The bottoms of ships, piles, and other timbers exposed to the action of the sea, are liable to be destroyed by marine animals, which attack them in every vulnerable part within their reach.

* ' Transactions of the Society of Arts,' etc, vol. xxi., p. 302. † Rees's ' Cyclopaedia,' art. Timber. ‡' Treatise on Building in Water.'

The most destructive of these are the Teredo navalis, or the well-known ship-worm, and the Limnoria terebrans. The former converts the timber into the state of a honeycomb, while the latter destroys it by gradually eating in between the annual rings.

477. The Teredo navalis is first attached to the timber in the form of an egg, which is supposed to be washed against it by the action of the sea. It remains in this state for some time, probably until the salt water has thoroughly penetrated the substance of the wood, and vegetation has commenced to form on its surface. When the animal leaves the egg it is very small, but after entering into the wood it increases to a considerable size, often 3 or 4 inches in length, and sometimes a great deal more.

In a fir pile taken from the old pier-head at Southend, a worm was found 2 feet long and 3/4 inch in diameter, and they have been heard of as much as 3 and 4 feet in length, and 1 inch in diameter, according to the nature of the wood which they inhabit.*

To the casual observer there is frequently no symptom of the destruction that is taking place, apparent on the surface, nor are the animals themselves visible until the outer part of the wood has been cut or broken away, when their shelly habitations come into sight and show the perfect honeycomb they have formed. On a closer examination of the wood, however, a multitude of very minute perforations are discovered in the surface, generally covered with a slimy matter: and on opening the wood at one of these and tracing it, the tail, or posterior portion of the animal, is immediately found, and after various windings and turnings, the head is discovered, which in some cases has been found as much as 3 feet from the point of entrance. Sometimes it will happen, especially if the wood has been much eaten, that their shelly tubes are partly visible on the surface, but this is rare; they enter at the surface, and bore in every direction, both with and against the grain of the wood, growing in size they proceed. It is rarely that the animal bores completely through, though it frequently approaches to within a twentieth of an inch of the surface. The head of the teredo is provided with a hard calcareous substance, which performs the office of an auger, and enables it to penetrate the hardest wood.

* 'Min. Proc. Inst, of Civil Engineers,' 1849-50.

Although these animals often bore exceedingly close to each other, they never injure one another's habitation.

Fir and alder are the two kinds of wood they seem to destroy with the greatest ease, and in which they attain their greatest size.

In oak and other hard woods they make slower progress, are smaller, and do not appear to be so well nourished.

In some situations they carry on their depredations with great rapidity, and the parts of the wood most subject to their attacks are those between the bottom and low water, and they appear gradually to relax in their destructive habits from low water towards high water. Some of the Memel fir piles of the old pier-head at Southend, which had been well coated with pitch and tar previously to being fixed, and parts covered with copper, showed signs of the teredo within six months after the completion of the work, and in twelve months was reported to have been seriously injured, and in four years the worm had eaten some of the piles completely through - both above and below the copper-sheeting.

The teredo, it has been said, was originally brought from India, but this is a mistake, as it has been discovered in this country in a fossil state, and its ravages have been known in Scotland for more than 300 years.*

* Stevenson's ' Design and Construction of Harbours.'

It seems, however to bo very abundant in the East and West Indies, on the coast of Africa, and in the Mediterranean, It is said to avoid bitter woods. The "Jarrah" timber of Western Australia has also been known to resist its ravages, and so has the Greenheart timber of British Guiana.