The description of Polyp structure given above includes all the general features of the sea-anemone; but for the better explanation of the figures, it may not be amiss to recapitulate them here in their special application. The body of the sea-anemone may be described as a circular, gelatinous bag, the bottom of which is flat and slightly spreading around the margin. ([Fig. 2.]) The upper edge of this bag turns in so as to form a sac within a sac. ([Fig. 6.]) This inner sac, s, is the stomach or digestive cavity, forming a simple open space in the centre of the body, with an aperture in the bottom, b, through which the food passes into the larger sac, in which it is enclosed. But this outer and larger sac or main cavity of the body is not, like the inner one, a simple open space. It is, on the contrary, divided by vertical partitions into a number of distinct chambers, converging from the periphery to the centre. These partitions do not all advance so far as actually to join the wall of the digestive cavity hanging in the centre of the body, but most of them stop a little short of it, leaving thus a small, open space between the chambers and the inner sac. ([Fig. 1.]) The eggs hang on the inner edge of the partitions; when mature they drop into the main cavity, enter the inner digestive cavity through its lower opening, and are passed out through the mouth.

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.

[Fig. 7]

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.

The eggs are not always laid in the condition of the simple planula described above. They may, on the contrary, be dropped from the parent in different stages of development, sometimes even after the tentacles have begun to form, as in Figs.[ 8], [9]. Neither is it by means of eggs alone that these animals reproduce themselves; they may also multiply by a process of self-division. The disk of an Actinia may contract along its centre till the circular outline is changed to that of a figure 8, this constriction deepening gradually till the two halves of the 8 separate, and we have an Actinia with two mouths, each surrounded by an independent set of tentacles. Presently this separation descends vertically till the body is finally divided from summit to base, and we have two Actiniæ where there was originally but one. Another and a far more common mode of reproduction among these animals is that of budding like corals. A slight swelling arises on the side of the body or at its base; it enlarges gradually, a digestive cavity is formed within it, tentacles arise around its summit, and it finally drops off from the parent and leads an independent existence. As a number of these buds are frequently formed at once, such an Actinia, surrounded by its little family, still attached to the parent, may appear for a time like a compound stock, though their normal mode of existence is individual and distinct.

[Fig. 8]

[Fig. 9]

Figs. 8, 9. Young Actiniæ in different stages of growth.

The Actinia is exceedingly sensitive, contracting the body and drawing in the tentacles almost instantaneously at the slightest touch. These sudden movements are produced by two powerful sets of muscles, running at right angles with each other through the thickness of the body wall; the one straight and vertical, extending from the base of the wall to its summit; the other circular and horizontal, stretching concentrically around it. By the contraction of the former, the body is of course shortened; by the contraction of the latter, the body is, on the contrary, lengthened in proportion to the compression of its circumference. Both sets can easily be traced by the vertical and horizontal lines crossing each other on the external wall of the body, as in [Fig. 5]. Each tentacle is in like manner furnished with a double set of muscles, having an action similar to that described above. In consequence of these violent muscular contractions, the water imbibed by the animal, and by which all its parts are distended to the utmost, is forced, not only out of the mouth, but also through small openings in the body wall scarcely perceptible under ordinary circumstances, but at such times emitting little fountains in every direction.