SPONGE, Sporgia, a genus which originally included all the numerous genera and species of the family spongiada, all of which are still commonly spoken of by natural ists as sponges, although in its more popular sense that term is limited to a few kinds, or to their fibrous framework. The sponges are creatures of very low organization, concerning which there has been much difference of opinion, whether they ought to be referred to the animal or to the vegetable kingdom. Naturalists are now gener ally agreed in regarding them as animals. They are, perhaps, the very lowest of protozoa. They are attached, like plants or zoophytes, to rocks or other sub stances in water; most of them are marine. They consist of a glairL or gelatinous substance (sareode), and of a framework, which is often formed of a horny, elastic substance (keratose), in fibers growing from a broad base, anastomozing and intimately connecting together, or consists of calcareous, or more generally, siliceous spieules, imbedded iu the gelatinous mass, and exhibiting great variety of form and arrangement. These spicules do not consist of mere mineral matter, but in part of animal matter, by the growth of which their form is determined. They arc most beautiful microscopic objects, and spicules of different forms are sometimes found in the same species, sometimes lying close together in bundles, sometimes straight or slightly curved, sometimes in the shape of needles pointed at one cud, or at both; sometimes of needles radiating from a center; while some have a head at one end like a pin, some have grapnel-like hooks at the ends. Some of the species with horny frame work have spicules imbedded in it; some have them implanted in the fibers; some are destitute of them. There is a beautiful West Indian species, dielyocalyx puniiceus, in which the siliceous matter becomes itself a fibrous net-work, and is so fine and trans parent as to resemble spun glass. In a living state many sponges exhibit lively colors, from the presence of some coloring matter, or from iridescence. Their gelatinous sub stance has a fish-like odor. If detached portions of it are examined under the micro scope, variable processes may be seen in motion, as in the amada or prams (q.v.). Sponges may be regarded as aggregations of amcetce, or as still lower in the scale of animal life, because individuality is soon lost when individuals of the same species arc brought together. They coalesce into one. And if a sponge is divided by the knife,
the parts placed together very quickly reunite, even if not in their former relation to each other. But parts of different species never unite in this way, however closely placed together. Sponges have never been observed to exhibit irritability. At first, they are like separate amaba; but after they become fixed to a spot, increase by a kind of gemmation, like zoophytes. They assmne very various forms, which, as well as the peculiarities in the structure of the framework, are characteristic of the different genera and species. Some are nearly globular; some cup-shaped, top-shaped, conical, cylin drical, thread-like, etc.; some are simple, and some branched.
_ The surface of a living sponge is generally covered with minute pores, through which water is imbibed, carrying with it both the air and the organic particles necessary for the support of life. The pores are supposed to be permanent in ninny of the sponges, and the currents which enter through them to be produced by cilia, although these have as yet been detected only in a few species. But in those of the very lowest organization the pores seem to be formed for the occasion, just as the onyx& opens anywhere to admit food within its substance, In spongilla fluriatilis, a small fresh-water species found in Britain, the opening and closing of each pore occupies less than a minute, and the pores do not open simultaneously, but in irregular succession, and apparently never again in precisely the same spot. No trace of the pore remains for an instant after its closing, nor is there • any indication of the point where a new one is to be open. The water which enters by the pores passes out of some sponges by a single orifice, which serves for the whole mass; others have numerous orifices (oscula) which are permanent, and are much larger than the pores by which the water is imbibed, the whole mass being pervaded by canals which lead from the pores to these orifices, from which, under the microscope, a constant discharge of water may be seen taking place, minute opaque particles being carried along with its current. These particles are not only fecal mat ter, but geminules and ova.