Fig. 10. Spongia, half the natural size, attached to its rocky bed.
The physiological function of those tubes and orifices which present themselves on all parts of the sponge has been interpreted in various ways. Ellis, writing in 1765, supposes that they were the orifices of the cells occupied by the polypi. In 1816, Lamarck still advocated this opinion; and even now we find the observer, whose notes M. Frédol has edited with so much judgment, asserting that "the inhabitants of the sponge are a species of fleeting, transparent, gelatinous tube, susceptible of extension and contraction; young polypes, as we may call them, without consistence, without cilia; incipient polypes, in short, of very simple but sufficient organization. The animalcule of the sponge is a stomach, without arms, very simple, very elementary—in short, an animal all stomach!"
This mode of considering the sponge is not conformable to the views of the leaders of modern science, however. Mr. Milne Edwards, for instance, in place of seeing in the sponge a collection of united beings, forming as it were a colony, considers each to be an isolated being, an unique individual. The innumerable canals by which the sponge is traversed, according to that author, are at once the digestive organs and breathing pores of the zoophyte. The vibratile cilia are necessary to the renewed aeration of the water required as a respiratory fluid in the interior canals of the sponge. The currents in these channels have one constant direction. The water penetrates the sponge by numerous orifices of minute dimensions and irregular disposition; it traverses channels in the body of the zoophyte, which reunite somewhat like the root of a plant, in order to constitute the trunk and increase its substance; finally, the water makes its escape by special openings. According to this view, the channels of the sponge have a kind of cumulative physiology, performing the two functions of digestion and respiration. The rapid currents of aerated water which traverse them lead into them the substances necessary to the nourishment of these strange creatures, rejecting all excremental matter. At the same time, the walls of these canals present a large absorbing surface which separates the oxygen with which the water is charged, and disengages the carbonic acid which results from respiration.
Sponges contain true eggs, from which embryo polyps are produced; these have not cilia at first. In the interior of these eggs the contractile cellules have their birth; then the spiculæ; and when they are finally covered with the vibratile cilia, aided by them these larvæ of ovoid form swim, or rather glide, through the water. The species of infusoria born of the sponge resemble the larvæ of various polypes at the moment they issue from the egg. "They soon attach themselves to some foreign body," says Mr. Milne Edwards, "and become henceforth immovable; no longer giving signs either of sensibility or of contractibility, while in their enlargement they are completely transformed. The gelatinous substance of their bodies is channeled and riddled with holes—the fibrous framework is completed—the sponge is formed."
We may add, however, that other zoologists, and among them MM. Paul Gervais and Van Beneden, take a different view of the development of the sponges, and Dr. Johnston omits them altogether from his great work on "British Zoophytes." "If they are not the production of polypi," he says, "the zoologist who retains them in his province must contend that they are individually animals, an opinion to which I cannot assent, seeing that they have no animal structure or individual organs, and exhibit not one function usually supposed to be characteristic of the animal kingdom." Gervais and Van Beneden consider, as Milne Edwards does, that the embryos are at first movable, then fixed, many of them uniting together, and melting, as it were, into one common colony, which become a sponge, such as we see it. An isolated embryo might also, by throwing out germs, produce a similar colony, which would thus become a product of agamous generation. Thus it appears that Science is far from being settled in its views as to the organization and development of these obscure and complex formations; nor is it more advanced in its knowledge of the duration of life and the quickness of growth in sponges. It is agreed, however, on one point—namely, that the sponge-fisher may return to the same fishing-ground after three years from the last fishing. At the present time sponge-fishing takes place principally in the Grecian Archipelago and the Syrian littoral. The Greeks and Syrians sell the product of their fishing to the Western nations, and the trade has been immensely extended in recent times, when the sponge has become an almost necessary adjunct of the toilet as well as the stable, and in other cleansing operations.
Fishing usually commences towards the beginning of June on the coast of Syria, and finishes at the end of October. But the months of July and August are peculiarly favourable to the sponge harvest, if we may use the term. Latakia furnishes about ten boats to the fishery, Batroun twenty, Tripoli twenty-five to thirty, Kalki fifty, Simi about a hundred and seventy to a hundred and eighty, and Kalminos more than two hundred. The operations of one of these boats fishing for sponges on the Syrian coast is represented in Plate II.
The boat's crew consists of four or five men, who scatter themselves along the coast for two or three miles in search of sponges under the cliffs and ledges of rock. Sponges of inferior quality are gathered in shallow waters. The finer kinds are found only at a depth of from twelve to twenty fathoms. The first are fished for with three-toothed harpoons, by the aid of which they are torn from their native rock; but not without deteriorating them more or less. The finer kinds of sponges, on the other hand, are collected by divers aided by a knife; they are carefully detached. Thus the price of a sponge brought up by diving is much more considerable than that of a harpooned sponge. Among divers, those of Kalminos and of Psara are particularly renowned. They will descend to the depth of twenty-five fathoms, remain down a shorter time than the Syrian divers, and yet bring up a more abundant harvest. The fishing of the Archipelago furnishes few fine sponges to commerce, but a great quantity of very common ones. The Syrian fisheries furnish many of the finer kinds, which find a ready market in France; they are of medium size. On the other hand, those which are furnished from the Barbary coast are of great dimensions, of a very fine tissue, and much sought for in England. On the Bahama banks, and in the Gulf of Mexico, the sponges grow in water of small depth. The fishermen, Spanish, American, and English, sink a long mast or perch into the water moored near the boat, down which they drop upon the sponges; by this means they are easily gathered.
Plate II.—Sponge Fishing on the Coast of Syria.