de Quatrefages: "The larva, which is at first almost spherical and entirely covered with vibratile cilia, may be compared to a very minute hedgehog, in which every spine acts as a natatory organ. It swims in all directions with extreme agility, and this first state continues about a day and a half. Toward the end of this time the external skin bursts, and, after being incrusted with calcareous salts, becomes a shell, which is at first oval, then triangular, and at last very nearly spherical. While the shell is being formed, the vibratile cilia disappear, but the little animal is not on that account condemned to inactivity. In proportion as the external cilia diminish, we observe that another equally ciliated organ becomes developed, which widens and extends in such a manner as to form a large collar or ruff margined with fringes. This new organ of locomotion may be entirely concealed within the shell, or may be extended from it, and acts in the manner of the paddle-wheel of a steamboat.
Fig. 1.
Rock perforated by Pholades.
"By means of this apparatus the young larva continues to swim with as much facility as in its earlier age; but it now, moreover, acquires another organ, a sort of fleshy foot, which can be extended and contracted at will. It has also organs of hearing similar to those of several mollusks, and eyes analogous to those of certain annelids." The last metamorphosis is when the Teredo takes its worm-shape, and is ready to commence its boring operations.
The Teredo is supposed to have been originally a native of tropical
