A branch of _living_ coral, if we may use the term, is an aggregation of animals derived from a first being by budding. They are united among themselves by a common tissue, each seeming to enjoy a life of its own, though participating in a common object. The branch seems to originate in an egg, which produces a young animal, which attaches itself soon after its birth, as already described. From this is derived the new beings which, by their united labours, produce the branch of coral or polypidom. [Illustration: Fig. 47. Living Bed of Coral after the entrance of the Polyps. (Lacaze-Duthiers.)] This branch is composed of two distinct parts: the one central, of a hard, brittle, and stony nature, the well-known coral of commerce; the other altogether external, like the bark of a tree, soft and fleshy, and easily impressed with the nail. This is essentially the bed of the living colony. The first is called the polypidom, the second is the colony of polyps. This bed (Fig. 47) is much contracted when the water is withdrawn from the colony. It is covered with salient mammals or protuberances, much wrinkled and furrowed. Each protuberance encloses a polyp, and presents on its summit eight folds, radiating round a central pore, which presents a star-like appearance. This pore as it opens gives to the polyps the opportunity of coming out. Its edge presents a reddish calyx, like the rest of the bark, the festooned throat of which presents eight dentations. [Illustration: Fig. 48. Three Polyps of the Coral. (Lacaze-Duthiers.)] The polyp itself (Fig. 48) is formed of a whitish membranous tube, nearly cylindrical, having an upper disk, surrounded by its eight tentacula, bearing many delicate fibres spreading out laterally. This assemblage of tentacula resembles the corolla of some flowers; its form is very variable, but always truly elegant. Fig. 49 (which is borrowed from M. Lacaze-Duthiers' great work) represents one of these forms of the coral. [Illustration: Fig. 49. Coral Polyp. (Lacaze-Duthiers.)] The arms of the polyps are at times subject to violent agitation: the tentacula become much excited. If this excitement continues, the tentacula can be seen to fold and roll themselves up, as shown in Fig. 50. If we look at the expanded disk, we see that the eight tentacula attach themselves to the body, describing a space perfectly circular, in the middle of which rises a small mammal, the summit of which is occupied by a small slit like two rounded lips. This is the mouth of the polyps, the form being very variable, but well represented in Fig. 50, where the organ under consideration is displayed.