Zoophytes are animals of a much higher organization than the Protozoa, inasmuch as they are furnished with special organs of prehension, offence and defence, of attachment, and in many of locomotion. For the most part they consist of numerous individuals called Polypes, united in a community, and living together in intimate sympathy and combined action, so as to form one single compound animal.

Zoophytes are divided into two groups, namely the Hydrozoa, whose type is the common fresh-water Hydra, and the Actinozoa, which are composite animals, including the reef-building corals, whose polypes are formed according to the type of the Actinia, or common Sea Anemone. The Hydrozoa consist of seven orders, the first of which are the Hydridæ, inhabitants of fresh water; the next constitute the oceanic Hydrozoa, some of which, though extremely varied in form, are connected by the most wonderful relations.

The solitary Hydra that lives in fresh-water pools and ditches, consists of a soft cylindrical muscular bag, capable of being stretched into a slender tube, shrunk into a minute globe, or widely distended at will. At one end there is a circular mouth, which is highly sensitive, opening, closing, or protruding like a cone, and surrounded at its base by six long flexible arms called tentacles, arranged symmetrically. The mouth opens into a cavity extending throughout the length of the body, which is the stomach; the other end of the sac is narrow, and terminates in a disk-shaped sucker, by which the Hydra fixes itself to aquatic plants, or floating objects, from whence it hangs down, and the tentacles float in the water.

The sac or body is formed of two layers, an inner and an outer layer, of firmer texture, formed of cells imbedded in a kind of sarcode, and the space between the two layers is filled with a semifluid substance, mixed with solid particles and full of vacuoles. The inner and outer layers are united at the mouth, and the tentacles are closed tubes in communication with the cavity of the stomach. The exterior layer of the tentacles is beset with wart-like excrescences, formed of clusters of cells, with a larger one in the centre filled with a liquid. In all of them a long spicula, or sting, often serrated at the edge, is coiled up like a thread, and fixed by one end to a kind of tube, like the inverted finger of a glove, that the animal can dart out in an instant.

Thus armed, the tentacles are formidable weapons; they are highly contractile and wonderfully strong, tenaciously adhering to the small worms and aquatic insects on which the Hydræ feed, and they are aided by the roughness of their surface. They transfix their prey, and are believed to infuse a liquid poison from the dart, or thread-cells, into the wound, then twisting their other tentacles round the victim, it is instantly conveyed to the mouth, and slowly forced into the digesting cavity, where it is seen through the transparent skin to move for a short time, but as soon as the nutritious juice is extracted, the animal ejects the refuse by its mouth. In the inner layer, enclosing the cavity of the stomach, there are cells containing a clear liquid with coloured particles floating in it, which is supposed to perform the part of a liver; and, as the Hydræ have no respiratory organs, their juices are aërated through their skin. They have no perceptible nerves nor nerve centres, yet they are irritable, eminently contractile, and are attracted towards the light—all these being probably sympathetic motions.

Though in general stationary, the Hydra can change its place; it bends its body, stretches to a little distance, and fixes its anterior extremity firmly by its tentacles; then it detaches its sucker and brings it close to its mouth, fixes it, and again stretches its fore part to a little distance along its path, and repeats the same process, so that it moves exactly after the manner of certain caterpillars. It can even move along the water by attaching the expanded disk of its sucker to the surface, where it soon dries on being exposed to the air, and becomes a float, from whence the Hydra hangs down with its tentacles extended like fishing lines, as in fig. 110; or it can use them as oars to row itself along under the surface of the water.

On account of their simple organization, the Hydræ are endowed with the most astonishing tenacity of life. As the whole animal is nourished from the surface of the digestive cavity, they appear to suffer no inconvenience from being turned inside-out, the new cavity performing all the functions of digestion as well as the old one. They may be cut into any number of pieces, and, after a little time, each piece becomes a perfect Hydra. The head may be cut off and they get a new one; or it may be split into two or three parts or more, and the animal becomes many-headed; and, what is still more marvellous, two Hydræ may be grafted together direct, or head and tail, and they combine into one animal.

These singular and voracious creatures increase like plants by budding. A little protuberance rises on the body by the bulging out of the double skin or wall, so that the interior of the bud is a clear cavity in communication with the stomach of the Hydra (fig. 110, b). The bud increases in length, opens at its extremity into a mouth, and gradually acquires the size and form of its parent (fig. 110, c); the communication is then by degrees closed, and at last the matured bud drops off and becomes an independent Hydra. Dr. Carpenter observed that this process, which so closely resembles the budding of plants, must be regarded as a modification of the ordinary nutritious process. The same may be said of the power of reparation, which every animal body possesses in a greater or less degree, but which is most remarkable among the lower tribes, for when an entire member is renewed, or even when the whole body is regenerated from a small fragment, which is the case in many polypes, it is by a process exactly analogous to that which takes place in the reparation of the simplest wound in our own bodies, and which is but a modification of the process that is constantly renewing, more or less rapidly, every portion of our frame.

There is but one species of the single colourless Hydra, but there are four compound fresh-water Hydræ in England—the rubra, viridis, vulgaris, which is of an orange brown, and the fusca. They have coloured particles, either imbedded in their external coat, or immediately under it. The Hydra viridis and H. vulgaris have short tentacles, whilst H. fusca, which is a rare animal, has arms from seven to eight inches long, and so contractile, that they can shrink into the space of small tubercules. All these four Hydræ are compound and permanently arborescent animals; each springs from one individual hydra of its own race, which increases in length and forms the stem, while young ones spring from it and from one another consecutively, like the compound branches of a tree. The numerous tentacles that hang down like fishing lines, thickly covered with thread-cells and their envenomed darts, catch prey for the whole colony, because the communication between the stomachs of the young polypes or Hydræ and that of their parent is never cut off, as it is when the offspring is deciduous; but tubes from the base of each individual Hydra or polype, passing through the stalks and branches of the living tree, unite their stomachs with the stomach or assimilating cavity in the main stem. Each individual polype, sometimes to the number of nineteen, after having digested its food or prey, ejects the refuse from its mouth, and the nutritious juice traverses the labyrinth of tubes to that general reservoir.

Since every portion of the bodies of the Hydræ is nearly of the same kind, and as every part of their surface inside and outside is in contact with the water in which they live, and from whence they derive oxygen to aërate their juices, no circulation is necessary in these simple animals, either for nutrition of their tissues, or to furnish them with oxygen.

If the Hydræ only produced deciduous buds which are developed into facsimiles of their parent, their race would become extinct, since they die in winter, unless kept artificially in water of mild temperature; but the animals are hermaphrodite, so that each individual produces fertilized eggs in autumn, which are hatched in spring, so that the Hydra is alternately propagated by deciduous buds and by eggs. The fresh-water hydræ are the only hydroids that are locomotive, all the others being fixed to some solid substance.

The oceanic Hydrozoa comprehend the three families of Corynidæ, Tubulariidæ, and Sertulariidæ. They are chiefly compound animals, numerous in genera and species, and have great variety of form. They may be simple and slender, they may be creeping or like a bush or tree, more or less compound and regularly branched according to the form of the polypary or tubular substance which unites their numerous hydra-form polypes into one animal. In general they are exceedingly small; three or four inches in height is quite gigantic. There is scarcely a still clear pool left by the retiring tide among the rocks along the British coasts, that does not abound with these beautiful creatures attached to stones, old shells, or sea-weeds. But they must be sought for amidst the luxuriant marine vegetation and profusion of animal life which adorn these rocky pools, otherwise they would escape notice; and even when large enough to be conspicuous, the eye must be aided in order to see the wonderful minuteness and delicacy of their structure. The aquaria have furnished an opportunity to study their forms, habits, and the marvellous circumstances of their lives and reproduction.

The compound oceanic Hydrozoa are essentially the same in structure as the compound fresh-water Hydræ. They differ, however, from them in often having a greater number of tentacles, and in being defended by a firm and flexible horny coat; notwithstanding which they increase in size by budding from the base of a single primary polype. The horny coat covers the bud and grows with it; but as soon as the polype is formed within it, the top of the bud opens and the young polype protrudes itself, so that a separation is effectually prevented; and while the stem and branches are being formed, and increase by the continual development of new buds, the communication between the stomachs of the whole brood of polypes with that in the parent stem is maintained by tubes from their bases passing through the interior fleshy matter in the branches.

In short these marine Hydrozoa consist of a ramified tube of sensitive animal matter, covered by an external flexible and often jointed and horny coat or skeleton, and they are fed by the activity of the tentacles and the digestive powers of frequently some hundreds of hydra-formed polypes, as in the Sertularia cupressina. The common produce of their food circulates as a fluid through the tubular cavities, for the benefit of the whole community, while the indigestible part is ejected from the mouth of each individual. The stomach of each polype has a more or less ciliated lining, containing cells with nutritive juices, which are supposed to perform the part of a liver. The liquid which circulates in these animals is colourless, with solid particles floating in it; and there is reason to believe that sea-water is admitted into the tubes, and that, mixed with the juices prepared by the polypes, it circulates through the ramified cavities, is sent into the hollow prehensile tentacles, and returns back into the digesting cavity after having contributed to respiration by its oxygen. The movements of this fluid appear to depend upon the delicate ciliated fibre which lines the cavities of the tentacles and those of the stem and branches of the compound animal, possibly aided by vital contraction. The soft skin of the tentacles contains cells full of liquid, with a thread and its sting or dart coiled up within it. These thread stings are protruded when the skin is irritated, which frequently gives the tentacles the appearance of being beset with bristled warts. In many instances these kinds of Hydrozoa are covered with a gelatinous substance, either as a film or thick coat.

The reproduction of many of these arborescent or compound Hydrozoa is one of the most unexpected and extraordinary phenomena in the life-history of the animal creation. For besides the system of consecutive budding from a single polype which builds up the compound animal, peculiar buds are formed and developed, which bear no resemblance whatever to the polype buds: on the contrary, when mature, they assume an organization exactly the same as that of the common jelly-fish or Medusoid Acalephæ, and swim freely away from their fixed parent as soon as they are detached. These medusiform zooids, which are extremely small, consist of a cup or umbrella-shaped bell of colourless transparent matter, which is their swimming apparatus; it is contracted and expanded by a muscular band under the rim, the water is alternately imbibed and forcibly ejected, and by its reaction the zooid is impelled in a contrary direction. From the centre of the bell a stomach hangs down in the form of a proboscis, with a mouth at its extremity, either with or without tentacles and sting-cells. Four canals, or a greater number, which begin in the stomach, radiate through the transparent matter of the bell, and are united by a circular canal round the rim; they convey the nutritious liquid from the stomach throughout the system. This general structure may be traced in the zooids of the three great families of the oceanic hydraform-zoophytes, in a greater or less degree, from deciduous perfect medusæ to such as are imperfect and fixed.

These medusiform zooids are male and female, and when detached from their parent they are independent creatures, each of them being furnished with nutrient and locomotive organs of its own. They produce fertilized eggs, which are developed into ciliated locomotive larvæ; after a time these lose their cilia and acquire a rayed sucking disc, with which they fix themselves permanently to a solid object, and, after various changes, each gets a mouth and tentacles and becomes a perfect young hydra. Thus a brood of young hydræ is produced, each of which acquires the compound form of its parent by budding, and as each of these compound animals in its turn gives off medusa-buds, there is a cycle of the alternate forms of hydra and medusa or jelly-fish, showing a singular connection between two animals which seem to have nothing in common. The analogy which so often prevails between plants and animals obtains here also, for the medusa-buds bear the same relation to the hydra or polype-buds that the flower-buds of a tree do to the leaf-buds: the flower-buds contain the germs of future generations of the tree, while the leaf-buds contain only the undeveloped stems, stalks, and leaves of the individual plant on which they grow.

The Corynidæ form the first of the three families of the oceanic hydra zoophytes. They comprise six genera, and many species of compound animals of various forms, each derived from a single animal by budding; and although they possess a thin flexible coat, the polypes are sheathed either in a thin membrane or bone. Their club-shaped tentacles form either a single or double circlet round the base of their conical mouth, and are also scattered over their bodies when bare.

The zooids are developed at once in the Syncoryna Sarsii, which is a long, thinly branched, and horny zoophyte, with a single naked, spindle-shaped polype at the extremity of each branch, as in fig. 111, A. The bodies of the polypes are studded with numerous tentacles, among which buds appear (fig. 111, a, b); these gradually expand into bell-shaped medusa-zooids (fig. 111, c), some being masculine and others feminine. They drop off their parent, swim away by the contraction of their bell, and their fertilized eggs are developed into single hydræ, which become arborescent like their parent by budding.

The family of the Sertulariidæ take branching forms, sometimes of perfect symmetry: they have a firm, horny coat, which not only covers the stem and branches, but becomes a cup for the protection of the polype. The most common form of the family of the Tubularia has no branches: it has an erect, hollow stem like a straw, sometimes a foot high, coated by a horny sheath. The polype which terminates each plant has a mouth surrounded by alternately long and short tentacles. The stomach of the polype is connected with the hollow in the stem by a muscular ring, by whose alternate dilatation and contraction, at intervals of eighty seconds, the fluid is forced up from below, enters the stomach, and is again expelled. Another liquid carrying solid particles circulates in a spiral through the whole length of the stem. Some of this family are propagated by perfect deciduous medusæ, others by imperfect fixed ones; both are developed on the polypes or among their tentacles. Like the fresh-water Hydræ, these creatures can restore any part of their bodies that is injured.

Numerous instances might be given to show that the minute medusiform zooids are only a stage or phase in the life of an oceanic hydra: conversely it will now be shown, that the single simple hydra is but a stage in the life-history of the highly organized medusa, jelly-fish, or sea-nettle of sailors, the Acalepha of Cuvier.

The medusæ vary in size, from microscopic specks that swim on the surface of the sea in a warm summer day to large umbrella-shaped jelly fish almost a yard in diameter. They abound in every part of the ocean and in all seas, often in such shoals that the surface of the water is like a sheet of jelly. Their substance is transparent, pure, and nearly colourless; chiefly consisting of water, with so little solid matter, that a newly caught medusa, weighing two pounds, dries into a film scarcely weighing thirty grains.

The Pulmograde Medusæ, which swim by the contractions of their umbrella-shaped respiratory disc, form two distinct groups, the naked-eyed medusæ and the covered-eyed group. Both are male and female; each has its own form of thread-cells; and the stinging power or strength of the poison is nearly in proportion to the size of the animal and the coarseness of its threads.

The disk, or umbrella-shaped swimming organ, in both groups consists of a large cavity included between two layers of gelatinous matter, which unite at the rim. The interior membrane, called the sub-umbrella, is encircled at its edge by a ring of highly contractile muscular fibre like the iris of our eyes, by which this swimming organ is expanded and contracted. From the centre of the sub-umbrella a stomach, in the form of a proboscis, is suspended, which is of a very different structure in the two groups.

The Thaumantia pilosella, a member of the naked-eyed group, is like an inverted watch-glass (fig. 112), less than an inch in diameter. The roof of this umbrella is much thicker than the sides, and gradually thins off towards the rim. The proboscis, or stomach, descends from the centre of the sub-umbrella, but not so far as to the edge of the rim: it ends in a mouth with four sensitive fleshy lips. Four slender canals, which originate in the cavity of the stomach, radiate from the centre of the roof of the umbrella and extend to its margin, where they unite at the quadrants with a canal which encircles the rim, and are prolonged beyond it in the form of tentacles armed with numerous thread-cells containing poisonous darts. These tentacles must be formed of muscular fibre, for they are very irritable: each of them may be extended and contracted separately or along with the others; they guide the medusa through the water, and can anchor it by twisting round a fixed object.

The prey caught is digested in the stomach, the refuse is ejected by the mouth, and the nutritious fluid that has been extracted is carried up through the base of the stomach into the four radiating canals, to supply the waste and nourish the system. The digestive cavity and canals are lined with a soft membrane, covered with cilia, whose vibrations maintain the circulation of the juices and perform the duty of a heart; for the medusæ have none, nor have they any special respiratory system: their juices are aërated through the under-surface of the rim of the umbrella, while passing through the circular canal lying either within the water or on its surface.

A fringe of filamental tentacles hangs down into the water from the rim of the disc or umbrella, which is studded at equal distances by fleshy bulbs, each of which has a group of fifty dark eye-specks, being the rudiment of an eye; and if the animal be disturbed when in the dark, each eye-speck shines with a brilliant phosphoric light, and the umbrella looks as if it were begirt with a garland of stars.

Close to the edge of the canal which encircles the margin of the umbrella, there are eight hollow semi-oval enlargements of the flesh, two in each quadrant formed by the four radiating canals: they are the eight ears of the medusa, for in these hollow organs there are from thirty to fifty solid, transparent, and highly refractive spheres, arranged in a double row, so as to form a crescent, those near its centre being larger than the more remote. The solid spheres are analogous to the otolites in the ears of the more highly organized animals. Mr. M‘Cready has discovered nerve-centres behind each tentacle, and under each marginal coloured speck in several species of the open-eyed medusæ, which places this group of Acalephæ in a higher grade than any of the preceding orders. The medusæ swim by the muscular energy of their umbrellas: at each rhythmical contraction the water, which enters by the mouth and fills the great central cavity within the umbrella, is forced out again through an orifice at the other end, and by its reaction the medusa is impelled with considerable velocity in the contrary direction, so that the top of the umbrella goes first, and all its tentacles are dragged after it.

The medusæ are diœcious: in the males four reproductive cells full of reddish or purple granular matter surround the cavity of the stomach, and appear like a coloured cross through the top of the gelatinous umbrella. In the females, at a point just before the four radiating canals enter the marginal canal, the flesh on the exterior of the umbrella swells out into bulbs, containing vessels full of clear eggs with minute globular yolks. These eggs, when fertilized, are hatched, and the young are developed within these ovaries, so that they come into the water as a kind of infusorial ciliated animalcule destitute of a mouth. One end of the creature acquires a suctorial disc, fixes itself to an object, and uses its cilia. The other end opens into a mouth, round which tentacles like fishing lines spring forth; the central part is converted into the cavity of the stomach, and thus a perfect hydra is formed, capable of being propagated naturally by budding, or artificially by being cut in pieces, each piece becoming a perfect hydra, differing in no respect from a common simple fresh-water Hydra.

From one of these, numberless successive generations of simple hydræ may be produced by budding, all catching their prey with their tentacles and digesting it in their stomachs. The limits to this budding-system seems to be indefinite: years may pass in this stage, but at length it ceases, and either the original hydra, or one of its descendants, undergoes a series of remarkable changes. The body of the hydra lengthens into a cylinder; it is then marked transversely by a number of constrictions beginning at the free end; these become deeper and deeper, till at length they break up the body into a pile of shallow cups, each lying in the hollow of the other, and leaving a kind of fleshy wall at the point of suspension or fixture. The edges of the cups are divided into lobes with a slit in each, in which the coloured rudiment of the eye is sunk. The cups are permanent, and characteristic of the group of naked-eyed medusæ. After a time, the cups begin to show contractile motions, which increase till the fibre of their attachment is broken, and then the superimposed cups are detached from the pile one after another, and swim freely away by the contractions of their lobes as young medusæ, leaving what remains of the parent hydra to repair its loss and again repeat this singular process. However, the young medusæ are not yet perfect. As they increase in size the divisions on the edge of the cup fill up; a proboscis-shaped stomach, with its four coloured cells and its square mouth, is developed from the centre of the sub-umbrella; the radiating canals extend from the central cavity, the encircling canal and fringe form round the umbrella-shaped cups, and the result is a highly organized Thaumantia pilosella, in whose life-history a simple hydra forms a singular stage.

Thus hydræ produce medusæ whose offspring are hydræ, and perfect medusæ produce hydræ whose offspring are perfect medusæ. However, the law of the alternation of generation is by no means peculiar to the Thaumantiæ. Many species of medusæ are subject to it, as the Turris neglecta, a beautiful little medusa not larger than a hempseed, common on the British coasts. It has a white muscular pellucid umbrella, a large proboscis of a rich orange colour at its upper part: in the orange-coloured flesh of it there are ovaries containing rose-coloured eggs, which are hatched within them, and come into the water as ciliated gemmules, which, after swimming about for a time, become fixed and are developed into small hydræ of a rich purple colour with sixty-four tentacles. From these hydræ others bud off indefinitely till the time comes when one of them becomes lengthened, constricted, divided into cups which drop off, and finally become a brood of the Turris neglecta.

The naked-eyed medusæ are extremely numerous. There are six orders of them and many genera, chiefly distinguished by the position and nature of their ovaries and the number of canals which radiate through their swimming organs. Both of the medusæ that have been described have four radiating canals; yet they belong to different orders, for the ovaries of the Thaumantia are in the edge of the umbrella, while those of the Turris are in the substance of the proboscis. Neither of these kinds have more than four ovaries, but some other kinds have eight ovaries and eight radiating canals. Most of the canals are simple, but in one genus they are branching. All are furnished with tentacles, some of them having stings, others none.

The covered-eyed group consists only of two natural divisions—the Rhizostoma, or many-mouthed medusæ, and the Monostoma, or one-mouthed medusæ. In both the coloured eye-specks at the margin of the umbrella are larger and more numerous, than in the naked-eyed group, and they are covered with a hood. The proboscis of the one-mouthed order terminates in a square mouth, the four angles of which are prolonged into tentacles with a solid hyaline axis. They have a fringed membrane along their under-surface, containing numerous stinging thread-cells. Sixteen canals, connected with the stomach or cavity of the proboscis, radiate over the flattish, cup-shaped umbrella; eight of these are branched, and terminate in the circular canal which runs round its fringed edge, and they form the nutrient and respiratory system of the animal, while the eight simple and alternate canals terminate in eight openings at the rim of the umbrella, through which the refuse or indigestible part of the food is discharged, thus forming an exception to the other pulmograde medusæ, and indeed to the Hydrozoa in general, which eject it at the mouth. All the canals are lined with cilia, whose vibrations maintain the circulation of the fluids, and perform the duties both of a heart and respiratory apparatus. Dr. A. Krohn has observed that in three species of the genus Pelagia belonging to the covered-eyed medusæ, the young are at once developed as medusæ without the intervention of the hydra form.

The disk of the Rhizostoma, or root-mouthed medusæ, is rather flat, and the large proboscis is unlike any other of the tribe. In the naked-eyed medusæ digestion is performed in the cavity of the proboscis; but in this order the proboscis is divided into four very long branches ending in club-shaped knobs (fig. 115), and nutrient tubes extend to their extremities from the great central cavity in the umbrella. Their broadish frilled borders are divided and subdivided along their whole lengths, and the nutrient canals, which follow all their ramifications, end in numerous fringed pores upon their edges and upon the club-shaped ends of the quadrifid proboscis. These numerous pores are mouths; they absorb minute animalcules, which are digested while passing through the united canals to the great central cavity of the umbrella, which receives the products of digestion. Eight canals radiate from that great cavity and traverse the umbrella; and the nutrient fluid, mixed with the sea-water, passes from the great cavity through these canals into an elegant network of large capillary tubes spread on the under-surface of the margin of the umbrella, which is always in contact with the water; and in this beautiful respiratory organ the carbonic acid gas is exchanged for the oxygen in the water of the sea. The indigestible part of the food is discharged through the mouths or pores, whose edges are prolonged into solid tentacles containing thread-cells, with their usual weapons of offence and defence. Besides these armed tentacles, which are very numerous in the covered-eyed group, the gelatinous umbrella has a multitude of oval thread-cells on its external coat, in each of which a very long filament is spirally coiled, which darts out to a considerable distance on the smallest touch, and stings severely.

A few only of the British pulmonigrade medusæ sting: the Cyanea capillata, one of the single-mouthed covered-eyed family, is most formidable. It has very long tentacles, which it can throw off if they get entangled, but they continue to sting, even after they are detached from the medusa.

This is one of the most remarkable instances of the inherent irritability of muscular fibre still in full force after the tentacles have been separated from the living animal. In many of the lower animals, as in the Hydra itself, vitality is so far from being extinguished in the severed members that it repairs the injury. Since the covered-eyed medusæ have eyes, ears, and very sensitive tentacles, it may be inferred that they possess nerves of sight, hearing, and touch, though none have been discovered, probably on account of the softness and transparency of their tissues. The stinging power by which they kill their prey and defend themselves may be classed among the consensual powers prompted by the sympathetic sensations of hunger or danger.

In all latitudes the medusæ are highly luminous, especially in warm seas. Professor Vogt remarked that flashes of light passed over their disk when they touched one another in swimming, and they appear at intervals like globes of fire among the lesser lights of the Noctilucæ; if from involuntary nervous contraction, as is most likely, the light must be electric.

The medusæ are infested by many parasites. Entozoa are often abundant in their gelatinous substance, and crustaceans of various kinds and colours, such as shrimps, sand-hoppers, and a galæmon of glassy transparency, move about in the substance of their disc and arms, entering unscathed by the poisonous darts which inflict instant death on others of their class. The Libanea crab, of gigantic size compared with its host, is in the habit of taking up its abode between the four columns of the Rhizostoma. But the most singular intruder is the Philomedusa Vogtii, which is a polype with twelve thick short tentacles, its whole body and tentacles being covered with cilia and thread-cells. These polypes live in the disk, arms, and stomach of the medusæ, and, when taken out, their stomachs are found to contain fragments of the tentacles of their host, and even the thread-cells with their stings. The larger polypes devour the smaller ones, and the latter live for weeks within the larger ones without apparent inconvenience to either.^[24]

Mr. M‘Cready mentions that the larvæ of the medusa Cunina octonaria swim as parasites in the cavity of the bell of the medusa Turritopsis nutricula, which not only furnishes a shelter and dwelling-place to the larvæ during their development, but it also serves as a nurse, by permitting the parasites, which adhere by their tentacles, to take the food out of its mouth by means of their long proboscides. They undergo many transformations, and become nearly perfect medusæ while within their nurse.

Medusæ of different species are met with in every sea from the equator to the poles. They are eminently social, migrating in enormous shoals to great distances. The largest shoal of young sea nettles on record was met with in the Gulf Stream, off the coast of Florida, by a vessel bound for England. The captain likened them to acorns; they were so crowded as completely to cover the sea, giving it the appearance from a distance of a boundless meadow in the yellow leaf. He was five or six days in sailing through them, and in about sixty days afterwards, on returning from England, he fell in with the same school, as the sailors call it, off the Western Islands, and was three or four days in sailing through them again. Mr. Piazzi Smyth, when on a voyage to Teneriffe in 1856, fell in with a vast shoal of medusæ. With a microscope he found part of the stomach of one of these creatures so full of diatoms of various forms—stars, crosses, semicircles, embossed circles and spirals—that he computed the whole stomach could not have contained less than 700,000. The flinty shells of the diatoms ejected in myriads by the medusæ, accumulate in the course of ages into siliceous strata, which, heaved up by subterranean fires, at length become the abode of man. Thus gelatinous transparent beings indirectly aid in forming the solid crust of the earth by means of the microscopic vegetation of the sea.

Ciliograde Hydrozoa.

The ciliograde Acalephæ, which form four orders and many genera, and which swim by means of symmetrical rows of long cilia, are represented on the British coasts by the Cydippe pileus and the Beroë Forskalia (fig. 116), little delicately tinted, gelatinous, and transparent animals that shine in the dark.

The Cydippe pileus is a globe three-eighths of an inch in diameter, like the purest crystal, with eight bands of large cilia, stretching at regular distances from pole to pole. A mouth, surrounded by extremely sensitive tentacles, is situated at one pole, the vent at the other. The Cydippes poise and fix themselves to objects by means of two very long tentacles, fringed on one edge by cirri, that is, short curled tentacles. These cirrated tentacles, which in some species stretch out to more than twenty times the length of the animal, can be instantaneously retracted into cavities at the posterior end of the body, while, at the same time, the marginal filaments are as rapidly coiled up in a series of close spirals. The whole of these complex organs are enclosed within the limits of a pin’s head.

The manner in which these little gems swim is beautiful; sometimes they rise and descend slowly, like a balloon, and when they glide along the surface of the water in sunshine, the cilia on the eight meridional bands exhibit the most brilliant iridescence. The long cirrated tentacles follow all their motions in graceful curves, or hang indolently down, and sometimes they are suddenly stretched to their full length, and as suddenly retracted, and in all their varied convolutions the cirri that fringe them are in constant vibration, and exhibit all the tints of the rainbow. Sometimes these creatures whirl round their axis with great rapidity, but, active as they are, no nervous system has yet been discovered in them.

The common Beroë is like an elongated melon, obtusely octangular, with eight rows of cilia, extending from a mouth at one end to a kind of ciliated star at the other. The Beroës are of a gelatinous transparent substance, which expands and contracts with great facility: it is always expanded when they swim.

The Cestum Veneris belongs to another genus of the same family. It is like a blue ribbon, the mouth and vent being on the opposite sides in the middle of the band, which is furnished throughout its whole length with active cilia for swimming. The ciliograde Hydrozoa are monœcious, and do not produce medusa-zoids.

Campanograde Acalephæ.

There is a group of oceanic Hydrozoa, consisting of several families, which are fed by numerous suctorial organs called polypites, with tentacula and thread-cells attached to their bodies, so that they are analogous to the marine hydræ, in being colonies of individuals united into a compound animal. Some have air-vessels, which enable them to float on the surface of the water; but the locomotive organs of this group are bells, so that they may be called Campanograde Acalephæ.

The family of the Diphyidæ are colourless, and of such transparency that they are all but invisible when in the water, and are gelatinous masses clear as crystal when taken out of it. They are chiefly inhabitants of the warmer parts of the Pacific and Atlantic Oceans, but many fine specimens are found in the Mediterranean. Of these the Praya diphys is one of the most extraordinary (fig. 117). It has two large swimming-bells, their mouths turned backwards, with which the whole community is connected. They are nearly equal in size, soft, gelatinous, transparent, and colourless, rounded in front, open and truncated behind. The adjacent sides are parallel, with a groove between them, into which one end of the long tubular filiform body of the animal is fixed by slender tubes, through which a nourishing liquid passes into radiating canals in the bells, and from them into a circular canal at their margins, which are surrounded by a muscular contractile iris, like that in our eyes, which shuts and opens the bells. By the alternate absorption and ejection of the water the bells go head foremost, and regulate the motions of the whole compound animal. When both bells are active it goes straight forward; when the right hand bell is alone in action, it goes to the left, and vice versâ; in fact, the bells act as a rudder.

The slender cylindrical body or axis of the Praya is so transparent, that the cavity and muscular fibres of its walls are distinctly seen. These animals are extremely contractile. Professor Vogt mentions an individual he met with at Nice more than three feet long, when extended on the surface of the water, which could contract itself into little more than a finger length. It was said to have had a hundred isolated groups of polypites with their appendages attached to it; but in general the Prayæ are not so long, and seldom have more than thirty or forty of these isolated groups, which are attached to the under-side of the long flexible body, and hang down like a rich and beautiful fringe. In the figure, the position of the numerous groups of polypites and their appendages are merely indicated by round marks and lines.

In the body of the Praya diphys (fig. 117), as in that of the whole family, there is a nutritious liquid, which, by means of cilia, flows on its interior surface in two directions: it enters the canals in the two large bells, and supplies them with nourishment.

The polypites which digest the food are vermiform double sacs communicating at one end by a valve with the canal in the body of the animal; and at the free end they are prolonged into a mouth with an everted lip, and the digesting apparatus lies in the centre. Each polypite is supplied with food by its own fishing-line descending from a point close to where the polypite is fixed to the long axis. It is a long, tubular, branched tentacle, each branch ending in a coloured, pear-shaped, or fusiform battery of thread-cells with their stings. A gelatinous plate is placed on the upper side of the common axis immediately over the isolated groups, to protect and separate them.

Such are some of the most general characters of the family Diphyidæ: the Praya diphys has something peculiar to itself.

In the Praya, each individual group has a swimming-bell of its own adjacent to the polypite, and lying parallel to the axis of the animal, with its mouth turned backwards. It is connected by tubes both with the general central canal, and with a helmet-shaped protecting plate. On the other side of the polypite, there is a tuft of vermiform buds with spiral terminations, bristled with thread-cells. From the centre of this tuft a tentacle, or fishing-line, descends with numerous branches, the whole forming a tubular system connected with the common canal in the axis. Each of the branches of the tentacle terminates in a vermilion-coloured tendril, coiled up into a minute capsule. The inside of the tendril is not only bristled with the points of sabre-shaped darts, but it conceals a filament crowded with thread-cells. On the slightest touch, the tendril stretches out like a corkscrew of red coral, and every dart springs forth. Such is, more or less, the complicated structure of the offensive and defensive weapons of many of this order of oceanic Hydrozoa, which appear to the naked eye as merely brightly-coloured points. The use of these tentacles, or fishing-lines, is the same in all; they seize, kill, and carry their victims to the mouth of the polypite by contracting their long lines.

In the Praya, the groups are individualized in the highest degree consistent with union; for, when the animal is at rest, each of the individual groups, amounting to thirty or forty, swims about by means of its little bell independent of the rest. Their motions can be compared to nothing but a troop of jugglers performing gymnastic exercises round a cord represented by the common body of the animal; except for adherence to which the life and will of each group are so perfectly independent, that the mutual dependence of the whole is only seen when the common trunk contracts to bring all its appendages towards the two principal bells, which then begin to move.^[25]

Thus each group has a special life and motion, controlled by a general life and motion; strong individual muscular power controlled by general muscular power; yet no nervous system has as yet been discovered, so this animal activity must for the present be attributed to a strong, inherent, contractile power in the muscular fibre. The Praya is seldom complete, on account of the ease with which it casts off its great bells.

None of the Diphyidæ have special organs for respiration; their juices are aërated through their delicate tissues. They are diœcious, and invariably produce perfect male and female medusiform zooids; they are situated among the groups of the polypites and their appendages, and are attached to the axis of the animal. When free, they swim away by the contraction of their bells; the eggs are fertilized, and produce young Diphyidæ, male and female; so these animals, like most of the oceanic Hydrozoa, have two alternate stages of existence.