Sponge, the common name applied to the order spongida, of the class of rhizopods, the most characteristic of the subkingdom protozoa. Sponges were for a long time regarded as plants, but the best naturalists are now agreed that they belong to the animal kingdom. Prof. H. J. Clark placed them nearest to the compound protozoans known as the flagellate infusoria, and it has been proved by him, and by others since, that the collar round the cilium must be regarded as the sponge animal; Kent classes them between the flagellate infusoria and the rhizopods; and Haeckel stands alone in placing them nearest to the corals or coelenterata. (See "Annual and Magazine of Natural History," London, January, 1870.) A sponge is really an aggregation of separate masses of an amoeba-like sarcode, secreting a supporting network of fibro-corne-ous, calcareous, or silicious matter, the compound mass being traversed by canals opening on the surface. The apparently homogeneous jelly, or sponge flesh, which covers the outside and lines the canals of the living sponge, is made up of an enormous number of sarcode masses, composed of separate sarcoids, each' capable of pushing out its pseudopodia, generally with a vibrating cilium, and, if detached, able to move and live independently.

Large rounded orifices, or oseula, are scattered over the surface of most sponges, which lead into sinuous canals permeating the substance in every direction; water is continually absorbed by the smaller pores of the sponge, tilling every part, and, having supplied air and food, is driven out through the oscula; the currents are kept up by the action of the minute vibra-tile cilia. In the words of Prof. Huxley, the sponge "represents a kind of subaqueous city, where the people are arranged about the streets and roads in such a manner that each can easily appropriate his food from the water as it passes along." Many sponges contain a large amount of silica, in the form of spicules of various shapes, both formed in their substance and introduced from without; two of the most beautiful of the silicious sponges will be found described under Glass Sponge and Venus's Flower Basket. - There is a gradual passage from the soft sponges of commerce to those of stiff and compact texture, with the fibres loaded with silicious spicula, crumbling easily when dry, and useless in the arts; others are rather of a felted character, usually grayish white. • Sponges vary much in form, being irregularly branched, round, pear-shaped, or cup-like, and are fixed by a kind of root at the base, or in-crust other bodies, growing mostly in groups attached to all kinds of objects, living or dead, fixed or floating; most are marine, but spongilla (Lam.) grows in fresh water; they often have brilliant colors.

Some, like cliona, instead of incrusting other objects, excavate branching cavities in shells, which they inhabit. Sponges are propagated sometimes by ciliated gemmules, yellowish and oval, arising from the sarcode mass and carried out by the currents; they are mostly formed in the spring, and, after swimming freely about for some time, become fixed and grow. They also produce internal, unciliated, oviform bodies, resembling winter ova, which, when thrown out, swell, burst, and give issue to the locomotive germs within; they are said also to grow by division, or growth of detached portions of the parent body; they are believed to be nourished by minute alga) drawn within their pores. Some live in shallow, others in very deep water; scarce and small in cold latitudes, they increase in size and number toward the tropics, being most abundant in the Australian seas. According to Dr. Bowerbank, there are 24 genera on the shores of Great Britain. While spongia is the type of the corneous sponges, thethys (Cuv.) and Grantia (Flem.) are types of the silicious and calcareous sponges respectively. (See Protozoa.) - For the latest researches on the sponges see the papers now in course of publication (1876) by Prof. A. Hyatt, in the " Memoirs of the Boston Society of Natural History," with figures and bibliography.

Haeckel (Monographic der Kallcschwdmmer, 1872) regards the sponges and acalephs as having been evolved from a common ancestor, which he calls protascus, described as a body cavity surrounded by two layers of cells; he compares the sponge to the embryos of higher animals, both verte-brato and invertebrate. In his view, the germ of all animals, and the adult of such forms as hydra, may be reduced to the simple form of the young of a calcareous sponge, which he calls gastrula; this he considers the "truest and most significant embryonal form of the animal kingdom." - The sponges of commerce are procured chiefly in the Mediterranean and the Bahama islands; most of them are obtained by diving, to which persons are trained from childhood in the Greek islands; the adhesion to the bottom is generally firm, and the growth slow. To bleach sponges, the finest and softest are selected, washed several times in water, and immersed in very dilute hydrochloric acid to dissolve out the calcareous matters; having been again washed, they are placed in another bath of dilute hydrochloric acid to which 6 per cent, of hyposulphite of soda dissolved in a little warm water has been added; the sponge is left in this bath 24 hours, or until it is as white as snow.

Smyrna is the chief place for the export of fine sponges. The coarse sponges used for horses and carriages, etc, are obtained chiefly from the Bahamas; when taken from the water they have a sickish, disagreeable odor, which soon becomes disgusting, like that of decomposing animal matter; they are first buried in dry sand, and when decomposition has ceased are exposed in wire cages to the action of the tide for purification. - Fossil sponges are found in the Trenton limestone, and, if scolitlius be a mining sponge, even as low as the Potsdam sandstone, and probably were in existence long before the oldest Silurian epoch. Brachiospongia, discovered by the Rev. Mr. Hovey in the Birdseye group of the lower Silurian, is characterized by armlike processes radiating from a central cup. Eospongia of Billings has been found in the lowest Potsdam.

Diagrammatic Section of Spongilla (after Huxley).

Diagrammatic Section of Spongilla (after Huxley). a a. Outer or superficial layer of sponge. b b. Inhalant apertures, or pores, c c. Ciliated chambers, d. An cx-halant aperture, or osculum. The arrows indicate the direction of the currents.

Sponge attached to its rocky bed.

Sponge attached to its rocky bed.