Dr. Grant, to whom we are chiefly indebted for the physiology of Sponge, was the first to establish the fact of a continued cur rent (except when interrupted by the will of the animal,) moving from without into the interior of its body, and thence passing through large channels to find an exit by the oscula or mouths again, from the exterior surface. Dr. Grant observes :—" I put a small branch of Spongia coalita, with some sea water, into a watch glass, so as to bring one of the apertures on the side of the Sponge fully into view under the microscope, and I beheld for the first time the splendid spectacle of this living fountain vomiting forth, from a circular cavity, an impetuous torrent of liquid matter, and hurling along, in rapid succession, opaque masses, which it strewed every where around. The beauty and novelty of such a scene in the animal kingdom long arrested my attention ; but, after twenty-five minutes of constant observation, I was obliged to withdraw my eye from fatigue, without having seen the torrent for one instant change its direction, or diminish in the slightest degree the rapidity of its course. I continued to watch the same orifice, at short intervals, for five hours—some times observing it for a quarter of an hour at a time—but still the stream rolled on with a constant and equal velocity ; the cur rent then gradually diminished, and ceased in about an hour." Dr. Grant, in referring to experiments made with the crumb of bread sponge, (1Ialichondria panicea,) a common species on the British coast, remarks :—" Two entire round portions of this Sponge were placed together in a glass of sea water, with their orifices opposite to each other at the distance of two inches ; they appeared to the naked eye like two living batteries, and soon covered each other with feculent matter. I placed one of them in a shallow vessel, and just covered its surface and highest orifice with water. On strewing some powdered chalk on the surface of the water, the currents were visible at a great distance ; and on placing some small pieces of cork, or dry paper, over the apertures, I could perceive them moving by the force of the cur rent at the distance of ten feet from the table on which the speci men rested. Naturalists have long suspected the presence of very minute cilia in the gelatinous flesh of the sponge, and were inclined to attribute the currents to their agency ; but as yet, they had been unable to detect them by the closest scrutiny, when Dr. Dobie, and afterwards Mr. Bowerbank, discovered them in motion in living Sponges. It was in the sack Sponge, (Grantia compressa,) which has the form of a little flattened bag, of an angular outline, of a whitish hue, with an orifice at each angle, that the experienced eyes of these gentlemen detected the moving cilia. " By tearing specimens in pieces, (for the use of the keenest cutting instruments so crushed the texture as to de stroy the parts,) and examining the separated edges, with high powers, Mr. Bowerbank found that the sides are composed of a number of hexagonal cells, defined by the peculiar arrangement of the triradiate spiculx, and having their walls formed by a number of nucleated granules; these angular cells are laid at right angles to the long axis of the Sponge, extending from the outer surface to the inner, and they are crossed near the middle by a thin partition, perforated in the centre. In this perforation, several long, whip-like cilia were seen lashing with energy, and the same organs were afterwards found to be connected with the granules of which the cell walls were composed. By means of the
waving of these cilia, then, the water is made to flow through the cells from without, being discharged into the interior of the sack and poured out in streams through the oscula or openings which terminate the angles of the Sponge." This experiment, we think, leaves no doubt of the animal na ture of Sponge. " If a Sponge be mechanically divided into several pieces, every portion becomes a distinct animal. The multiplication of Sponges, however, is effected in another man ner, which is the ordinary mode of their reproduction, and forms a very interesting portion of their history. At certain seasons of the year, if a living Sponge be cut to pieces, the channels in its interior are found to have their walls studded with yellowish gelatinous granules, developed in the paranchymatous tissue; these granules are the germs or gemmules from which a future race will spring; they seem to be formed indifferently in all parts of the mass, sprouting, as it were, from the albuminous crust that coats the skeleton, without the appearance of any organs espe cially appropriated to their development.
As they increase in size they are found to project more and more into the canals ramifying through the Sponge, and to be provided with an apparatus of locomotion, of a description such as we shall frequently have occasion to mention.
The gemmule assumes an ovoid form, and a large portion of its surface becomes covered with innumerable vibrating hairs, or cilia, as they are denominated ; these are of inconceivable mi nuteness, yet individually capable of exercising rapid movements, whereby they produce currents in the surrounding fluid. As soon, therefore, as a gemmule becomes sufficiently mature, it be comes detached from the nidus where it was formed, and, being whirled along by the issuing streams, is expelled through the fecal orifices of the parent, and escapes into the water around.
Instead, however, of falling to the bottom, as so apparently helpless a particle of jelly might be expected to do, the cease less vibration of the cilia upon its surface propels it rapidly along, until, being removed to a considerable distance from its original, it attaches itself to a proper object, and, losing the now useless locomotive cilia, it becomes fixed and motionless, and develops within its substance the skeleton peculiar to its species, exhibiting by degrees the form of the individual from which it sprung.
It is curious to observe the remarkable exception which Sponges exhibit to the usual phenomena witnessed in the repro duction of animals, the object of which is evident as the result is admirable. The parent Sponge, deprived of all power of move ment, would obviously be incapable of dispersing to a distatice the numerous progeny which it furnishes. They must inevitably have accumulated in the immediate vicinity of their place of birth, without the possibility of their distribution to other locali ties.
The seeds of vegetables, sometimes winged and plumed for the purpose, are blown about by the winds, or transported by various agencies to distant places ; but in the present instance, the still waters in which Sponges grow would not have served to trans port their progeny elsewhere ; and germs so soft and delicate could hardly be removed by other creatures. Instead, therefore, of being helpless at their birth, the young Sponges can, by means of their cilia, row themselves about at pleasure, and enjoy for a period, powers of locomotion denied to their adult state.