THE next group of animals to be considered consists of the Porifera, or pore-bearing animals; also known as the Sponges, although the word sponge should probably be restricted to one only of these animals—the Bath-sponge. Indeed, in the minds of most people there is only one thing meant by this word : the familiar article of toilet use. But in actual fact, and using the word in its more liberal sense, it includes some ten thousand or more different kinds of animals. All of these live in water, and most of them in salt water. They are found in large numbers all over the world, in the
shallow seas as well as the ocean deeps; in ponds, streams and rivers, even to the lakes formed in the craters of extinct volcanoes ten thousand feet or more above sea-level. Their form is varied, from thin crusts growing on rocks and seaweeds, to irregular masses, often several feet across. Many are fan-shaped, or branching and looking like leafless trees. Others are cup-shaped, funnel-shaped or tubular. Those growing in the shallow waters are coloured in a variety of hues and rival in this respect the better-known corals.
The bath-sponge, as we know it in the home, is merely the skeleton of a once living animal, which in life was covered by a jelly-like flesh and a tough purple skin; and although so many different kinds of sponges are known, only the one is of commercial value. In most of them the skeleton is built up of minute bodies, known as spicules, composed either of lime or of silica, and these spicules, although exhibiting a great beauty of form when examined under a microscope, render them useless for the usual purposes. To wash with them would be akin to washing with a handful of very minute tin-tacks. There are many, it is true, which have a fibrous skeleton, but only in one case, that known and used as a bath-sponge, are the fibres sufficiently fine and delicate to allow of vigorous contact with the human skin.
Sponges in life show little sign of activity and are, moreover. extremely plant-like in appearance. Consequently, it was a very long time before scientists could make up their minds whether they should be regarded as plants or animals, and it was not until the discovery that they were in a state of constant activity that anything approaching a decision was arrived at on this point. Anatomically, sponges differ markedly from any other group of animals. In place of the usual mouth and digestive tract, which is a feature of practically all other animals, the body is fed through a complicated system of fine canals. Water is drawn in through numerous minute pores in the skin and makes its way through the body of the sponge and out again through a number of larger, crater-like vents at other points on the surface. The motive power is supplied by numbers of cells grouped at various points along the system of canals, which, by the lashing movements of long protoplasmic threads extending from their upper surfaces, draw water in through the pores and drive it out again through the vents. In other words, sponges are living pump-ing-stations, and the water during its course through the
body of a sponge gives to the animal oxygen and food particles and carries out with it all effete matter.
HOW SPONGES GET NEW BODIES FOR OLD ONE of the most interesting features of the behaviour of sponges is their truly remarkable power of regeneration. If we take a piece of living sponge and squeeze it through fine silk, the cells composing the tissues will become completely separated from each other and will flow through the pores of the silk in the form of a fine emulsion. When this emulsion is examined under a microscope the separate cells can be clearly seen, but a remarkable change has taken place in them. Instead of having its distinctive shape, according to its function, as it has in the undamaged sponge, each cell has become irregular in outline and is seen to be wandering about on its own.
A still more wonderful thing is yet to happen, however. If we take a few drops of this emulsion as it flows through the silk, place it in a small vessel containing sea-water, and watch it at intervals under the microscope, we shall see that after a time the cells have ceased their disordered wanderings and that they have begun to congregate together in a loose mass. In the course of a few hours this loose mass gives place to a small irregular mass of jelly, and if we now leave it for a longer period, we shall find on our return that this small mass has become a complete sponge. Guided by some influence which we can no more than doubtfully guess at, the various cells have come together again, after a brief spell of independence, have each taken on a definite place and a function, with the result that a complete new sponge is created out of the disintegrated remains of the piece originally taken.
In other words, the cells of sponges are capable of assuming the form and behaviour of independent Amoebas for a brief space of time, but ultimately are compelled to come together for mutual well-being. If then the first animals in the world were something in the order of the present-day Amoeba, it is probable that we have in the behaviour of sponge cells a suggestion as to how the many-celled animals were evolved from the one-celled, by a co-operation on the part of separate and independent cells and a living in harmony for the welfare and progress of the race.
THE MANY-ARMED HYDRA OF OUR PONDS THE recuperative powers of sponges are shared to a hardly lesser degree by a few other of the lower animals. There is, foi example, a small creature to be found living on the leaves and stems of the plants growing in our ponds which goes by the short and elegant name of Hydra. Whether so-called because it lives in water or whether on account of a resemblance to the Hydra of Greek mythology is not known. If, however, the latter is the case the name is doubly apt.
According to the legend, it fell to the lot of Hercules to conquer a loathsome creature living in the marshes which, in addition to its other terrifying attributes, possessed nine heads, each capable of inflicting considerable injury on anyone attacking the owner. Each head was carried at the end of a long neck and, to make matters worse for those who incurred the creature’s wrath, as fast as one head was cut off another grew in its place. The Hydra of our ponds, though doubtless as formidable an enemy to the smaller creatures with whom it shares the pond, is from a human point of view inoffensive and innocuous. It can, on the other hand, replace lost parts with something approaching the ease displayed by the monster of the legend. This power, which is called regeneration, is strongest among the lower animals and is gradually lost as we ascend the animal scale—that is, as the organism becomes more complex and highly organised, until the only vestige of it left is the ability of wounds to heal themselves.
The body of Hydra is built slightly more on the plan familiar to us in the higher animals than is that of a sponge, but even so is sufficiently peculiar to merit some attention being paid to its structure. There is a definite digestive cavity and a mouth, but that is all, and the waste products of digestion are passed out through the mouth. Further, instead of three sets of tissue being present, as in practically all multicellular animals, there are only two. Hydra may, therefore, be described as having a tubular body, the wall of which is composed of two layers of cells. The cavity of the sac so formed constitutes the digestive cavity, and while there is a mouth at the upper end, the lower end is completely closed. Food is captured and conveyed to the mouth by tentacles grouped in a ring on the summit of the body around the mouth.
CREATURES THAT POISON THEIR FOOD
PERHAPS the most distinctive feature of the Coelenterata, the group to which Hydra belongs, is the means by which it is able to catch its food. The tentacles, more particularly, and the surface of the body are armed with stinging cells; each of these contains a reservoir of poison and a coiled-up tubular thread connected with it. The base of the thread is armed with barbs, and at the summit of the cell, protruding outwards, is a small pointed trigger. The lightest touch on the trigger and the cell is shot out, the thread uncoils rapidly and with some force. Therefore, whenever one of the smaller animals inhabiting the same water as Hydra comes in contact
with it, numerous stinging cells are released; their threads penetrate the body of the victim, the barbs take a firm hold, the poison flows down the threads into the victim’s body, and the unfortunate creature is paralysed by the poison. At the same time, the tentacles of Hydra begin to curve over towards the mouth, carrying the victim with them. The victim is then pushed in through the mouth and digested.
Among the numerous relatives of Hydra, and having the same fundamental structure, however much they may differ in appearance, are the corals, sea-anemones, jelly-fish and a host of other animals of less familiar names. In most of these the resemblance to Hydra is obvious. A sea-anemone is, except for certain superficial details, little more than a gigantic Hydra, having the same fundamental form and, like it, having the skin armed with stinging cells. Corals, too, have this same superficial resemblance. The hard, stony masses we are accustomed to seeing in museums or in glass cases elsewhere are nothing more than the skeletons formed by colonies of what are popularly referred to as ‘coral-animals,’ each one very similar in form to Hydra or sea-anemones. On the other hand, the relationship of jelly-fish to any of these is not at all obvious until we come to study their life-history.
THE INFANT JELLY-FISH
To appreciate fully the close relationship between a jellyfish and any other of the Coelenterates, such as Hydra or the sea-anemones, two things must be borne in mind : that there are two distinct phases in the life-cycle of a jellyfish—a sedentary stage when the animal resembles them strongly, at least superficially; and a free-swimming stage, the jelly-fish proper which, appearances notwithstanding, is built on the same radial plan and has essentially the same organs as they. The familiar sting of a jelly-fish is due to stinging cells also. The sedentary stage of the life-history of a jelly-fish begins with a small white opaque object about three-eighths of an inch long, consisting of a compact cylindrical body and a short stalk at the base of it. The surface of it has a scaled appearance reminiscent of that of a half-opened cedar cone, and at either end is a ring of tentacles, the ring at the base being situated at the junction of the scaled body and the stalk. The only sign of life is a somewhat lethargic movement on the part of the tentacles. After about twenty-four hours in this sedentary stage the body increases in size,
becomes more transparent, the scaled appearance becomes more pronounced and the upper ring of tentacles disappears.
Strong pulsations begin to take place spasmodically at different points in the body, as though it were made up of a number of separate parts each of which was trying to free itself from the rest. This, indeed, is the case, for after a while, beginning at the top, a number of small eight-pointed stars become detached from the body, the process going on until only the stalk is left.
The reason why jelly-fish are found in such abundant shoals is that this process is repeated again and again, until the sea around is infested with them. To make matters worse, the rate of multiplication is increased by budding. The stalk remaining does not die but grows longer and swollen in the upper part, and fresh stars are budded off. In addition, the stalk may give rise to others like it, by budding, and each of these is capable of producing its own crop of eight-pointed stars. Thus each scyphistoma, as these stalked bodies are called, is capable of producing several hundreds of medusa;, or eight – pointed stars, simply by the repeated division of its own body or the production of other scyphistoma; budded off from it. The small eight-pointed medusae gradually lose their star-like appearance and, by a process of growth extending over several months, become converted into the familiar umbrella-shaped jelly-fish.
In the life of a jelly-fish, therefore, there are two distinct phases : the hydroid phase, represented by the scyphistoma, and the medusoid. In the first, multiplication is asexual, by budding and fission, and in the second it is a sexual process wherein male and female cells, carried on the under-surface of the umbrella of the jelly-fish, unite to form a larva, which in turn becomes fixed to the sea-bottom and is converted into a scyphistoma. This alternation of generations is, generally speaking, found throughout the Coelenterates. In corals and sea-anemones the hydroid phase has become dominant and the medusoid phase almost completely suppressed. In the jellyfish the medusoid phase is dominant. Thus the scyphistoma or stalk of a jelly-fish corresponds to the familiar sea-anemone in anatomical details, to a lesser extent in appearance, but differs considerably in size, so that although sea-anemones and jelly-fish appear to differ so markedly that a blood-relationship would hardly be suspected, closer study of them reveals the fact that they are very closely related.
THE STRANGE FOSTER-CARE OF THE JELLY-FISH FADS and fancies are not the monopoly of human beings, and all living things whatever their rank in the animal or vegetable kingdom have their likes and dislikes. Some of these fads involve the intimate and often lifelong association of two totally unlike creatures. In some cases a mutual benefit results and to this we give the name of commensalism. In many cases of commensalism the two partners are never found apart and, we may assume, are unable to exist for very long on their own. Often the special blessings derived from living together are far from obvious. Stranger still, however, are the cases where creatures live together in an association from which only one partner derives any benefit : stranger because it is difficult to see why the partner that gains nothing should tolerate such a state of affairs. A striking case is that concerning the association of certain large species of jelly-fish and certain small fishes that seek their protection. It is no uncommon sight, to those who study the life in the oceans, to see a large jelly-fish floating about the surface with a crowd of small fishes sheltering beneath the ample expanse of its umbrella-shaped body. From time to time the fishes sally forth to feed or exercise, but as often as danger threatens they scuttle back into the safety of the jelly-fish’s body. On the face of it there is nothing remaikable about this, for fish are past-masters at the art of utilising any nook or cranny—the spaces between rocks or the interstices in a coral bank—for a refuge, but the strange part of this association is that jelly-fish often feed largely on fish; even if they do not feed on them they have the power to make it extremely unpleasant for even a big fish to come within range of their trailing tentacles.
Like the sea-anemones, jelly-fish are armed with batteries of stinging cells, as bathers in the sea have often found to their cost. If the sting of a jelly-fish can be painful to human beings, even fatal in the case of some of the larger forms found in tropical seas, how much more deadly must it be to the smaller creatures? A mere touch is sufficient to call forth a fusillade of these poisoned darts and it would be difficult to believe that these crowds of little fishes, constantly swimming around and under the jelly-fish, scuttling this way and that among the bush of tentacles suspended from the underside of its body, never at any time touch the
tentacles. Yet the fact remains that, so far as can be seen, they are never harmed by their strange guardians.
WHEN REAL CHIPS OFF THE OLD BLOCK COME TO LIFE IN animals that exercise no parental care multiplication is attended by an enormous mortality in the offspring. In the sea there are not only living enemies to be reckoned with, but the elements take toll to a greater extent than is the case on land. The familiar example of the codfish which is said to lay millions of eggs in one season, out of which only one or two are destined to give rise to adult individuals, the rest being eaten or otherwise destroyed at some stage of their career, is only too true of practically all the lower invertebrates (animals without backbones). Constantly, the breeding season sees the production of enormous quantities of larva?, most of which are practically certain to be destroyed by one means or another before reaching maturity. In the lower invertebrates, too, it is not unusual to find fragmentation used as an auxiliary means of reproduction, the animal breaking up into a number of fragments each of which is capable of producing a new individual. This method is very common among sponges. The more fragile sponges are torn asunder or lacerated by wave action, or pulled to pieces by crabs, the fragments so formed giving rise to fresh sponges.
Some sea-anemones have the power of reproducing themselves by unconscious self-laceration. In other words, they are able to take ‘cuttings ‘of themselves. Cases are known of sea-anemones which as they move about—even sea-anemones do this occasionally, strange as it may seem—leave behind fragments of their own bodies adhering to the surface of the rocks, and these fragments grow into new anemones. The early naturalists thought that eels dashed themselves against the rocks and that young eels were produced from the bits of skin rubbed off in the process. This was the only way they could think of to explain why eels’ eggs could never be found. We smile at such notions now that the breeding-grounds of the eel have been traced to the mid-Atlantic, but the old ideas were not so extravagant after all, if we are to believe some of the modern investigators. It is said that some sea-anemones will choose the sharp edges of rocks and stones, almost purposely it would seem, in order to lacerate their bodies for the purposes of multiplication.