the Laomedeæ it is very common to find branches torn off, but the specimens show evident signs of great violence in the destruction of their cells, and other marks of depauperization. The Sertularia nigra, a remarkably stout and rigid species, which grows near the Eddystone in the English Channel, is remarkably exposed to strong and constantly changing currents, yet the shedding of the pinnae is comparatively rare in it, and the ovarian vesicles are constantly produced in rich profusion. If the sheath were subject to no change after it was once formed, the younger pinnæ would suffer equally with the older ones, which is not the case. When either the pinnæ or vesicles disappear, they leave behind well-defined cicatrices, which gradually become nearly obliterated, and not jagged and irregular marks, as if produced by violence. They appear to fall, after having performed their functions, from a process very similar to that by which trees shed their leaves and fruits.
The sheath also, under certain conditions, affords signs of irritation which could not be expected from a dead membrane. In the Campanularia volubilis, in which the cells stand on long slender footstalks, which are annulated at each extremity and plain in the centre, the whole length of the peduncle will become corrugated, as if ringed throughout. Ellis figures them in this condition at pl. 14, fig. A, in his Essay on Corallines; and I have seen them assume this appearance, and again change it, while under the microscope: in either state, and while the change was going on, the polype appeared to be very little influenced by it, or to have any effect in producing it. The extremities of the branches and trunk are always more tender than other parts, from being more recently formed: these points frequently give way, and the pulp is forced through the ruptured parts. This extravasation is sometimes so extensive that in the knotted sea-thread (Laomedea geniculata), sea-bristles (Plumularia setacea), &c, scarcely a vestige of the pulp remains. This effect can be produced by immersion -in fresh' water, by allowing the animal to remain in deteriorated water, or by the warming of the water by the manipulation used in examination. In what way can this be produced but by the pressure of the horny envelope on the pulp? This, together with the corrugation of the stalks of the cells in Campanularia volubilis, shows a degree of irritability in the sheath entirely incompatible with an inorganic and extravascular nature. The two parts are intimately connected, governed in their growth by one principle, which not only regulates their formation, but appears to be the source of the specific differences. The polypes are but of secondary importance, and ex-
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