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Encyclopedia Britannica - Main :: TAV-THE |
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TEREDO , a genus of Lamellibranchiate Mollusca," of the order Eulamellibranchia, sub-order Adesmacea, family Teredinidae. The animals included in this genus are commonly 638 known as " ship-worms," and are notorious for the destruction which they cause in ships' timbers, the woodwork of harbours, and piles or other wood immersed for a long period in the sea. They inhabit long cylindrical holes, which they excavate in the wood, and usually occur in great numbers, crowded together so that often only a very thin film remains between the adjacent burrows. Each burrow is lined with a layer of calcareous substance secreted by the mollusc; this lining is not usually complete, but stops short a little distance from the inner end of the burrow, where the boring process continues to take place. In some burrows, however, the lining is complete, either because the animal has reached its full size or because some cause pre-vents it continuing its tunnel; in such cases the calcareous tube has a hemispherical termination. The burrows are usually driven in the direction of the grain of the wood, but not in-variably so. When a knot or nail or the tube of a neighbour is reached, the course of the burrow is altered so as to bend round the obstruction. One burrow is never found to break into another. The adult Teredo, when removed from its burrow and calcareous tube, is from a few inches to 3 ft. in length, according to the species to which it belongs, and is cylindrical and worm-like in appearance. The anterior end, which lies at the bottom of the burrow, is some-what enlarged and bears a pair of shells or valves, which are not connected by the usual ligament, but are widely separated dorsally. The valves are triangular in shape and very concave on the side which is in contact with the animal. In front their edges are widely separated, and the mantle tube, which is elsewhere closed, has here a slight median aperture, through which the short sucker-like foot can be protruded. The next portion of the body
body
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the handle is fixed in a deep socket or pit lined by epidermis. These calcareous plates are called pallets (Fr. palmules). Behind the pallets the tubular body bifurcates, forming two siphons similar to those of other Lamellibranchs; the siphons can be contracted or expanded within wide limits of length. The principal organs of the bodystomach, heart, generative organs and nephridiaare situated in the anterior part of the body, forming a visceral mass, which extends some distance behind the valves. The heart is above the intestine and not perforated by it. There are two adductor muscles of which the anterior is rudimentary and situated just above the mouth, while the posterior is large and passes between the middle parts of the shell-valves. The visceral mass extends some distance behind the posterior adductor, and behind the rectum, and the visceral ganglia, which in most Larnellibranchs are attached to the ventral surface of the posterior adductor, are in this case at the end of the visceral mass and at the anterior end of the gills. Besides the visceral ganglia a cerebral and a pedal pair are present. The stomach is provided with a large crystalline style. The function of the pallets is to form an operculum to the calcareous tube when the siphons are with- drawn
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The Teredo is dioecious, and the males are only in the proportion of 1 : 500 of he females. As in the case of the oyster
Concerning the later changes of the larva and the method by which it bores into wood little or nothing is known from direct observation. Much has been written about the boring of this and other marine animals, but even yet the matter cannot be said to be satisfactorily elucidated. Osier
paper in Phil. Trans., 1826, argued that the Teredo bores by means of its shells, fixing itself by the surface of the foot, which it uses as a sucker, and then rasping the wood with the rough front edges of the shell-valves. This view was founded on the similarity of the arrangement of the shells and muscles in Teredo to those occurring in Pholas, in which the method of boring described was actually observed. W. Thompson, in a paper in the Edinb. New Phil. Journ., 1835, supported the view that the excavation is due to the action of a solvent secreted from the surface of the animal. Albany Hancock, again (Ann. and Mag. Nat. Hist., vol. xv.), thinks that the excavating power of Teredo is due to silicious particles imbedded in the anterior portion of the integument, in front of the valves. But the actual existence of either silicious particles or acid secretion has been denied by others.Jeffreys believes that the foot is the organ by which the animal urrows. In the larger number of Lamellibranchs the foot is doubt-less a burrowing organ, and it is difficult to see how the limpet hollows out the rock to which it is attached if not by means of the surface of its foot. At the same time it is difficult to explain how the soft muscular foot can penetrate into hard wood. The process is of course slow, and Jeffreys supposes that particles are detached one by one from the moistened surface to which the foot is applied. In any case the valves are covered by an epidermis, which could scarcely be there if they were used in burrowing. Teredo grows and burrows at an extremely rapid rate: spawning takes place in the spring and summer, and before the end of the year the animals are adult and their burrows of large size. Quatre-fages relates that at Guipuzcoa (N. Spain) a ferry-boat was sunk accidentally in the spring , and was raised four months afterwards, when its timbers were already rendered useless by T. pedicellata. How long the animals live is not accurately known, but Quatref ages found that they nearly all perished in the winter. This cannot be generally the case, as the size of the tubes varies so greatly. In Holland their greatest ravages are made in July and August. Iron ships have nothing to fear from their attacks, and the copper sheathing now almost universally used protects wooden hulls. A great deal of loss is, however, caused by Teredo in harbour works and shipping stages, and the embankments in Holland are continually injured by it. The most efficient protection is afforded by large-headed nails driven in in close proximity. Soaking wood in creosote is not a certain safeguard; Jeffreys found at Christiania in 1863 that a large number of harbour piles previously soaked in creosote had been completely destroyed by T. navalis. Coal tar and the silicate of lime used for coating stonework have been suggested as protective coverings, but they do not seem to have been adequately tested.Species of Teredo occur in all seas. The animal was known to the ancients and is mentioned by Theophrastus, Pliny and Ovid. In 1715 it is mentioned by Valisnieri, in 1720 by Deslandes. In 1733 great attention was drawn
Jeffreys, in his British Conchology, gives the following species as British : Teredo norvegica, Spengler; T. navalis, Linn.; T. pedicellala, Quatrefages; T. megotara, Hanley. T. norvegica occurs chiefly on the west coast of Great Britain. It was taken by Thompson at Portpatrick in Wigtownshire, and occurred in Jeffreys s time in abundance at Milford Haven. This species has been de-scribed by Gmelin and a number of British authors as T. navalis, Linn. It is distinguished by having the base of the pallets simple, not forked, and the tube semi-concamerated at its narrower posterior end. The length does not usually exceed a foot. It is the T. navium of Sellius. T. navalis has been identified from the figures of Sellius, to which Linnaeus referred; Sellius called it T. marina. It occurs on all the western and southern coasts of Europe, from Christiania to the Black Sea, and is the species which causes so much damage to the Dutch embankments. The pallets of this species are small and forked, and the stalk is cylindrical. The tube is simple and not chambered at its narrow end. T. pedicellata was originally discovered by Quatrefages in the Bay of Los Pasages on the north coast of Spain; it has also been found in the Channel Islands, at Toulon, in Provence and in Algeria. In T. megotara the tube is simple and the pallets like those of T. norvegica; it occurs at Shetland
Wright
Twenty-four fossil species have been recognized in the Lias and succeeding beds of Europe and the United States. The sub-genus Teredina, Lam., is a fossil of the Eocene of Great Britain and France. (J. T. C.) End of Article: TEREDO If you wish, you can link directly to this article.
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