Page:Popular Science Monthly Volume 13.djvu/567

Rh chance (which, by-the-way, only secures such good fortune to very careful observers) to seize the moment when an annelide, coming out of one of the openings of wood, at once took possession of a teredo that he had placed on the bottom of the vessel which held the wood. He saw the annelide seize the teredo with his jaws, draw him into the canal which he occupied, and devour him so completely that there only remained the two valves of the shell.

It is in an entirely different manner that the cirripeds (Balanus sulcatus) aid in preserving wood. When these animals, to which sailors and the inhabitants of our coasts give the name of Pustules of the Sea, or Sea-Thorns, multiply to such an extent on the surface of wood that their disks touch, without leaving the least vacant space, the natural consequence is, that the young teredo cannot find any place where it can attach itself, and hence it is impossible for him to penetrate the wood. This preservative effect is produced even when the shells have fallen, provided the disks adhere to the wood.

.—The commission gave in its first report an historical epitome of the injuries done by the teredo at different epochs in Holland.

When the teredo was remarked for the first time, an idea prevailed that it was imported from abroad; vessels coming from the East Indies were accused of having brought that destructive guest. Two facts show the incorrectness of this idea. On the occasion of the deepening of the Dumbart Dock at Belfast, William Thompson found, twelve feet below the surface of the earth, in a blue, argillaceous soil, the trunk of a tree entirely riddled by the teredo. Considering the depth at which this débris was found, and the fact that it lay beneath a series of strata of shells, it is certain that it was deposited there ages ago, long before a vessel, coming from the East or West, could touch the coast at Belfast.

Fossil wood, perforated by the teredo, has been found in different localities: for example, in the London clay, in the Eocene formations at Brussels, where Van Beneden discovered fossil wood, inclosing the remains of the teredo; and at considerable depth, also, near Ghent, at the time of the construction of the citadel.

The teredo existed in a geological period earlier than our own, and he appears to have been always an inhabitant of our coast. Why is it, then, that at certain epochs, as in the years 1730, 1770, 1827, 1858, and 1859, he multiplied so prodigiously as to destroy entire dikes in a very short space of time? Even as early as 1733, Massuet assigned as a cause an increase of the degree of saltness of the water, resulting