[Illustration: Fig. 6. Vertical section of an Actinia, showing a primary _(g)_ and a secondary partition of _g'_; _o_ mouth, _t_ tentacles, _s_ stomach, _f f_ reproductive organs, _b_ main cavity, _c_ openings in partitions, _a_ lower floor, or foot.] The embryo bears no resemblance to the mature animal. It is a little planula, semi-transparent, oblong, entirely covered with vibratile cilia, by means of which it swims freely about in the water till it establishes itself on some rocky surface, the end by which it becomes attached spreading slightly and fitting itself to the inequalities of the rock so as to form a secure basis. The upper end then becomes depressed toward the centre, that depression deepening more and more till it forms the inner sac, or in other words the digestive cavity described above. The open mouth of this inner sac, which may, however, be closed at will, since the whole substance of the body is exceedingly contractile, is the oral opening or so-called mouth of the animal. We have seen how the main cavity becomes divided by radiating partitions into numerous chambers; but while these internal changes are going on, corresponding external appendages are forming in the shape of the tentacles, which add so much to the beauty of the animal, and play so important a part in its history. The tentacles, at first only few in number, are in fact so many extensions of the inner chambers, gradually narrowing upward till they form these delicate hollow feelers which make a soft downy fringe all around the mouth. (Fig. 7.) They do not start abruptly from the summit, but the upper margin of the body itself thins out to form more or less extensive lobes, through which the partitions and chambers continue their course, and along the edge of which the tentacles arise. [Illustration: Fig. 7. View from above of an Actinia with all its tentacles expanded; _o_ mouth, _b_ crescent-shaped folds at extremity of mouth, _a a_ folds round mouth, _t t t_ tentacles.] [Illustration: Figs. 8, 9. Young Actiniæ in different stages of growth.]