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TEREDO
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TEREDO , a genus of Lamellibranchiate See also:Mollusca," of the See also:order Eulamellibranchia, sub-order Adesmacea, See also:family Teredinidae. The animals included in this genus are commonly 638 known as " See also:ship-See also:worms," and are notorious for the destruction which they cause in See also:ships' timbers, the woodwork of harbours, and piles or other See also:wood immersed for a See also:long See also:period in the See also:sea. They inhabit long cylindrical holes, which they excavate in the wood, and usually occur in See also:great See also: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 See also:complete, but stops See also:short a little distance from the inner end of the burrow, where the See also:boring See also:process continues to take See also:place. In some burrows, however, the lining is complete, either because the See also:animal has reached its full See also:size or because some cause pre-vents it continuing its See also:tunnel; in such cases the calcareous See also:tube has a hemispherical termination. The burrows are usually driven in the direction of the See also:grain of the wood, but not in-variably so. When a See also:knot or See also:nail or the tube of a See also:neighbour is reached, the course of the burrow is altered so as to See also:bend See also: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 See also:species to which it belongs, and is cylindrical and See also:worm-like in See also: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 See also:ligament, but are widely separated dorsally. The valves are triangular in shape and very See also:concave on the See also:side which is in contact with the animal. In front their edges are widely separated, and the See also:mantle tube, which is elsewhere closed, has here a slight median See also:aperture, through which the short sucker-like See also:foot can be protruded. The next portion of the See also:body behind the See also:shell-bearing partis naked, except for the shelly lining of the burrow, which is secreted by this See also:part. Anteriorly this portion contains part of the body proper; posteriorly it forms a tube divided internally by a See also:horizontal See also:partition into two See also:chambers and representing the two tubular outgrowths of the mantle called siphons, here See also:united together. In the See also:lower chamber are the elongated gill plates, which have the typical lamellibranchiate structure. In the upper chamber anteriorly is the rectum. A thick See also:muscular See also:ring terminates this region of the body, and bears two calcareous plates shaped like spades or battledores. The See also:expanded parts of these plates are See also:free and project backwards; the handle is fixed in a deep socket or See also: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 See also:principal See also:organs of the body—See also:stomach, See also:heart, generative organs and nephridia—are situated in the anterior part of the body, forming a visceral See also:mass, which extends some distance behind the valves. The heart is above the See also: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 See also: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 See also:surface of the posterior adductor, are in this See also: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 See also:present. The stomach is provided with a large crystalline See also:style. The See also:function of the pallets is to See also:form an operculum to the calcareous tube when the siphons are with-See also:drawn into it. In some species the See also:external or narrower end of the calcareous tube is provided with transverse laminae projecting into the lumen; and in some the external aperture is divided by a horizontal partition into two, one for each See also:siphon. The Teredo is dioecious, and the See also:males are only in the proportion of 1 : 500 of he See also:females. As in the case of the See also:oyster, the ova are retained in the branchial chamber during the See also:early stages of their development. The segmentation of the ovum is unequal, and leads to the formation of a gastrula by epibole. By the growth of a preoral See also:lobe provided with a ring of See also:cilia, and by the formation of a mouth and an anus, the trochosphere See also:stage is reached. A pair of thin shells then appear on the sides of the larva, connected by a See also:hinge on the dorsal median See also:line, and the foot grows out between mouth and anus. By the See also:time the larvae " swarm," or leave the branchial cavity of the See also:parent to live for a time as free-See also:swimming pelagic larvae, the valves of the shell have grown so large as to See also:cover the whole of the body when the velum is retracted; the foot is also long, cylindrical and flexible, and can be protruded far beyond the shell. The valves of the shell at this stage arehemispherical in shape, so that the whole larva when its organs are retracted is contained in a globular case. Concerning the later changes of the larva and the method by which it bores into wood little or nothing is known from See also:direct observation. Much has been written about the boring of this and other marine animals, but even yet the See also:matter cannot be said to be satisfactorily elucidated. See also:Osier, in a See also: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. See also:Thompson, in a paper in the Edinb. New Phil. Journ., 1835, supported the view that the excavation is due to the See also:action of a solvent secreted from the surface of the animal. See also:Albany See also:Hancock, again (See also:Ann. and Mag. Nat. Hist., vol. xv.), thinks that the excavating See also: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 See also:acid secretion has been denied by others. See also:Jeffreys believes that the foot is the See also: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 See also: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 See also:rate: spawning takes place in the See also:spring and summer, and before the end of the See also:year the animals are adult and their burrows of large size. Quatre-
fages relates that at See also:Guipuzcoa (N. See also:Spain) a See also:ferry-See also: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 See also:winter. This cannot be generally the case, as the size of the tubes varies so greatly. In See also: A great See also:deal of loss is, however, caused by Teredo in See also:harbour See also:works and See also:shipping stages, and the embankments in Holland are continually injured by it. The most efficient See also:protection is afforded by large-headed nails driven in in See also:close proximity. Soaking wood in See also:creosote is not a certain safeguard; Jeffreys found at See also:Christiania in 1863 that a large number of harbour piles previously soaked in creosote had been completely destroyed by T. navalis. See also:Coal See also:tar and the silicate of See also: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 See also:Theophrastus, See also:Pliny and See also:Ovid. In 1715 it is mentioned by Valisnieri, in 1720 by Deslandes. In 1733 great See also:attention was drawn to it on See also:account of the See also:discovery that the wooden dikes of Holland were being rapidly destroyed by ship-worms, and that the See also:country was in danger of inundation. Three See also:treatises were published concerning the animal, by P. Massuet, J. Rousset and See also:Godfrey Sellius. The See also:work of the last-named, which was the best, described the See also:anatomy of the creature and showed that its See also:affinities were with bivalve molluscs. The truth of Sellius's view was not grasped by See also:Linnaeus, who placed Teredo together with Serpula in the genus Dentalium; but its proper position was re-established by See also:Cuvier and See also:Lamarck. See also:Adanson, unaware of the work of Sellius, in 1757 believed himself to be the first to discover the molluscan affinities of T redo. It will not be necessary to give here a See also:definition of the genus taken from any systematist; it will be sufficient to point out that the long cylindrical body with its two small anterior polygonal valves, the See also:absence of a ligament and See also:accessory valves, the muscular ring into which are inserted the calcareous pallets, and the continuous calcareous tube lining the hole bored by the animal are the diagnostic features. Jeffreys, in his See also:British Conchology, gives the following species as British : Teredo norvegica, Spengler; T. navalis, Linn.; T. pedicellala, Quatrefages; T. megotara, See also:Hanley. T. norvegica occurs chiefly on the See also:west See also:coast of Great See also:Britain. It was taken by Thompson at Portpatrick in See also:Wigtownshire, and occurred in Jeffreys s time in abundance at See also:Milford Haven. This species has been de-scribed by See also:Gmelin and a number of British authors as T. navalis, Linn. It is distinguished by having the See also:base of the pallets See also: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 See also:southern coasts of See also:Europe, from Christiania to the See also: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 See also:Bay of Los Pasages on the See also:north coast of Spain; it has also been found in the Channel Islands, at See also:Toulon, in See also:Provence and in See also:Algeria. In T. megotara the tube is simple and the pallets like those of T. norvegica; it occurs at See also:Shetland and See also:Wick, and also on the western See also:shore of the See also:Atlantic, where its range extends from See also:Massachusetts to See also:South Carolina. T. malleolus, See also:Turton, and T. bipinnata, Turton, belong to the West Indies, but are often drifted in floating See also:timber to the coasts of Europe. Other occasional visitants to the British shores are T. excavata, bipartita,. spatha, fusticulus, cucullata, and fimbriata. These were described by See also:Gwyn Jeffreys in Ann. and Mag. Nat. Hist., I86o. T. fimbriata is stated to be a native of See also:Vancouver's See also:Island. A See also:kind of ship-worm, the Nausitora dunlopei of See also:Perceval See also:Wright, has been discovered in See also:India, 70 M. from the sea, in a stream of perfectly fresh See also:water, namely, the See also:river Kumar, one of the branches of the See also:Ganges. T. corniformis, Lam., is found burrowing in the husks of coco-nuts and other woody fruits floating in the tropical seas; its tubes are extremely crooked and contorted for want of space. Fossil wood and See also:palm-fruits of See also:Sheppey and See also:Brabant are pierced in the same way. Twenty-four fossil species have been recognized in the See also:Lias and succeeding beds of Europe and the United States. The sub-genus Teredina, Lam., is a fossil of the See also:Eocene of Great Britain and See also:France. (J. T. Additional information and CommentsThere are no comments yet for this article.
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