in natural history, a genus of vermes belonging to the order of teredos. The animal is a terebella; there are two valves, calcareous, hemispherical, and cut off before, and two lanceolated. The shell is tapering, bending, and capable of penetrating wood. There are only three species; the navalis, utriculus, and clavus.
The navalis, or ship-worm, which has a very slender smooth cylindrical shell, inhabits the Indian seas, whence it was imported into Europe. It penetrates easily into the stoutest oak-planks, and produces dreadful destruction to the ships by the holes it makes in their sides; and it is to avoid the effects of this insect that vessels require sheathing.
The head of this creature is well prepared by nature for the hard offices which it has to undergo, being coated with a strong armour, and furnished with a mouth like that of the leech; by which it pierces wood, as that animal does the skin; a little above this it has two horns which seem a kind of continuation of the shell; the neck is strongly provided for the service of the creature as the head, being furnished with several strong muscles; the rest of the body is only covered by a very thin and transparent skin, through which the motion of the intestines is plainly seen by the naked eye; and by means of the microscope several other very remarkable particulars become visible there. This creature is wonderfully minute when newly excluded from the egg, but it grows to the length of four or six inches, and sometimes more.
When the bottom of a vessel, or any piece of wood which is constantly under water, is inhabited by these worms, it is full of small holes; but no damage appears till the outer parts are cut away: Then their shelly habitations come into view; in which there is a large space for inclosing the animal, and surrounding it with water. There is an evident care in these creatures never to injure one another's habitations; by this means each case or shell is preserved entire; and in such pieces of wood as have been found eaten by them into a fort of honeycomb, there never is seen a passage or communication between any two of the shells, tho' the woody matter between them often is not thicker than a piece of writing-paper.
They penetrate some kinds of wood much more easily than others. They make their way most quickly into fir and alder, and grow to the greatest size. In the oak they make small progress, and appear small and feeble, and their shells much discoloured.
Since each of these animals is lodged in a solitary cell, and has no access to those of its own species, it has been matter of surprize how they should increase to so vast a multitude. Upon dissecting them, it appears that every individual has the parts of both sexes, and is therefore supposed to propagate by itself.
The sea-worms, which are pernicious to our shipping, appear to have the same office allotted them in the waters which the termites have on the land (see Termites). They will appear, on a very little consideration, to be most important beings in the great chain of creation, and pleasing demonstrations of that infinitely wise and gracious Power which formed, and still preserves, the whole in such wonderful order and beauty; for if it was not for the rapacity of these and such animals, tropical rivets, and indeed the ocean itself, would be choked with the bodies of trees which are annually carried down by the rapid torrents, as many of them would last for ages, and probably be productive of evils, of which, happily, we cannot in the present harmonious state of things form any idea; whereas now being consumed by these animals, they are more easily broken in pieces by the waves; and the fragments which are not devoured become specifically lighter, and consequently more readily and more effectually thrown on shore, where the sun, wind, insects, and various other instruments, speedily promote their entire dissolution.