Fig. 55. Very young Polyps, attached to a Bryozoa.
It is only at a certain period after birth that the coral polyp fixes itself and commences its metamorphoses, which consist essentially in a change of form and proportions. The buccal extremity is diminished and tapers off, whilst the base swells, and is enlarged—it becomes discoid; the posterior surface of this sort of disk is a plane, the front representing the mouth, at the bottom of a depression edged with a great cushion. Eight mammillations or swellings now appear, corresponding to the chambers which divide the interior of the disk: the worm has taken its radiate form. Finally, the mammals are elongated and transformed into tentacula. In Fig. 55 a young coral polyp is represented fixed upon a bryozoa, a name employed by Ehrenberg for zoophytes having a mouth and anus. It forms a small disk, the fortieth part of an inch in diameter, and having its spicula already coloured red. Fig. 56 shows the successive forms of the young polyps in the progressive phases of their development—being a young coralline polyp fixed upon a rock still contracted. Fig. 57 is a similar coralline attached to a rock and expanding its tentacula. Fig. 58 represents a small pointed rock covered with polypi and polypidoms of the natural size and of different shapes, but all young, and indicating the definite form of development which the collective beings are to assume.
Fig. 56. A young Coral Polyp fixed upon a Rock. (Lacaze-Duthiers.)
The simple isolated state of the animal, whose phases of development we have indicated, does not last long. It possesses the property of producing new beings, as we have already said, by budding. But how is the polypidom formed? If we take a very young branch, we find in the centre of the thickness of the crust a nucleus or stony substance resembling an agglomeration of spicula. When they are sufficient in number and size, these nuclei form a kind of stony plate, which is imbedded in the thickness of the tissues of the animal. These laminæ, at first quite flat, assume in the course of their development a horse-shoe shape. Figs. 59 and 60 will give the reader some idea of the form in which the young present themselves. Fig. 59 represents the corpuscles in which the polypidom has its origin; Fig. 60, the rudimentary form of the coralline polypidom.
Fig. 57. Young Coral Polyp attached to a Rock and expanded. (Lacaze-Duthiers.)
Our information fails to convey any precise notion of the time necessary for the coral to acquire the various proportions in which it presents itself.
