’s length or less, we have a sort of idea of the crinoid “in the rough.”

A polype supported on a stem branching out in feathery, grassy-looking arms represents the Encrinites, the remains of which are found as fossils. The arms of the crinoids are subdivided, and quite a flowery crown may in time result. The animal obtains its food by the motion of cilia. The stem and the branches are jointed, as it were, and capable of flexible movement in any direction. The crinoids remain stationary during their lives.

The care taken by the star-fish of its young is remarkable. It carries the eggs about in its suckers and with great caution. The young remain attached to the mother until they are able to go about alone, and then the physical attachments die off, and the asteroidea goes forth to seek its fortune in the sea.

The echinus, already mentioned, is a most elaborately constructed animal; the plates being secreted from the soft parts are ever being renewed as the animal gets older and larger. The whole subject is well worth a long independent study, of which it is here impossible for us to give the results.

The sea-cucumbers are more like the familiar garden slug than any other animal, and are surmounted by a fringe round the mouth which looks like leaves. The surface is moist, and has no horny covering like the “urchin,” or star fish, but the suckers are present and are used for locomotion. The tentacles round the mouth serve as prehensile organs. The “alimentary canal” is most curiously curved, and of great length, and the animal can turn itself “inside out” with great facility if alarmed. It possesses a kind of breathing apparatus, and may be classed as the most highly organized of all the Echinodermata. These cucumbers are much esteemed by the Chinese, and “trepang,” as they are called, are caught by thousands in Australian waters.

Annulosa.

The worm-like animals are divided into sections, which include intestinal worms, entozoa, annelida, and crustacea, with the worms, spiders, and insects classed in each section. We may at once perceive what a very extensive division the Annulosa is, and we must devote some space to it.

The Rotifers, or “wheel animalculÆ,” are included in this class; they stand almost alone, and certainly invisible to the naked eye. They are very curious animals, as will be seen from the accompanying illustration. The motion of the cilia around the mouth gives the whirling movement from which their name is derived (fig. 841).

The Annelides, or worms, include earth-worms, water-worms, leeches, etc. The appearance of the earth-worm is so common that few people comparatively studied it until Mr. Darwin’s book took the amateur reader by surprise and delighted him, and to that volume we must refer our readers for details of these very interesting animals, termed annelides because of the rings appearing upon their bodies.

The common leech is well known in medicine. It is curiously enough an inhabitant of ponds and lakes, and in such conditions has no opportunities for tasting the warm blood for which it develops such a liking when opportunity offers. This is really a remarkable fact, that the animal should be placed in a position naturally in which its most natural tendencies should remain unsatisfied.

The progression of the leech is performed by undulating movements and the prehensile action of the suckers—head and tail. The eyes are ten in number, near the mouth and at the top of the head. The mouth is furnished with numerous tri-radiate teeth, but in some leeches they are not sharp, as in the “medicinal” variety. It is known that both sexes are represented in every leech, but they are not self-reproductive.

The earth-worm, so familiar to all, has been lately raised into importance. It lives in clay, and bores its way through the ground. It feeds upon organic matter contained in the earth, and when it has assimilated the nourishing particles, it ejects the remainder in small heaps of soft dirt, which are visible after rain particularly. The worm, by its burrowing, turns up the land, and vastly increases its fertility.

The earth-worm in its outer structure resembles the leech, but, as any one will at once perceive, the worm is not furnished with suckers by which it can assist itself to move. Instead of these rounded terminals the worm is finely pointed, and thus capable of boring its way through the earth. Progression is accomplished by moving first the head portion and then the next, so that a regular series of movements is necessary. The minute spines or bristles of the worm prevent its body being retracted by muscular effort.

The vital organs are rather forward of the centre of the body, and so if a worm be cut behind them it will survive and reproduce a tail. But the portion cut off will not be found alive, nor is it capable of forming a new perfect worm as generally supposed.

There are many other orders of worms which we can only indicate—viz., the TubicolÆ which surround themselves with a hard case; the Errantia, or sea-worms, sand-worms, etc., like the lobworm used for bait, and the naiads of our fresh-water ponds, all of which are suited to the aquatic life they lead. Indeed, of all the annelides, the earth-worm is the only specimen that is suited for living upon land. As regards the last mentioned, we may add that worms do not prey upon dead bodies as is so generally imagined. They are vegetable feeders, and do not burrow very deeply.

The transparent condition of the Rotifers renders them easy of observation under the microscope, and we find a nervous system, intestines, and a developed stomach. They are fresh-water inhabitants.

The Entozoa or “intestinal” worms claim a brief notice at our hands. The entozoa are those beings which inhabit, as parasites, the intestines and other parts of animals. Their history is still obscure, but there seems to be about twenty varieties of these creatures, and a great number of animals have their peculiar entozoa. The best known in the human subject are the “Ascaris” or thread-worm, the “Lumbricus Teres” or long-worm, and the “TÆnia” or tape-worm; this last is jointed, and grows to several yards in length.

The development of these TÆnia is one of the most curious performances of nature. Each of the joints shown in the illustration below (fig. 842) is a perfected and mature proglottis, containing the ova or eggs, which can only be brought to perfection when swallowed by a warm-blooded animal (not the same from which they emanated). The head within the embryo then holds to the tissues and penetrates to the alimentary canal, where only it can redevelop joints from the so-called head, which has no organs and merely pushes out immature joints which are continued, and they become more mature the farther they are pushed out by the new ones. The “measles” of the pig are produced by the ova of these worms.

Myriapoda.

The “many-footed” annulosa include the centipedes and millipedes, and may be regarded as a connecting link between the worms and the insects. The heads of these animals are distinct from the body.

The millipedes can be any day found under a large stone in a field which has not been tilled, or any place where a stone has been suffered to remain for some time undisturbed. These specimens are of the pill-millipede order, because they roll themselves up into a ball when disturbed. The myriapods of this country are not of large dimensions, but in tropical climates they attain a great size. The giant centipede has been found in South America more than a foot long, and is capable of inflicting severe wounds, its tenacity being extraordinary and equalling that of the bull-dog when once it has gripped its enemy.

The myriapods have no wings; they possess antennÆ, and numerous, never less than eighteen, feet,—frequently twenty pairs, but never a thousand, much less “ten thousand” feet, as the class name indicates. They are provided with strong forceps or “foot-jaws,” which supply a poison for killing their enemies. The millipedes and centipedes are known scientifically as IulidÆ and ScolopendridÆ respectively, and in most points of internal arrangement resemble insects, such as breathing by spiracles or (stomates), and trachÆ or tubes. Some of the centipedes possess electric qualities, and can administer a shock to an opponent.

Insecta.

Insects inhabit the world around us in myriad forms in air and earth and water. Some exist for a very brief space in the air; others live under water, or in trees, or in the ground; some burrow and hide in chinks of rocks and under stones. The numbers are countless, and all have some function to perform as palpably as the busy honey-bee, or as mysteriously as the giddy, careless butterfly.

Insects are divided into three distinct parts,—viz., the head, the thorax, and the abdomen, and each of these parts has a pair of legs attached to it, as will be perceived from the accompanying diagram. Along the body are tubes called trachÆ,—for insects do not breathe by lungs,—by which the air is carried into the system of the insect, by the “spiracles” or openings of fine network, to prevent dust entering the air-passages. The head is joined to the body by a constricted neck, the part of the body to which it is joined is called the thorax, and to this is added the posterior part or abdomen; this part is extremely various in form in different insects; in some it is round and full, in others long and extended. The antennÆ arise from the head, and are generally composed of eleven pieces variously disposed; these wonderful organs are possessed of great sensibility, and they certainly serve to convey information to the insect, of the nature of one of the special senses; it was formerly thought to be simply that of touch very much refined, or of smell, but it is now generally considered to be that of hearing, or a modification of it. The forms of the antennÆ are very various; fig. 845 represents that of the cockchafer (Melolontha vulgaris). The legs proceed from the thorax as do the wings, the abdomen giving rise to none of the extremities; the feet of insects are all pretty much upon the same model, some being more developed than others, they have a pair of hooks or claws for catching and clinging to rough surfaces, and a pair of cushions or pads, covered in some cases with suckers.

The foot of the common house-fly is most beautifully fitted for its progression and support. We have often wondered how the fly manages to support itself back downwards on the ceiling, or walk up glass. We give a cut of the fly’s foot (fig. 846).

The eyes of insects are also marvellous. There are only two, but each one is composed of numerous cells (ocelli), and look like a honey-comb. (See illustration, fig. 847.)

Insects swarm in innumerable companies, and no one who has not seen the locusts descending upon the earth can form more than a faint idea of the devastation they occasion in an incredibly short time. These, as well as thousands of other insects, exist in myriads, and we must content ourselves, on this occasion, by merely noting the different orders and their characteristics, after we have mentioned some of the attributes common to all.

The term “insect” means cut into or divided, and so the insecta are divided, as already mentioned, into three parts,—head, thorax, and abdomen,—the thorax being subdivided into three rings, pro-thorax, meso-thorax, and meta-thorax—beginning, middle, and end. All insects have six legs, and usually two or four wings, though some have no wings at all. The legs are united to the thorax, the antennÆ and eyes to the head. The abdomen contains the important sexual organs, a sting or defensive weapon, and in females the egg chamber.

Insects breathe by tubes in the sides, and consume a great quantity of air. Their powers of flying and leaping are too familiar to need dwelling upon. The wings display beautiful colours like those observable in the soap-bubble, others are covered with scales or hairs. The mouths vary very much with the species, as the manner of obtaining food is by suction or gnawing. The blood of insects is pale and thin.

The various transformations which insects undergo are always a subject of interest for the young student. The ugly forms which develop in beautiful creations are more astonishing than the change of the “ugly duckling” into the graceful swan.

Insects come to maturity only after undergoing successive changes from the egg to the perfect animal. The eggs (some of which are very beautiful) are first deposited in some safe place, either attached to a leaf or tied up in a small bundle by silken threads spun by the parent insect, and in some nutritious substance, so that when it comes to life it may at once have food; this is sometimes in manure, sometimes in flesh, and sometimes under the skin of a living animal (few are exempt from this infliction), where they remain for a time and then come forth as maggots, caterpillars, etc.; in this state they are called larvÆ,—these are generally active creatures and eat most voraciously, which seems to be the principal act of this state of their existence. These larvÆ frequently change their skins as they grow, and at last they assume the next stage of their life, the pupa or chrysalis state, which is one generally of complete inactivity; many of these larvÆ weave themselves a covering of a sort of silk, to defend them while in the pupa state,—such as the silkworm, whose covering (cocoon) is the source of all the silk of commerce,—others merely place themselves in a situation of security. The pupa remains dormant for a certain time, and then becomes the imago or perfect insect (the last state of its existence), such as a moth, a butterfly, a beetle, etc. These are of different sexes, and in due time produce a batch of eggs and then die; these eggs are often incredible in numbers, amounting to many thousands—but few escape the watchful eyes of other insects and of birds who feed upon them.

But there are some of the insecta which do not undergo metamorphosis; the Aptera or wingless insects include these, as the flea and such parasites which bore into other animals, and deposit their eggs within them.

Insects have very little means for making themselves audible, at least so far as can be ascertained. The humming of bees and flies and other insects are, of course, not intended to represent the voice. The cricket’s “chirp,” as people commonly imagine, but the sound is attributable to the rubbing together of the wings or wing-cases, as is the noise produced by the field-cricket. There is a very peculiar sound attributable to the “Death-Watch,” a ticking, and to nervous people terrible warning of dissolution. It may reassure some one, perhaps, to know that this “unearthly sound” is caused simply by the insect beating its head against a piece of wood to attract its mate, as the female glow-worm lights her lamp to guide her lord to her bower.

The Insecta may simply be divided into nine orders:—

The Coleoptera are well represented in England as beetles. They have four wings, but the outer pair serve as coverings to the inner ones. They are termed Elytra, and are horny in texture. These beetles are short-lived, but useful as scavengers, and serve to manure the ground by burying objectionable matter. The larvÆ of beetles eat tremendously. The stag-beetle is a formidable-looking animal, and the lady-bird is well known as an enemy to the aphides on our rose trees. The tiger-beetle, cockchafer, and various water-beetles belong to this family. The scarabee, or sacred Egyptian beetle, also will be found classed with the coleoptera. Many of these beetles are excellent scavengers, and some called burying beetles remove the soil underneath the carcases of birds and other small dead animals, letting them fall down below the ground level; the beetles then lay their eggs in the body, so that sustenance may be at hand for the young when hatched.

The Orthoptera include our cockroaches, miscalled “black beetles,” the locusts, crickets, etc. The ravages of the locust are well known. The larvÆ of the orthoptera has no wings, but otherwise is very like the grown insect. They change their skins frequently before they become perfect insects.

Passing the “nerve-winged” dragon-flies and caddis, whose larvÆ case is so familiar and useful as bait, we come to the very important and interesting order of Hymenoptera, with four membranous wings. In this rank we find bees, wasps, and ants, the first and last named being proverbial for industry and examples of almost superhuman reasoning powers, and a similitude to man’s arrangements in labour and house-building marvellous to contemplate. A study of the habits of ants, bees, and wasps will reveal a state of society existing amongst them which more nearly resembles man in feelings and habits, for these insects possess means of oral communication.

All these insects are armed with a sting, or other offensive weapon. The ant possesses the “formic” acid, which derives its name from the possessor. The destructive white ants will eat away a wooden house very quickly, sapping and mining it in all directions till it is a mere skeleton. The habits of bees are so well known and have been so often described that we need not detail them. The manner in which the ants “milk” the aphides is curious and interesting.

The Strepsiptera order includes very few species, so we may pass quickly to the Lepidoptera, the butterflies and moths, whose beautiful colourings and markings have attracted us all from childhood. There are about 12,000 species of the lepidoptera, and they are divided into “moths” and “butterflies,” the former being seen in twilight, or darkness, the latter in sunlight. They can readily be distinguished by the antennÆ, those of the butterfly being tipped, or knobbed. The silkworm belongs to this family. These insects undergo complete metamorphosis. The remaining two orders of insects include the house-flies and gnats; and the flea, and jigger, or chigo, which penetrates the skin and lays its eggs in the flesh, causing thereby dangerous inflammation.

Crustacea.

This class includes a number of familiar animals such as the barnacle, the crab, the lobster, shrimp, etc.; and curious as it may appear are closely related to our spiders. Their cases or coverings are all articulated or disposed in distinct segments. They breathe through gills or by tubes, and possess legs, or appendages for walking, eating, or guidance. They are generally marine creatures.

The shell of the crustacea is composed largely of lime, and of course becomes very hard in time. It is formed from the skin. The body, like that of an insect, is composed of head, thorax, and abdomen, divided into twenty-one segments, of which seven occupy the head, seven the thorax, and the remainder the abdomen. Twenty segments are furnished with legs, or feelers, or claws—a pair to a segment. The lobster or crayfish will give excellent examples of the anatomy of the macrura or lobster kind of crustacea. The heart is situated in the back.

The following table given by Professor Nicholson will explain the “segments and appendages” of the lobster:—

The tail is, as may be supposed, the great aid to locomotion in the lobster family, and they can swim backwards with great rapidity by its assistance. Lobsters shed their claws when alarmed, and are easily caught by a glittering bait.

The hermit crabs are interesting creatures, but do not possess the horny coat of the crab or lobster. They are therefore compelled to inhabit an empty shell, into which they thrust themselves, holding to the bottom of it by their tail, while a large claw guards the entrance. When the animal gets too big for his house he moves to another, leaving the old home for another hermit of the shore.

The crabs have no developed tails, and are therefore called brachyura—“short-tailed,” and they are walking creatures. There are king crabs, land crabs, and the common swimming crab. These animals can shed their shells as other crustaceans, and a curious fact is they shed them whole. How the claws come out must remain more or less a mystery. RÉaumur investigated the action of the crayfish, and noticed that as the casting time approached the crustacea retired to some hiding-place and remained without eating. The shell becomes gradually loosened, and at last by putting its feet against a stone and pushing backwards the animal jerks himself away. It must be a painful operation, for the mill-like teeth of the stomach are also rejected, and the joints do not give way. After a while a new shell appears, and is cast in due time as before.

The eyes of the crustacea are situated in front, and are composed like the insects, or are simple, like spiders. They possess a sense of hearing evidently. The eggs of the lobster are carried by the female, and they are termed the “coral” in consequence of their red and beadlike appearance. Our space will not admit of our saying much more concerning the interesting crustacea, though the barnacles, so well known by sight by all dwellers at the sea, and called Cirripedia, which fix themselves to rocks and ships, deserve notice. The young are capable of movement, and this fact was first discovered in 1830. It resembled a mussel, but when kept in sea-water it adhered to the vessel which retained it. The cirripedia are so called from the cirri or arms which they possess, and by which they are enabled to entangle or catch their food, as in a net. They hold themselves by a “foot stalk.” The goose-mussel, or barnacle, is very common, but must not be confounded with the limpet.

Dr. Baird gives the following description of them:—“The cirripeds are articulated animals contained within a hard covering composed of several pieces and consisting of calcified chitine. The body of the animal is enclosed in a sac lined with the most delicate membrane of chitine, which in one group is prolonged into a peduncle and contains the ova; the body is distinctly articulated and placed with the back downwards.

Arachnida—Spiders.

There are many families of arachnida besides the well-known garden and house spiders. The sea spiders, though classed with the arachnida, are sometimes placed amongst the crustacea. We have the “tick” and the cheese-mite and the scorpions; all of which belong to the spider family. But the true spiders are known by the joining of the two upper segments, the thorax and head being united (cephalo-thorax). The pretty and marvellous webs are spun from abdominal glands through small apertures. The fluid hardens in its passage sufficiently to be woven into threads to resist the struggles of the captured prey. The forms of these webs vary, but some spiders do not catch their victims in the net; they pounce upon them cat fashion. The large house spider is well known to all. The garden spider is seen in the illustration (fig. 855) with the scorpion. The habits of spiders will be found a very interesting study, and many volumes have been devoted to them. The water spider is a frequent inmate of an aquarium, and the bubble of air he takes down with him to breathe serves as a means of living while he is seeking his aquatic prey.

We will close our rapid survey of the invertebrate animals with a glance at the Mollusca, which are divided into six classes (see page 703). The first is the Tunicata, which have no shell or hard covering, and come under the denomination of molluscoids, and belong to a lower order. The true mollusca include the Brachiopoda, which have a pair of shells. They are called “arm-footed” because a long cord or tendon passes through one of the shells, and fixes the mollusc to the rock. The Lingula of this class have been discovered in very old formations such as the Devonian period, and indeed appear to have been amongst the first created animals.

The Lamellibranchiata include the oyster, cockle, mussel, etc. They are well known, and scarcely need description. The Pteropoda have no shell. The Gasteropoda are very numerous, and periwinkles, whelks, snails, etc., belong to this class. They progress by a muscular “mantle,” which is extended and contracted. The “horns” have eyes at the extremities. When they retire into their “houses” they can close the door by a kind of lid called the “operculum.”

The Cephalopods include the nautilus and the cuttle fish, the terrible squid, or octopus, etc. Wonderful tales are told of the tenacity and ferocity of the “Poulpes,” and no doubt in long-past ages these animals attained a gigantic growth. They are very unpleasant enemies, and the cold, slimy grasp of the long tentacles is apt to give one the “horrors,” while the terrible head and beak fill one with dismay. The poulpes are very formidable opponents, and discretion will certainly be the better part of valour when they appear in our vicinity.

We must here close our sketch of the Invertebrates, and we regret that the limits of our volume will not permit us to continue this interesting subject, nor can we find space, at present, for even the barest description of the Vertebrate animals.