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 shores of our own country. It is true that they do not form large masses, like the sponges grown in warmer seas, which we import; but the smaller growths, massed together, often cover a considerable space 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. Masses of dead sponge, consisting of branching parallel fingers a few inches long, may often be found in the dead state, washed up on the shore; these are the usual drab colour of a dead sponge.

The encrusting sponges which grow on rocks present a mass, 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 mass 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 spaces, which communicate with the exterior by means of microscopic pores. Through the latter, water is drawn in, and passes, 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 passes 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 pass 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 canal-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 passes 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 spaces 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 passage. 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 canals and chambers is seen cut through at various points; the cells lining them and dividing them may be individually studied. The passage 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 passed 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 canals 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 asexual buds, or "gemmules," in the interior of its substance, which are liberated in the spring, and become young sponges.

TABLE SHOWING THE CLASSIFICATION OF SPONGES

Some of the marine sponges are parasitic. Most people have doubtless found on the sea-shore now and then a dead oyster-shell, 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 shell; 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 shell 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 shore, 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 classed 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 larvÆ, are turned upside down, the mouth being, as above stated, originally situated at the fixed end.