When a living sponge is examined atten tively in its native element, the water is per aeived entering at the pores and issuing from the fmcal orifices, its courSe being indicated by the Motion of any floating particles that may be present. The issuing currents are stronger than the entering, and are rendered con spicuous by excrementitious matters or some times ova, conveyed out at the fwcal orifices.
When sections of the sponge, including a greater or less extent of the internal canals, are placed in water, the fluid, according to Dr. Grant's observations, is still evidently moved along the internal, surface of the portions of canals, although their continuity with the rest is destroyed. Dr. Grant could not detect cilia either in these canals or the pores vvhich lead to them, but he discovered these organ's on the oi7a of the sponge, which there by execute remarkable spontaneous motions, and he is inclined to attribute the currents' in the adult siionge also to'cilia, which he conceives may probably exist, though, from their small ness, he has not been able to perceive them. At any rate he has shewn by' most satisfactory observations, that the current cannot be ascribed to contractions-in the canal, for in none of his numerous experiments instituted for the pur pose, could he discover any sign of irritability, at least any sign of contraction of the tissue of the sponge on the application of stimuli.
Naturalists even of the earliest times, whose attention was directed to the phenomena exhi bited by- the living sponge, have remarked that water. entered and passed out from its porous subStance, but the true course of the fluid seems to have. been unknown, it having been erroneously supposed to enter and issue by the same orifices. Dr. Grant,* to whose labours We owe Most of the correct information ob tained respecting the structure and functions of the sponge, demonstrated that the current is continuous, and flows always in one direction as abOve described, and proved that the motion of the water was not produced by contraction and dilatation of the tissue of the sponge, which he showed to be destitute of irritability. Dutrochet had made observations on the same subject, which were published subsequently* to those of Dr. Grant, and not anteriorly as he
supposes; he perceived the constant direction of the current, and ascribed the phenomenon to endosmosis and exosmosis.
3. Ciliary nzotion of the ova of Polypi and Sponges.—The ova or gemmules of several of these zoophytes execute independent move ments, and produce currents in the surrounding water. This singular fact was, it appears, first noticed by Mr. Ellis in 1755,t in ex amining a species of Sertularia, the Campanu laria dichotoma ; but he described the ova or embryos which he had seen in motion, as young polypi, already somewhat advanced in their formation. Cavolini,t in 1784 and 1785, observed the same phenomenon in the ova of the Gorgonia and Madrepore, and investigated it more fully. He saN,v the egg-shaped gem mules or ova, on quitting the parent, rise to the surface, and swim with their large end for wards, in a horizontal direction, till they fixed themselves OD some spot where they were deve loped. Dr. Grant,§ in 1825, discovered similar motions in the ova of the sponge, and detected the moving cilia. The cilia covered the whole surface of the ovum, except the pos terior tapering extremity, and in its motions the large end of the ovum was always directed forwards. When an ovum fixed itself, its cilia still continued to play, by which a current along its surface was kept up for some time. Dr. Grant also investigated the movements of the ova of the Campanularia, previously seen by Ellis, and of the Plumularia falcata. The ova of both these zoophytes are .contained within transparent capsules, two or more being in each capsule, surrounded by a clear fluid. Dr. Grant distinctly perceived cilia vibrating on the surface of the ova, and causing, while within the capsule, an eddying motion of the surrounding fluid, but propelling the ova through the water when extracted from_ their capsule, as in the sponge. The ciliaryJriotion has also been found in the ova of fresh-water polypi, having been discovered by Meyenil in those of the Alcyonella stagnorum, which is probably the same with, or at least nearly allied to the Bell-flower Polype.