The coral of commerce, so beautiful and so appreciated by lovers of bijouterie, is the polypidom. It is cylindrical, much channeled on the surface, the lines usually parallel to the axis of the cylinder, the depressions sometimes corresponding to the body of the animal. If the transverse section of a polypidom be examined, it is found to be regularly festooned on its circumference. Towards its centre certain sinuosities appear, sometimes crossing, sometimes trigonal, sometimes in irregular lines, and in the remaining mass are reddish folds alternating with brighter spaces which radiate from the centre towards the circumference (Fig. 53). In the section of a very red coral, it will be observed that the colour is not equally distributed, but separated into zones more or less deep in colour, containing very thin preparations which crack, not irregularly, but parallel to the edge of the plate, and in such a manner as to reproduce the festoons on the circumference. From this it may be deduced that the stem increases by concentric layers being deposited, which mould themselves one upon the other. In the mass of coral certain small corpuscles occur, charged with irregular asperities, much redder than the tissue into which they are plunged. These are much more numerous in the red than in the light band, and they necessarily give more strength to the general tint. [Illustration: Fig. 54. Birth of the Coralline Larvæ. (Lacaze-Duthiers.)] To the mode of reproduction in the coral polyps, so well described by Lacaze-Duthiers, we can only devote a few lines. Sometimes, according to this able observer, the polyps of the same colony are all either male or female, and the branch is _unisexual_; in others there are both male and female, when the branch is _bisexual_. Finally, but very rarely, polyps are found uniting both sexes. The coral is viviparous; that is to say, its eggs become embryos inside the polyp. The larvæ remain a certain time in the general cavity of the polyp, where they can be seen through its transparency, as exhibited in Fig. 54. Aided by the magnifying powers of the microscope, coral larvæ may here be perceived through the transparent membranous envelope. From this position they escape from the mouth of the mother in the manner represented in the upper branch. The animal then resembles a little white grub or worm, more or less elongated. The larva is, however, still egg-shaped or ovoid; moreover, it is sunk in a hollow cavity, and covered with cilia, by the aid of which it can swim. Sometimes one of its extremities becomes enlarged, the other remaining slender and pointed. Upon this an opening is formed communicating with the interior cavity: this is the mouth. The larvæ swim backwards; that is to say, with the mouth behind. [Illustration: Fig. 55. Very young Polyps, attached to a Bryozoa.]