Circulation of Water.— In the living sponge, as was first remarked by Professor Thomas Bell, and subsequently by other observers, a constant circulation of the surrounding ele ment, is, by some mysterious agency, kept up throughout its substance, the water being perpetually sucked in, as it were, through all the rninute pores, upon the periphery of the mass, and again emitted in continuous streams through the larger orifices (oscula) of the sponge.
Dr. Grant put a small branch or Spongia coalita with some sea water into a watch-glass, in order to examine it with the microscope, and thus describes the phenomena it pre sented :—" On moving the watch-glass, so as to bring one of the apertures on the side of the sponge fully into view, 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 atten tion ; but after twenty-five minutes of constant observation I was obliged to withdraw my eye, from fatigue, without having seen the tor rent, 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. About the end of this time, however, I observed the current become perceptibly languid ; the opaque floc culi of feculent matter, which were thrown out with so much impetuosity at the begin ning, were now propelled to a shorter dis tance from the orifice, and fell to the bottom of the fluid within the sphere of vision, and, in one hour more, the current had entirely ceased." Subsequently, - two round portions of the Spongia panicea were placed together in a vessel of sea-water, vvith 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. Dr. Grant then 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 of dry paper over the aper tures, he could perceive them moving by the force of the currents at the distance of ten feet from the table on which the specimen rested. A portion of soft bread pressed be
tween the fingers into a globular form was not moved away in a mass by the stream. but was gradually worn down by the current beating on its sides, and thus propelled to a distance in small flakes. A globule of mercury of equal diameter with the orifice, let fall upon it from a glass tube, was not removed or shaken, and completely stopped the current. In this condition, on piercing the sponge with a needle, a new current was established through the artificial canal thus formed, which con tinued even after removing the obstruction from the original orifice.
A globule of mercury of any smallness placed over the orifice of a living sponge, is too heavy to be affected by the small column of vvater which impels against its smooth round surface, flowing at the rate with which it issues from that orifice, and is useful in enabling us to stop up the currents of certain orifices, in order to direct the stream with greater force through a particular aperture wbich we vrish to examine through the mi croscope. By adopting this plan with some sponges, which have very few and large orifices on the surface, it is distinctly per ceptible with the naked eye, that the current never enters by the same apertures through which it issues, and we might thus measure the whole strength of the forces employed to produce the currents in any particular speci men.* Various hypotheses have been suggested to account for the production of these streams of water which constantly percolate the body of the sponge, but all of them have been rejected in turn as unsatisfactory. Ciliary movement might be supposed to be the cause of this phenomenon, were it not that no observer has been able to detect, even with the most powerful microscopes, the presence of cilia in the inter;or of the aquiferous canals. At certain seasons, indeed, when the ciliated re productive gemmules described by Dr. Grant are abundantly disseminated through the living cortex of the sponge, it would seem possible that they might have Wine influence ; but as the currents appear to be equally strong at all periods, even when these gemmules are not developed, this supposition is untenable. Lastly, the laws of endesmosis have been ap pealed to as capable of' explaining the phe nomenon in question, yet even here there are difficulties not easily got rid of.