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Show 14 ON CLASSIFICATION. are all conjoined in a single lay.er, so as to form a continuo.us lamellar m-embrane, which constitutes the outer and superficial layer of the body. Beneath this is a. wide cavit!, co.mmunicating with the exterior by means of minute holes In the superficial layer (b), and, of course, filled with water. ~he cavity separates the superficial layer of the sponge from It~ deeper substq,nce, which is of the same character as the superfimal layer, being made up of a number of aggregated sponge particles, each of which has a nucleus and is competent to throw out nun1orous pseudopodia! prolongations if detached. While the living spon~e is contained in water, a great number of currents of water set m to the wide cavity beneath a, a, through the n1inuto apertures (b), which have thence been termed" inhalent." In the floor of the cavity, there are a nun1b r of apertures which lead into canals ramifying in the deep layer, and eventually ending in the floors of certain comparatively lofty funnels, or craters. The top of each of these presents one of those larger and less numerous apertures, which have boon referred to as existing on the surface of the sponge, and which are fitly termed "exhalent" apertures. For, as Dr. Grant discovered, many years ago, strong, though minute, currents of water are constantly flowing out of these large apertures; being fed by the currents which as constantly set in, by the small apertures and through the superficial cavity, into the canals of the deeper substance. The cause of this very singular system of currents remained for a long time unknown. It was Tendered intelligible by Dr. Bowerbank's discovery of the xistence of vibratile cilia in the genus Grantia, but it is only quite recently that the precise nature of the arrangement of tho apparatus which gives rise to these currents in ordinary sponges, has been made out by Lieberkiihn and by Carter. The canals which enter the deep substance of the sponge become dilated into spheroidal chambers, lined with sponge particles (Fig. 3, A, c, 0), each of which is provided with a vibratile cilium; and as all these cilia work in one direction-towards the crater-they swe p the water out in that direction, and its place is taken by fresh water, which flows in through the small apertures and thro11gh the superficial chamber. The currents of water carry along such mat- GREGARINIDA, RHI?.:OPODA, ~PONGIDA, AND INF SORIA. 15 ters as are suspended in them, and those are appropriated by the sponge particles ]juing tho passag ~, in just the saino way as any one of the Rhizopoda appropriates the particles of food it finds in the water abont itself. So that \Ve must not compare this system of aperture3 and canals to so 1nany mouths and intestines; but the sponge represents a kind of subaqueous city, where the people are arrano·ed about the streets and road , in such a manner, that each can asily appropriate his food from the water as it passes along. In the sponges two reproductive proces os are known to occur: the one of them, asoxna], corresponding with the encysting .process of the Gregarinida; and the other, truly s xual, and answ i'ing to the congress of the male and femaLe element in the higher animals. In the common fresh-water Spongilla, towards the autumn, the deeper )ayer of the sponge becomes full of exceedingly small bodies, sometimes called "seeds" or "gen1- mules," which are spheroidal, and have, at one point, an opening. Every one of these bags-in the walls of which are arranged a great number of very singular spicula, each resembling two toothed wheels joined by an axle-is, in point of fact, a mass of sponge particles which has set itself apart-gone into winter quarters, so to speak-and becoming quite quiescent, encysts itself and re1nains still. The whole Spongilla dies down, and the seeds, inclosed in their case, r<:main uninjured through tho winter. When the spring arrives, the encysted 1nasses witl1in the "seed," stimulated by the altered temperature of the water, creep out of their nests, and straightway grow up into Spongillm like that fron1 which they proceeded. But there is, in addition, a true sexual process, which goes on during the summer months. Individual sponge particles become quiescent, and take on the character of ova; while, in other parts, particular sponge particles fill with granules, the latter eventually becoming converted into spermatozoa. These sacs burst and some of the spermatozoa, coming into contact with the ova, impregnate them. The ova develop and grow into ciliated germs (D, fig. 3), which make their way out, and, after swimming about for a while, settle themselves down and grow up iuto Spongillm. |