On Molecular and Microscopic Science, Volume 2 (of 2)

The Annulosa, which are the lowest grade of articulated animals, consist of four distinct orders: the Entozoa, which are muscle and intestine parasites; the Turbellariæ, fresh and salt-water animals covered with cilia; the Annelida, or Worms; and the Rotifers, or Wheel animalcules.

Entozoa.

There are three genera and numerous species of Entozoa. Every animal has one or more species peculiar to itself; fourteen infest the human race. They have a soft, absorbent body of a white or whitish colour, in consequence of being excluded from light, and living as they do by absorbing the vitalized juices of the animals they infest. Their nutritive system is in the lowest state of development; yet there are some of a higher grade. All are remarkable for their vast productiveness.

The Tænioïdæ, which belong to the inferior group, are intestinal, many-jointed worms, which have neither mouth nor digestive organs; and what is called the head has only hooks and suckers to fasten it to the internal membrane of the animal at whose expense it lives. The common Tænia, or Tape-worm, sometimes ten feet long, which is the type of the order, has four suckers and a circle of hooklets round a terminal proboscis to attach it to its victim. Though destitute of the organs of nutrition it is extremely prolific, for each segment of its long flat body is a reproductive monœcious zooid, which forms and lays its own eggs exactly as if it were a single independent animal, thus furnishing a very remarkable instance of the law of irrelative repetition, which is a series of organs performing the same functions independently of one another. Two pairs of canals containing a clear colourless liquid extend throughout the body of the worm.

Bags, or vesicles called cysts, had been found in the glands and muscles of various animals, afterwards discovered to contain young worms, which attain their perfect development within such creatures as eat the flesh containing the cysts. Under circumstances so unprecedented, it required no small skill and patience to determine the life-history of these singular creatures. The cysts differ in size and form according to the genera, and are embedded singly or in groups in the flesh of their victim, on whose ready prepared juices they live.

The greater number of the Tænia genus begin their lives as sexless cysted larvæ, and on entering their final abode, segments are successively added till the worm has arrived at its adult state. The tape-worm of the cat has its origin in the encysted larvæ found in the livers of the mouse and rat. One species of Entozoa, while in its primary state, inhabits the stomach of the stickleback, and only comes to perfection within the aquatic birds that feed on this fish. Another species infests the livers of the salmon tribe, and gets its perfect form in the pike and perch.

Sheep and the hog are more tormented with cysted worms than any other domestic animals used for food. If introduced into the human intestines by eating raw ham or sausages, the larvæ soon acquire the perfect form. The eggs of the Tænia may be introduced into the human or animal stomach; for dogs and other carnivora which eat raw unwholesome meat are infested by full grown tænia, which fix themselves to their entrails by their hooks and suckers, while at the same time egg-bearing segments separate successively from their posterior extremity, and being voided scatter the eggs far and wide on land and in water.

The young of some Entozoa undergo various transformations, as those of the Distoma of the Lymnæa. When full grown that entozoön is like a sole, flat, broad, and long, with a kind of head at the broad end, and two suckers on its under-surface, in one of which there is a pore serving as a mouth, whence an alimentary canal extends, which spreads in branches almost throughout the whole body. This animal has a filamentary nerve round its gullet, from which minute fibres pass to the mouth, and two filaments extend backward on each side as far as the second sucker. The eggs which occupy the whole margin of the body are developed into worms, each of which seems to be merely a mass of structureless cells enclosed in a contractile case. By a second change each of these cells is transformed into a freely swimming ciliated zooid endowed with eyes. Having escaped from their contractile case, they remain for a time in that state, and then imbed themselves in the mucus on the foot of the fresh-water mollusk Lymnæa, or pond snail, where they are transformed into true Distomata, and ultimately enter into the body of the Lymnæa itself, where they lose their eyes and cilia, which are no longer of use in their dark and permanent abode. The Fluke found in the livers of sheep that have the rot is a Distoma.

The Nematoid order, or thread-worms, that live in the muscles of men and animals, are long, smooth, and cylindrical, with a structureless skin covering layers of longitudinal and circular fibres, by means of which they can stretch and contract themselves. They are generally pointed at both ends with a mouth at one extremity and an orifice at the other. The Filariæ are slender, sometimes of great length, as the Guinea worm, which varies in length from six inches to two, eight, or even twelve feet. In Persia they are believed to be introduced into the system by drinking water in which their eggs have been deposited. This worm may grow in the muscles of a man to the size of five or six feet without giving much annoyance, but when its head bores through the skin it produces a painful sore unless extracted. In Persia, where the worm is common, the natives seize it by the head, draw it carefully out, and wind it round a bit of wood, an operation which may require several days to accomplish. It has a numerous viviparous progeny, which come out through the mouth. There are certain very small slender species of Filaria which attack the eyes both of men and horses; some bury themselves close to the eye, and a very minute kind enters the ball itself.

The Ascaris lumbricoïdes, a common intestinal thread-worm of the hog, ox, and the human race, is sometimes of great length. The sexes are distinct, and their fertility enormous. The ovaries are two tubes sometimes several feet long, in each of which the eggs are arranged in whorls round a central stem, like the flowers of a plantago. By counting the number of microscopic eggs in a whorl, and the number of whorls, Dr. Eschricht ascertained that in a full grown female the average number of eggs amounted to sixty-four millions. In this species of worm the embryo is not developed from the egg while within the victim, so that most of the eggs perish.

Different species of Anguillulæ, which are minute eel-like worms slender as a hair, inhabit the alimentary canal of fresh-water snails, frogs, and fishes, but many species are not parasitic. These are often united in swarming masses that nestle in mud, wet moss, wet earth, and aquatic plants. One species causes the cockle in wheat, appearing like a living tuft of white wool in the blackened grains. They appear in sour paste and in other decomposing substances, and are so tenacious of life that, after being completely dried for months, and apparently dead, they revive on being moistened.

Turbellariæ.

The Turbellariæ are fresh- and salt-water animals, distinguished by having the whole surface of their bodies covered by cilia, under which in some species there are thread-cells containing six, eight, or a greater number of darts. Most of the members of this tribe have elongated flattened bodies, and move by a sort of crawling or gliding motion over the surface of aquatic plants and animals. Some of the smaller kinds are sufficiently transparent to allow their internal structure to be seen by transmitted light. The mouth, which is situated at a considerable distance from the rounded end of the body, opens into a sort of gullet leading into the stomach, which has no other orifice, but a great number of branching canals are prolonged from it, which carry its contents into every part of the body. A pair of oval nerve-centres are placed near the rounded end of the animal, whence nerves extend to various parts of the body; and near to these there are from two to forty rudimentary eyes according to the species, each of which has its crystalline lens, its pigment layer, nerve bulb, and its cornea. The power of the Planaria to reproduce portions which have been removed is but little inferior to that of the Hydra.

The Annelids are the most highly organized of all the worm tribe. They are exceedingly numerous and varied; some are inhabitants of fresh water, others are terrestrial, but by far the greater number and most highly endowed are marine. They generally have a long, soft, and smooth body, divided or marked by transverse rings into a succession of similar segments. In many the first and last segments are alike; in others the first segment can scarcely be called a head, though it exercises several functions, while in the highest two orders the head is the seat of several senses. On each side of the bodies of the Annelida there are one or two long rows of tufted bristles or feet, which may be regarded as the earliest form of symmetrical locomotive organs. Most of the Annelids have ocelli or eye-specks, and in many of them the head supports soft cylindrical tentacles, which are obviously organs of touch. These worms are divided into four orders, the Suctorial, Terrestrial, Tubercular, and Errantia, or Wandering Worms.^[32]

The first order consists of Leeches of different kinds: their body is long, slightly segmented, with a suctorial disc at each end. Their skin is smooth, whitish, and translucent; beneath it are cells filled with brown or greenish matter, and three layers of muscular fibres follow; the first are transverse, the second cross one another diagonally so as to form a network, and the third are longitudinal. The mouth, which occupies the centre of the principal sucking disk, varies in form with the genera. In the common leech it has an enlarged lip, and opens into a short gullet leading into a capacious and singularly complicated stomach, divided by deep constrictions into eleven compartments, the last of which is connected with an intestinal canal, which ends in a vent in the middle of the terminal sucker.

Within the mouth there are three crescent-shaped jaws, presenting their convex edges towards the cavity of the mouth, beset with from seventy to eighty teeth, formed of a highly refractive crystalline substance resembling glass. The leech makes a vacuum with its sucker, which forces the part to which it is applied into contact with the three-toothed jaws, which are moved sidewise by strong muscles, and saw through the skin and small bloodvessels below it.

The leech, like the other Annelids, has two distinct systems of circulating liquids, one red, the other colourless. The red liquid or blood is kept in circulation by the pulsations of a heart, or rather a contractile vessel behind the head. It is carried away from the heart by a pulsating canal passing along the back of the leech, and is brought back to the heart by a similar canal extending along its ventral side. During this course, portions of the liquid are sent off through veins to different parts of the body. The respiratory organs of the leech are pores arranged at regular distances on each side of the body which open into little sacs having capillary bloodvessels distributed under the skin through which the blood is aërated.

The colourless liquid which contains many organic molecules, occupies the space between the alimentary canal and the inner wall of the body, from whence it passes into canals which ramify extensively, but are not furnished with returning passages. This liquid forms a support to the muscles of the skin, and is kept in circulation by the motions of the leech.

Fig. 131 shows the highly developed nervous system of the leech. From the double lobe of the brain ten optic nerves go to the bases of ten black eye-specks, which mark at equal distances the upper margin of the expanded lip. A nerve-centre below the gullet supplies the lip and jaws with strong nerves. A double longitudinal cord, united at equal distances by twenty-one double nerve-centres, extends from a ring round the gullet throughout the whole length of the body, supplies the different organs with nerves, and ends near the vent in a nerve-centre, from whence nerves radiate through the terminal sucker.

The circulation of the blood and of the colourless liquid, as well as the nerve system, prevail generally in the Annelids, modified by the structure of the individual.

The leech, though greedy of blood, lives in fresh-water ponds, wet grass, and damp places, where it never can meet with warm-blooded animals. It probably lives on minute aquatic insects.

The common Earth-worm, which is a principal member of the second order of Annelids, has a more important part assigned to it in the economy of nature than its humble appearance leads us to suspect. It has a long, soft, cylindrical body tapering to a point at both ends, divided into numerous rings. The mouth is furnished with a short proboscis, or snout, without teeth. A long salivary glandular mass surrounds a short wide gullet, which leads to a digestive organ similar to a gizzard, whence a canal is continued to the vent. The circulation of the two fluids, and the nervous system modified at head and tail, are like those of the leech. Four rows of minute bristles extend longitudinally along the ventral surface of the worm, two on each side. With a low magnifying-power they appear to be minute points regularly pushed out and drawn in; but when more highly magnified each point is seen to consist of two transparent glassy rods having their points bent backwards: on these feet the worm crawls very rapidly.

While making its cylindrical burrow a slimy mucus exudes from the body of the worm, which cements the particles of earth together and renders the walls of the burrow perfectly smooth and slippery. When the worm pierces the earth it stretches its snout into a fine point that it may penetrate more easily, and when it is fixed, it draws its ringed body towards its head by a muscular effort; and to prevent it from slipping back again, it fixes the hooks of its posterior feet firmly into the ground. Having thus secured a point of support it penetrates deeper into the earth, draws up its body, fixes the hooks of the posterior feet into the smooth surface of the burrow, and continues the same process till the burrow is deep enough. Thus the feet are employed as points of resistance for the exertion of muscular force. This worm swallows earth mixed with decaying animal and vegetable matter, assimilates the nutritive part, and casts out the refuse in the form of fine mould, which may be seen in little heaps at the edges of their burrows. In fact, nearly all the fine vegetable mould so precious to gardeners and farmers has passed through the intestines of the common earth-worm.

There is a colourless little fresh-water species of the genus Naïs, remarkable for the beauty of its bristled feet. There are two pairs on each ring of the worm, consisting of wart-like perforated protuberances, through which a number of microscopic bristles protrude, arranged in a radiating pencil like a fan. They are very slender, bent at the tip, and so transparent that they look like threads of spun glass; the worm thrusts them out and draws them in with extreme rapidity.

A blood-red Naïs lives in burrows in the mud at the bottom of springs and pools in immense multitudes; large tracts of the mud of the Thames are red with a species of them; half of their bodies stretched out of their burrows maintain a constant oscillating motion on its surface, but, like the earth-worm, they instantly shrink into their burrows on the least alarm. They have no respiratory organs; but their blood is aërated through their skin, which is so transparent that, with a microscope, the whole of the internal structure, the motions of the liquids, and the particles they contain are distinctly visible. The blood acts the part of internal gills, by aërating the colourless liquid contained in a set of vascular coils surrounding the organs of digestion.

The Tubicola are marine worms, forming the third order of Annelida, according to the system of M. Milne-Edwards. They live in tubes, either of a shelly calcareous substance, which forms naturally on the tenacious mucus of their skins, or in tubes artificially constructed by themselves of sand and particles of shell glued together. All the Tubicola can protrude their gills and the anterior part of their bodies, and some can leave their tube and return to it again. These worms, which form beautiful objects for the microscope, have ringed bodies with tubular bristled feet, and respiratory organs or gills fixed either on the head or near it. They have an alimentary canal loosely attached to the ventral wall of the body, and two systems of circulating liquids, one red, the other colourless. In the Tubular Annelids the principal organs of respiration are the contractile plumes on the head.

In the Terebella there are distinct organs for the aëration of both liquids, which form a beautiful plume when expanded, as in fig. 133, which shows the animal when out of its tube. What may be called the head is fixed upon the first ring of the body. The mouth has a lip like a funnel-shaped cup with numerous long slender tubular tentacles; and two delicate arborescent branches or gills are fixed immediately behind the head. The colourless liquid which occupies the space between the alimentary canal and the ventral wall of the worm, is sent by the contractions of the body into the slender tubular filaments round the mouth, which are covered by cilia, whose action continually renews the stratum of water in contact with them. The blood in its usual course enters the capillary tubes of the arborescent gills, where it is oxygenized, and, after being distributed to the different parts of the body, returns to the heart and gills again.

The slender filaments which radiate from the head of the tubicular worms are flattened, sometimes tortuous, always ciliated, and are often barred and variegated by bright purple, green, and yellow tints, forming a rich and gorgeous crown.

The mucus, which cements together the particles of sand and shell for the artificial tubes of this kind of worms, is believed to be secreted from glands in the first segment of the body; but the long slender filaments of the head are the active agents in the structure. The tentacles are hollow bands with strong muscular edges, which the worm can bring together so as to form a cylinder, at any point of which it can take up a particle of sand, or a whole row of particles, and apply them to its glutinous body. The fibres at the free ends of the tentacles act both as muscular and suctorial organs; for when the worm is going to seize a particle of sand or food, the extremity of the tentacle is drawn in by the reflux of the colourless liquid in its interior, so that a cup-shaped cavity is formed in which the particle is secured by atmospheric pressure, aided by the power of the circular muscular fibres at the extremity of the tentacle.

The Serpula and its allies are richly-coloured worms, living in contorted tubes with lids, frequently seen encrusting rocks, the shells of oysters, and other mollusca. By a peculiar mechanism of their bristly feet they can open the lid of their tube, push out their fan of gorgeous tentacles, pull it in again, and shut up the tube. As the protrusion of the worm from its tube is slow, cautious, and gradual, the retreat swift and sudden as lightning, there are two distinct sets of organs in the feet by which these motions are performed.^[33]

On the back of the worm there is a sort of shield, the sides of which bear seven pairs of wart-like feet, which are perforated for the working of protrusile microscopic bristles (fig. 134). Their upper parts are double-edged, with a groove between them, and serrated with close-set teeth. The organs of retreat are much more complicated and numerous. Mr. Gosse has computed that there are about 1,900 blades on the seven pairs of feet, each movable at the will of the worm, and that there are nearly 10,000 teeth hooked into the lining of the tube when it wishes to retreat. The manner in which it comes out of its tube and retires into it again is the same as that employed by the earth-worm.

There are twenty-four genera of the order Errantia, or wandering sea-worms. Multitudes swarm on every coast; they have considerable muscular strength, and are highly irritable; some are called sea-centipedes, from the number of their feet and length of their segmented bodies, which are slender, and vary from a few inches or less to thirty-five or forty feet. They are generally coiled up under stones, or wander by the slipperiness of their smooth skins through masses of sea-weeds or shells at low tide. In most of them the rings are decidedly marked; the first and last segments are unlike, while the rest are mere repetitions one of another. Their locomotive organs are a pair of perforated fleshy warts on each of their numerous segments, through which groups of rigid, simple or barbed bristles are protruded and retracted.

The Errant Worms have a distinct small head with a mouth, or rather an orifice, on the upper side of it, through which a cylindrical gullet is from time to time turned inside out, forming a kind of pear-shaped bag, whose surface is studded with secreting glands; and its extremity, which is perforated, is surrounded by a muscle that contracts strongly on whatever it is applied to, and holds it firmly while the re-inversion of the sac draws it into the body to be digested. This apparatus is unarmed in the genera Arenicola, Phyllodoce, and others, but in the Nereis it has one pair of strong curved horny jaws. In the Eunice there are three toothed jaws on one side and four jaws on the other side of the gullet, each pair having a different form, and the tiny Lombrinereis has eight little black hooks which are seen through its pellucid tissues, snapping like so many pairs of hooked scissors. The Errant Worms are voraciously carnivorous, and when the gullet is turned inside out the toothed jaws project, seize the prey, and drag it into a ciliated alimentary canal, for there is no proper stomach in these worms. The canal is generally straight, and terminates in a vent at the posterior end of the body.

The respiratory organs of the Errantia are external gills of great variety of forms: they are chiefly like branching trees, or filamentary bushes, traversed by capillary bloodvessels. They are sometimes small, and arranged on every segment along both sides of the back; sometimes they are large and fixed only at intervals. Like the lower Annelids, they have two liquid systems, one red and the other colourless, and the circulation of the blood is the same; but as the pulsations of the vessel behind the head are too feeble to send the blood through the labyrinth of capillary vessels in these long worms, there is a supplementary heart, or pulsating vessel, in each segment of the worm, which partakes in and facilitates the general circulation.

The Eunice and other very long worms may have hundreds of these centres of propulsion, which make the circulation rapid; and it is increased by the restlessness and activity of the worms themselves, which bring their gills perpetually into new strata of water.

The nervous system of the Errantia consists of a double cord extending along the ventral side of the body, and united at equal intervals by double nerve-centres, as in fig. 131; but in the Annelids the two cords diverge below the gullet, surround it, unite again above that tube, and form a principal bilobed nerve-centre or brain. Each segment of the worm is occupied by a small double nerve-centre. In some of these marine worms there are hundreds of segments and as many nerve-centres. There are more than a thousand of these pairs of nerve-centres on the ventral cord of the Nemertes gigas, or Great Band Worm, which is sometimes forty feet long and an inch broad. The head is like a snake, and the bristled feet are jointed to enable it to move over hard surfaces.

The movements of the bristly feet of the Errantia are reflex, depending on the nerve-centres in their segments; but they are controlled and connected by the double cord which passes through them.

Every hair, cirrus, and tentacle on the bodies of the Errant Worms is a living organ of feeling, shrinking at the smallest touch, but enabling them to select their food, to move towards and retreat from objects, and to thread their way through the most intricate labyrinths with unerring certainty, which seems to render them independent of eyes; yet many of them have multitudes of eyes, or rather eye-specks, according to the genera. Some have but one eye-speck placed in the forehead; one genus has a double row throughout their whole length, two in each segment, while the Amphicora has two in its tail. All these eye-specks have their crystalline lens, pigment-layer, and nerve-bulb, so that the Errant Worms must see objects, and their motions show that they do; but we can form no idea of the kind of vision.

Besides the variety of organs on the skin of the Errantia, some of these worms have two rows of flat plates on their backs overlapping each other at their edges like the scales of a fish. They are well developed in the Aphrodita hystrix, or the Sea Mouse of fishermen, and its congeners. That Annelid, which is an inhabitant of European coasts, is thicker and broader than other sea-worms. The two rows of overlapping shields on its back, and the quantity of iridescent hairs, cirri, and other appendages covering the body, is so great as to form a kind of felt or fur like the skin of a mouse. The members of this genus of sea-worms have no gills properly so called; the only external sign of respiration is a periodical elevation and depression of the shields on their backs by the action of a complex system of muscles. The thick covering of felt on the body of the worm below the shields becomes filled with water during their elevation, which is ejected forcibly at the posterior end of the body during their depression. Although the water does not penetrate the thin skin on the back of the worm, its oxygen does, and is accumulated in the colourless liquid in which the stomach floats; and from it the blood, which is of a pale yellow colour, receives its oxygen. The feet of the worm are fan-shaped groups of sharp glassy bristles enclosed between two plates, which prevent them from hurting the animal when it puts them out or draws them in. The Aphrodita is male and female: the eggs escape through pores in the female, and are received in a kind of pouch beneath the dorsal shields till hatched. The embryo is an oval locomotive mass, with groups of cilia, and indications of an eye-speck: after swimming about for twenty-four hours, the segments begin to be developed.

Worms of the genus Polynoë have also two rows of shields on their backs, but they are studded with transparent oval bodies on short stems, supposed to be organs of touch. The filiform tentacles and antennæ that are developed between the shields, as well as the cirri or curly bristles of the feet, are likewise covered with similar sensitive organs. Fig. 135 shows the foot, cirri, and bristles of a Polynoë, which are enclosed in plates which preserve them from hurting the worm. These glassy bristles are beautiful objects under the microscope; still more so are the jointed feet, transparent as the purest flint glass, of the Phyllodoce viridis, one of the most beautiful Annelids on our coasts, where it threads its way among young mollusca like a slender green cord, exhibiting foliaceous gills in the highest perfection.

In the marine Annelids the embryo, on leaving the egg, is a gelatinous globular mass of cells furnished with strong cilia. In a few hours the mass elongates and divides into four parts, a head, a large ciliated segment, a smaller one without cilia, and a ciliated tail. After a time a succession of new segments are interposed, one by one, next to the tail segment, and the corresponding internal organs of each are developed till the worm arrives at its adult state. In many Annelids the embryo is highly developed within the parent; that of the Eunice has from 100 to 120 segments before it leaves her; and in the Nereis diversicolor the young, covered with cilia, come out by hundreds at an orifice in the side of the mother.

Many of the marine Annelids are luminous; electric scintillations are given out during the act of nervous contraction, which are increased in brilliancy and rapidity by irritation.

According to Professeur Quatrefages, the Annelida Errantia and Tubicola have no zoological regions characterized by one or more special types like the other classes of animals; they have representatives in all seas. But it is exactly the contrary with regard to species. The number of species common to any two seas, or the shores of two continents, is very small; there is not a single species common to the Atlantic coasts of France and the Mediterranean. The sea-worms are not affected by climate, but they are said to be more abundant on granitic and schistose coasts than on the calcareous.^[34]

With regard to fossil remains, worm-tracks are seen in the Forest marble, long calcareous tubes occur in the Upper Silurian and Carboniferous strata, and in all the later formations tubercular Annelids abound, especially of the genera Serpula, Spirorbis, and Vermilia.^[35]

Tardigrada.

The Tardigrades are slow creeping animalcules, which seem to form a link between the Worms and the Rotifers, though they are more nearly allied to the former in having a vermiform body divided transversely into five segments, the first of which is the head, and each of the others has a pair of little fleshy protuberances furnished with four curled hooks. They resemble the Rotifers in their jaws, in their general grade of organization, and in the extreme length of time they can remain dried up without loss of life. When in the dried state they can be heated to a temperature of 250° Fahr. without the destruction of life, although when in full activity they cannot endure a temperature of more than from 112° to 115° Fahr. When alive the transparency of their skin is such as to show a complicated muscular system, the fibre of which is smooth; and as no respiratory organs have yet been found, their respiration must be cutaneous. These animalcules have no nerve-centre in the head, but they have one in each segment of the body; and they are furnished with a suctorial mouth at the end of a retractile proboscis, on each side of which are two tooth-like styles, the rudiments of lateral jaws. The structure of these creatures is microscopic.

Rotifera.

Although the Rotifera are microscopic objects, their organization is higher than that of the Annelida in some respects. They are minute animalcules, which appear in vegetable infusions and in sea-water, but by far the greater number are found in fresh-water pools long exposed to the air: occasionally they appear in enormous numbers in cisterns which have neither shelter nor cover; a few can live in moist earth, and sometimes individuals are seen in the large cells of the Sphagnum or Bog-Moss.

The bodies of the Rotifers have no cilia; they are perfectly transparent, elongated, or vermiform, but not segmented; they have two coats, both of which in some genera are so soft and flexible that the animal can assume a variety of forms; while in others the external coat is a gelatinous horny cylindrical shell or tunic enclosing the whole body except the two extremities, which the animal can protrude or draw in. The soft kind can crawl over solid surfaces by the alternate contraction and extension of their bodies like a worm, and the stiff Rotifers are capable of doing the same by the contractility of their head and tail. All can swim by means of cilia or lobes at their head. The greater number possess means of attaching themselves to objects by the posterior end of their bodies and of removing to another place.

The wheel-like organs from which the class has its name, are most characteristic in the common Rotifer (fig. 137), where they consist of two disk-like lobes projecting from the body whose margins are fringed with long cilia. It is the uninterrupted succession of strokes given by these cilia, passing consecutively like waves along the lobes, and apparently returning into themselves, which gives the impression of two wheels in rapid rotation round their axes.

The Brachionus pala (fig. 136) affords another instance of the two-wheeled Rotifers. Though of unusually large dimensions in its class, it is just visible to the naked eye as a brilliant particle of diamond when moving in a glass of water. Its transparent horny tunic, when viewed in front with a microscope, is a cup of elegant form, bulging at the sides. One side of the rim is furnished with four spines, of which the middle pair are slender and sharp as needles, with a deep cleft between them; the other side of the rim is undulated but not toothed, and the bottom of the cup ends in two broad blunt points.

Between the terminal blunt points there is a round opening for the protrusion of the foot of the animal. The tunic is of glassy transparency, so that every organ and function of the animal can be traced with perfect distinctness.

The foot of the animal is long, rough and wrinkled, not unlike the flexible trunk of an elephant. It can be lengthened, shortened, drawn within, or pushed out of the tunic in an instant. It terminates in two short conical fingers or toes, which can be widely separated or brought into contact. By means of these, the Brachion has the power of mooring itself even to the smooth surface of glass so firmly, that it can stretch itself in all directions, shaking itself to and fro with sudden violence without letting go its hold. The Rotifers usually fix themselves before they set their wheels in motion in search of food.

From the anterior rim of the shelly cup, the Brachion protrudes a waved outline of limpid flesh which, as soon as it rises above the level of the sharp-pointed spines, spreads out into three broad flattish muscular lobes. On the edges of the middle one there are very strong cilia like stiff bristles, which do not vibrate, but are either erect or converge to a point, whereas the edges of the other two lobes are thickly fringed with long stout cilia, which, by striking the water in perpetual rapid succession, each cilium bending and rising again, produce the appearance of two circles of dark spots in rapid horizontal rotation, like wheels on their axis. It is merely an optical deception, for both the animal and its lobes may be at rest. The vibrations of the cilia can be instantaneously arrested, and the whole apparatus drawn out of sight, and as instantaneously protruded and set in motion.

In the flesh, on the ventral side of the Brachion, there is a deep cleft, the edges of which as well as the whole interior of a tube of which it is the orifice, are thickly covered with vibratile cilia. This tube leads to a mouth with powerful jaws of unwonted structure, which is so deeply sunk in the tissues of the body, that it never comes into contact with the water. It opens into a gullet leading to a stomach, intestine, and vent, at the posterior end of the body.

The vibrations of the cilia on the lobes of the animal’s head form two circular currents in the water, like whirlpools, which draw all floating particles into their vortices, and the streams from the two whirlpools uniting into one current, flow off horizontally and pass immediately over the slit on the ventral side of the animal. Some of the floating particles are arrested by the cilia on the edges of the slit, and are drawn into the sunken mouth by the vibrations of the cilia in the tube. The edges of the slit act like lips, and seem to possess the sense of taste, or of some modification of touch, which enables them to select from the particles presented to them, such as are fit for food; these are admitted into the mouth, where they are bruised by the powerful jaws. The mouth or masticating apparatus is the most extraordinary and complex part of this animal. It consists of two horny toothed jaws, acting like hammers upon an anvil. The two hammers, which approach each other from the dorsal sides of the body, are each formed of two parts united by a hinge; the first parts correspond to the handles; the second parts, which are bent at right angles to the first, resemble hands with five or six finger-shaped teeth united by a thin membrane. The teeth are parallel to one another when they meet on the anvil, and are seen through the transparent mass tearing the food into fragments. Some of the Rotifers resemble the Errant Annelids in being able to turn this complicated machine inside-out through the ciliated tube and slit, so as to bring it into contact with the water. When the food has been masticated it is sent into the stomach, where it is digested. The whole of this process is seen through the transparent and colourless body of the Brachion, because its favourite food is the Syncryn velox, a minute bright green plant, which from its active motions was at one time believed to be an animal.

The Brachion has four longitudinal muscular bands transversely striated, which move the ciliated lobes of the head, push them out and draw them in. From these muscular threads are sent to the different parts of the body, to the mouth especially, two strong bands, which bend and unbend the joints of the hammer-like jaws. The vigorous motions of the long serpentine foot and the firm hold of its anchors are owing to muscular bands fixed high up on the interior wall of the body, which extend throughout the whole length of the flexible organ. As long as the Brachion is fixed, the vibrations of the cilia on its lobes only produce whirlpools in the water, but the moment that it lets go its hold, these vibrations, in consequence of the reaction of the water, give the animal both a smooth progressive motion and a rotation round its axis.

Minute as the Brachionus pala is, it has several organs of sense. A sparkling, ruby-coloured, square eye-speck with a crystalline lens and crimson pigment layer is placed on a wart-like prominence on its back, and this prominence Mr. Gosse believes to be the brain of the animal. In the cleft between the spines and close to the eye-speck are two tubes, one within the other. The innermost tube, which can be protruded and withdrawn, has a bunch of bristles at its extremity that have the sensibility of antennæ. Nerves from the brain pass into these, to the various organs of the body, and to the lobes on the head.

The Brachion has no propelling vessel or heart to maintain the circulation of its liquids, but, like the Annelids, a colourless liquid occupies the general cavity between the alimentary canal and the internal wall of the body. It is believed to be connected with nutrition, and is furnished with oxygen by a complicated organism, and is kept in motion by the vibrations of long cilia. The determination of the whole structure and motions of a creature barely visible to the naked eye, is a wonderful instance of microscopic research, and of the perfection of the mechanism exhibited in the most minute objects of creation.

Fig. 137 represents the common Rotifer when its wheels are expanded and when they are retracted. The body is slender and flexible, it is stretched out by longitudinal muscles, and its girth is diminished by circular ones. The internal structure is similar to that of the Brachion, but there is a prominence or head between the wheels on which there are two crimson eye-specks, and the foot terminates in three concentric movable tubes that can be protruded and drawn in like the tubes of a telescope; each has a pair of claspers to enable the Rotifer to fix itself to any object.

The Rotifers are male and female, but, like the greater number of Infusoria, the males are only produced at intervals. The female Rotifers have their perfect form when they leave the egg: they even come out of the egg while it is attached to the tail of the mother, as in the Brachionus pala (fig. 136). The males, when hatched, have neither spines nor mouth, yet, during their short lives, their motions are very fleet on account of the vibrations of long cilia round their front.

Some Rotifers are remarkably fertile. Professor Ehrenberg estimated that, in the course of twenty-four days, the offspring of a single individual of the genus Hydatina might amount to seventeen millions. Female eggs laid in autumn are collected in heaps and covered with a gelatinous substance, which protects them from the cold in winter, though the Rotifers themselves are sufficiently protected by their great tenacity of life. They revive after being frozen; they may be dried for an unlimited time, but, as soon as they meet with warmth, moisture, and food, they resume their vitality.