Hence its name, fancifully derived from the seven-headed serpent of Greek Mythology, the Hydra killed by Hercules, which may be supposed to have presented a similar straggling appearance. The diagram on page 36 represents a section through the middle of the body, only without the arms.
Unlike the Sea-Anemone, the Hydra can walk about. This it does in a very awkward manner, much in the same way as the Caterpillar known as the "Looper," clinging first with the front and then with the back extremity of the body (for head and tail they can hardly be called in so simple an animal as the _Hydra_, although the Looper caterpillar boasts both head and tail).
The _Hydra_ is so small an animal that it appears to the unaided eye merely as a tiny speck. It may be found anywhere in British ponds and ditches, standing on water-weeds. Like the Sea-Anemone it preys on animals smaller than itself. Nature has provided it with minute stinging cells, which benumb its prey; and in this all the animals of the Coelenterate group resemble it.
One of the most curious things about the Hydra is, that it often throws out buds. It can, of course, produce eggs which are fertilized and hatched in the usual way of eggs; the buds are an additional way of multiplying itself.[C]
[C] We may recall in comparison the way trees may be propagated by slips independently of flowers producing the seeds of the trees.
These buds are at first merely swellings, in which both of the layers of the body join: they grow larger; become provided with tentacles and a mouth, like the parent, and finally are cast off as independent animals.
For this reason the group to which _Hydra_ belongs has received the name of Eleutheroblasteae, the animals with free buds. But Hydra has many near relations in which these buds are not so cast off, but remain attached to the parent; and they in turn may produce others which also remain attached.
In this way, groups or colonies are formed, consisting of large numbers of individuals, and possessing a common stalk or stock which is formed by degrees as the process of multiplication goes on. The corals and the corallines are familiar examples of this.
The matter is complicated by the fact that either the separate animals or the flesh of the stock, or both, may secrete within themselves a hard supporting structure forming what is known as Corals. This may be developed in such a complicated manner, that instead of the coral appearing to be the product of the animal, the animal seems to be inserted in the coral, into which indeed it can retract itself for shelter.
The Corallines, on the contrary, secrete a leathery coating or sheath outside themselves and the stock. The leathery case is fairly transparent, so that on magnifying the creature the flesh of the common stock, as well as of the stalks of individual animals, may be seen inside. The "heads" of the animals poke out at the end of each branch (see Fig. 9).
The _Hydra_, with which we started, had always the power of producing eggs; each animal could do so, besides producing buds. But in our Colonial Coralline this is not necessarily so. Some individuals lose the power of producing eggs. Others can do nothing else, and become greatly altered in structure, often losing the power of developing tentacles, and exhibiting other changes. So much are they altered sometimes that they seem to be mere buds, not separate animals at all.
In other cases a still more surprising thing happens. The bud that is destined to produce eggs falls off, and becomes quite independent of the colony; more than this, it becomes quite different in appearance from the members of the colony: and instead of being a Hydra-like animal it becomes a jelly-fish. But the eggs of this jelly-fish do not produce jelly-fishes: they produce a more or less Hydra-like animal which gives rise by budding to a fresh colony. This is what is known to Zoologists as "alternation of generations."
Now comes a puzzling question--Which part of this family group shall we select and call it an "animal"? Is each Hydroid of the colony an animal, and the jelly-fish another animal? Zoologists say "No": from the development of one egg, to the production of another, is the cycle that const.i.tutes an individual animal. So we have the puzzling result in nomenclature, that an "individual" consists of a very large colony of creatures in one place, together with a perfect shoal of creatures quite unlike it, floating miles away from it on the ocean. What name must we give to the units, so curiously connected with one another? Zoologists call them "Zooids" (animal-like parts) or "persons."
This is the story of the jelly-fish as originally told. But there are innumerable variations upon it. There are kinds of jelly-fish that produce jelly-fish and have no Hydroid stage at all. Sometimes the "persons" of the colony present many varieties, each taking up some different task for the community. Some may be "nutritive persons,"
_i.e._ commonplace Zooids that have mouths and eat food; some "protective persons," reduced to mere folds or sheathing processes to guard the others; some are "stinging persons" armed with enormous quant.i.ties of thread cells. Then the whole colony may be like the jelly-fish, a floating affair, and not fixed at all.
[Ill.u.s.tration: FIG. 9.--An example of the Hydrozoa. A, branch of a Coralline, _Sertularia Ellisii_, magnified. B, the same, more highly magnified.]
We have several times above referred to the animals known as corallines.
It may almost be a.s.sumed that the ordinary reader knows what these are; if not, a little search among the treasures of the sea-sh.o.r.e will almost certainly reveal some of them, living or dead. The texture and appearance of the dead stems remind one of soft horn or dried gelatine; the branching arrangement of the stems and the little cells disposed at the ends of the branches will easily be shown under slight magnification. Most people will remember the rage for dyed corallines, by which all the fancy shops and florists were possessed a few years ago. The corallines, dyed a bright emerald green, or a dull red, which were used for decorations at that time, were usually a variety of the Bottle-brush Coralline, found on English sh.o.r.es; but sometimes commoner kinds were employed.
Fig. 9 shows an example of a coralline, slightly magnified in A, and in B much more highly magnified, so as to show the individual hydra-like zooids, each with its circle of tentacula.
The Sea-Anemone and the Hydra respectively represent the two great groups of the Coelenterata, named after them, the Anthozoa (Flower-animals), and the Hydrozoa (Hydra-animals). The corals are forms of the Anthozoa, single or colonial, which possess a skeleton.
[Ill.u.s.tration: FIG. 10.--_Gorgonia verrucosa_, from Guernsey, nearly one-third of the natural size.]
[Ill.u.s.tration: FIG. 11.--Corals. _A_, _Acanthoporia horrida_.
_B_, _Meandrina strigosa_. _C_, _Madrepora divaricata_. _D_, _Fungia papillosa_. _E_, Red Coral, _Corallium rubrum_. _F_, _Stylaster sanguineus_.]
The above diagram shows examples of the Anthozoa. Fig. 10 is _Gorgonia_, the Sea-Fan; while Fig. 11 represents corals of six different kinds.
Besides the two great groups we have named, the Hydra-like animals and the Sea-Anemone-like animals, the Coelenterata contain a third group, the Ctenophora, or Comb-bearers, so called on account of their possessing bands of cilia, fancifully compared to the teeth of a comb.
At first sight most of them somewhat resemble jelly-fishes, being transparent forms swimming near the surface of the sea. They are carnivorous, and some of them highly phosph.o.r.escent at night. The gastric cavity is divided up into branches. The representatives of the Ctenoph.o.r.es, most often seen on our own coasts, are small rounded forms.
Two remarks must be added before quitting the subject of the Coelenterata.
Firstly, the description of them as two-layered Animals is one that only applies typically and to the simpler forms. In others, such as the jelly-fishes, there is an intermediate layer of jelly, which appears to acquire a cellular structure by the immigration of cells derived from the primary layers. Thus we see, within the group of the Coelenterata, the gradual establishment of that third body-layer, which is found in all animals of higher structure. Scarcely indicated in _Hydra_, as a faint trace of a boundary-line (lamella) between the ectoderm and endoderm, it attains a good thickness in the Jelly-fish and Ctenophora.
In animals of higher structure the third body-layer, being now fully established, is cellular from its beginning in the embryo; in the Coelenterata its gradual formation is to be traced.
Secondly, it must be remarked that the colonial structure and the arrangement sometimes concomitant with it of "alternation of generations," is by no means confined to the Coelenterata. Both are seen in other forms of life, in which the units, or zooids, differ greatly in structure from those of this group.
TABLE SHOWING THE CLa.s.sIFICATION OF THE COELENTERATA
{ HYDROZOA, or { HYDRA-LIKE { ANIMALS.
=Grade II.= { { THE TWO-LAYERED =COELENTERATA.= { ACTINOZOA, or ANIMALS. { SEA-ANEMONE-LIKE { ANIMALS.
{ { { CTENOPHORA.
CHAPTER VI
THE SPONGES
Many who are familiar with the domestic sponge have never seen a sponge in a growing state, and would find it almost impossible to realise that a sponge may be a thing of beauty. And yet sponges are quite common on the rocky sh.o.r.es of our own country. It is true that they do not form large ma.s.ses, like the sponges grown in warmer seas, which we import; but the smaller growths, ma.s.sed together, often cover a considerable s.p.a.ce of rock, and are conspicuous by their beautiful colouring. Some sponges are crimson, and some green; while one of the commonest is a brilliant orange-yellow. The latter may often be found near low-tide mark, on a shelf of rock under growing seaweed. If the explorer has any doubt what the object is, it may easily be identified by the touch, which though moist and firm in the growing state, is still the unmistakable "feel" of sponge. Where the receding tide exposes a large surface of steep rock, for instance in caves, sponges may be found covering the rocks as thickly as mosses do on land. Ma.s.ses of dead sponge, consisting of branching parallel fingers a few inches long, may often be found in the dead state, washed up on the sh.o.r.e; these are the usual drab colour of a dead sponge.
The encrusting sponges which grow on rocks present a ma.s.s, so to speak, of little hillocks: in kinds which attain a larger growth, these may almost be described as branches. Each little hillock or branch has a hole at the top; and on the exterior of the rounded ma.s.s of the bath-sponge may be found numbers of such holes. We should naturally suppose that these holes were the mouths of the various sponge branches, especially since they lead to the central cavity of the branch, and thus to that of the whole sponge; and indeed they are known by the Latin name of "oscula," little mouths. They are, however, nothing of the sort; the sponge once had a mouth, a single one, when it was young, but the adult sponge has lost it. For the young sponge is at first a little free-swimming, two-layered animal of the type which has been described above as the gastrula larva. When it gets old enough to settle down in life, it sinks upon some suitable surface, and becomes fixed to it, mouth downward: the mouth is thus lost. How, then, is the animal to be fed? As it grows, there is developed in its substance a system of hollow s.p.a.ces, which communicate with the exterior by means of microscopic pores. Through the latter, water is drawn in, and pa.s.ses, after devious wanderings, to the central cavity of the animal, whence it is expelled by the so-called osculum. At first, the young sponge has but one cavity and one osculum; but by degrees the sponge branches and spreads, the cavity of each new portion remaining in connection with the main cavity.
If, as they grow in size, the branches touch one another, they sometimes coalesce--a fact which renders the growth of the sponge in some cases a very complicated matter.
It will be seen from the above description that the sponge is a sort of living filter. As the water pa.s.ses in through the pores, it deposits in the substance of the sponge all the little organisms that it contains; on these the sponge feeds.
It will naturally be asked, how does this living filter work? Water will not pa.s.s through small holes to flow out again at large ones in an upward direction, unless helped by some mechanism. How is this supplied?
By the industry of the cells of the sponge. Its ca.n.a.l-system includes a set of wide chambers, lined with cells which have long cilia, called flagella. These flagella, constantly moving in one direction (like the fan of a ventilator), create a current, which pa.s.ses the water on with such force that it reaches the central cavity, whence it is expelled through the oscula. These chambers do not communicate directly with the exterior. They are closed, except at certain small holes, the "prosopyles," where they take in the water that enters from s.p.a.ces connected with the pores. At the main end of the chamber is an aperture called the "apopyle," capable of being partly closed, and leading into an excurrent pa.s.sage. This last communicates with the central cavity of the sponge.
It will be seen that the topography of the sponge is a very complicated business. All its details have been studied by means of thin sections specially prepared and placed under the microscope (see p. 183); in these the labyrinth of ca.n.a.ls and chambers is seen cut through at various points; the cells lining them and dividing them may be individually studied. The pa.s.sage of water through the sponge was first observed by Robert Grant; many of the most recent discoveries regarding the structure of sponges we owe to Professor Sollas.
We have not yet explained what our living filter does with its food when it gets it. The ciliated cells of the internal lining take in solid particles just as Amoeba does; and from these they may be pa.s.sed on to the cells of the middle layer, amoeboid cells, which can move about.
These cells are considered to be derived from the primary layers of the body, especially the inner one, and to have wandered into a cellless middle layer, comparable in nature with that of some Coelenterates.
The sponge is full of firm or gritty particles, which form its skeleton, and remain when the sponge is dead, and the softer parts decayed. These, when magnified, often present beautiful and curious shapes. The use of them is not only to support the body, but also to prevent the sponge from being eaten by other animals.
There is found in the English ca.n.a.ls and rivers a small, fresh-water sponge, usually greenish in colour. This is named _Spongilla fluviatilis_, the River-sponge, and affords an exception to the usual marine distribution of sponges. In the winter it dies gradually away, at the same time forming as.e.xual buds, or "gemmules," in the interior of its substance, which are liberated in the spring, and become young sponges.
TABLE SHOWING THE CLa.s.sIFICATION OF SPONGES
{ CALCAREA, WITH CALCAREOUS { =PORIFERA.= { SKELETON.
=Grade II.= { { Two-Layered { { Animals, or { { NON-CALCAREA, WITH Acoclomata. { { SKELETON ABSENT OR { { FLINTY.
{ =COELENTERATA.=
Some of the marine sponges are parasitic. Most people have doubtless found on the sea-sh.o.r.e now and then a dead oyster-sh.e.l.l, completely riddled with small round holes, very similar in appearance to those seen in "worm-eaten" wood. These are the work of _Clione_, a parasitic sponge which is very fatal to the oyster. At first sight it seems a puzzle how the sponge made its way into the hard sh.e.l.l; it has no mouth to bite or suck its way into the solid substance. The cells of the sponge, however, wear away the lime of the sh.e.l.l by means of some acid chemical action.
Not only so, but they can attack stones as well, when these consist of limestone; and on some parts of the coast bits of sponge-eaten limestone washed up on the beach are quite common objects. They are pierced all through by holes, so that their appearance would suggest a sponge carved in stone, but for the fact that the holes are fairly uniform in size.
Such stones, lying on the sh.o.r.e, often puzzle the finder, when they contain no apparent trace of the tenant that has worked its way through them.
The sponges have received the name of Porifera, on account of the structure above described. They are often cla.s.sed with the Coelenterata, because, among other reasons, they practically belong to the two-layered type of structure, and because they form a complex organism that may almost be called a colony. But some prefer to place them in a group by themselves, apart from the Coelenterata. The chief reason of this is that the sponges, as compared with a primitive two-layered type indicated by their own larvae, are turned upside down, the mouth being, as above stated, originally situated at the fixed end.