Popular Science Monthly/Volume 13/August 1878/The Teredo and its Depredations I
|THE TEREDO AND ITS DEPREDATIONS.|
DURING a period of about twenty-five years previous to 1858, the injuries caused to the timber of marine constructions by the Teredo navalis were rarely noticed in Holland, when, during the summer of that year, public anxiety was awakened afresh on that subject. Some repairs, undertaken at that time, of the marine works of the port of Nieuwendam, a village situated on the Y, brought to light the fact that all the piles broke off at the slightest blow, and were found to be entirely eaten off by the teredo.
The late secretary of the Royal Academy of Sciences of Amsterdam, Prof. W. Vrolik, called the attention of the Academy to this subject at a meeting held November 27, 1858, and the Academy appointed a commission from its own members, composed of Messrs. W. Vrolik, P. Harting, D. J. Storm Buysing, J. W. L. van Oordt, and E. H. von Baumbauer, charged with the duty of collecting and examining into all facts known concerning the natural history of the teredo, and, at the same time, to inquire into the best means for preserving wood from destruction by that mollusk.
Considering the great importance of this question to our country, bathed on all sides by the sea, the commission asked the assistance of the Government in its work, which was readily granted. This subject being of equal importance to other countries situated on the sea, and researches into the means of preventing the ravages of the teredo having been undertaken and the results published, especially in England, France, and Belgium, I have thought that a brief communication of the results we reached would be interesting and perhaps useful abroad, especially as our work was conducted on a large scale.
Before relating the experiments conducted by the commission, I propose to give a sketch of the examinations made by Mr. Harting, on the structure of the teredo and its mode of life, which have been very carefully studied by M. P. Kater, at Nieuwendam.
On the Mechanism of the Apparatus with which the Teredo perforates its Galleries.—The researches of several leading naturalists into the habits and structure of mollusks which perforate hard substances, such as wood and stone, have shown that some of them, which are found in calcareous rocks, make their excavations through some chemical means, i. e., by the dissolving action of an acid secretion, while the teredos and some others employ in their work purely mechanical means only. The manner in which the teredos proceed in their work has not, however, been clearly pointed out. In fact, while Hancock does not consider the shell, but the fleshy foot, as the boring instrument, and Quatrefages attributes that rôle to a part of the mantle of the animal, extending like a fold to the margin of the shell, Corilland has indicated the shell itself as the perforating instrument. By fastening the shell of a teredo on the end of a small stick of wood with gum, and turning it between the thumb and finger, he has succeeded, after four and a half hours' labor, in boring a hole in wood thirty millimetres deep. Mr. Harting arrived at the same conclusions by a careful microscopic examination of the shell and the muscular system of the teredo. We will point out the principal results of his studies, with illustrations to make them clear:
The shell is composed of two valves of equal size, which are not fastened together with a hinge; this is also the case with all other species of the genus Teredo and Pholas. The valves are maintained in place by a fold of the mantle in form of an arc (Fig. 1, b), which encircles them posteriorly. Moreover, the posterior part of the mantle has a
|Fig. 1.||Fig. 2.|
prolongation (Fig. 1, a, and Fig. 2, a) which covers, to a certain extent, the dorsal side of the valves, and extends on each side to their margin, forming two lobes (Fig. 1, c, and Fig. 2, b), which nevertheless do not adhere to the shell; by this mode of union the relative position of the valves is maintained. With other bivalve mollusks, which do not perforate, this relation is firmly fixed by a hinge; but, with the teredo, the valves have a certain play, which allows a slight displacement in their relative positions. The valves are, moreover, connected by two adductor muscles, which we will soon examine more closely.
The shell presents, even when the valves are brought closely together, three large openings.
The first, on the dorsal face (Fig. 3), is occupied in part by a pallial prolongation, a continuation of which is introduced by this opening into the interior of the shell, in part by the small adductor muscle.
|Fig. 3.||Fig. 4.|
The second opening is posterior (Fig. 4, a), and serves to open a passage to the internal organs contained in the cavity of the mantle.
Finally, the third, placed obliquely in front (Fig. 5, a a; Fig. 4, a), is the largest, and remains always gaping open to allow the foot to pass out (Fig. 5, b).
Each of these valves, which form the shell, is formed of three parts, viz.:
|Fig. 5.||Fig. 6.|
1. A posterior part (Fig. 3,f; Fig. 6, f; Fig. 7 f), which we can call the neck part; this posterior is the least arched, and thinner than the rest of the shell: its posterior edge is embraced by the folds of the mantle, which we have already mentioned, and thus the mantle is solidly attached to the shell.
2. The middle part (Fig. 3, b; Fig. 6, b; Fig. 7, b), which is the largest, is strongly arched, and presents, when seen from the side, the form of a half-moon; its ventral portion is a little more pointed, curved inwardly, and terminated by a small swelling or tubercle (Fig. 4, b'; Fig. 7, b'), which, when the shell is closed, comes in contact with the similar tubercle on the opposite valve.
3. The anterior part, which is a continuation of the upper portion of the preceding part, and is more or less spiral in form (Fig. 6, c; Fig. 7, c; Fig. 3, c c; Fig. 4, c c), and its edge makes, when seen from the side (Fig. 6), an angle of a little more than 90° with the free edge of the middle part. The limit of these two parts is marked by a zigzag line, which resembles a sort of suture (Fig. 3, e; Fig. 6, e e). This part of the shell curves backward and inward, and there terminates in a small rounded tubercle (Fig. 4, d; Fig. 3, d; Fig. 7, d), situated opposite the corresponding tubercle of the other valve. This point is the axis of rotation of the two valves; that is to say, when the shell opens or closes, the tubercles retain their relative positions, while all the other portions of the valves describe about them an arc of circle more or less large.
On each of these tubercles is a short, pointed projection, on which are implanted at about a right angle two other large projections, which extend into the interior of the shell a third or half its length (Fig. 4, e e; Fig. 7, e). These projections are slightly curved and flattened; they penetrate among the soft parts, so that their inner face reposes upon the visceral mass; their outer face comes in contact with the thin lining or mantle, which covers the interior of the valves and extends to their extreme edge.
Examining the shell with a magnifying-glass, one sees (Fig. 6) a large number of curved lines of growth, parallel, as is usual, with the free margin of the shell; a closer examination shows that those lines differ in each of the three parts of the valve, although in fact they form a continuous whole.
On the back part of the neck of the valve (Fig. 6, f), the lines seem to be simply curved lines without any especial peculiarities. This is equally true of those on the posterior and largest portion of the middle part of the shell (Fig. 6, b); they seem to be only linear thicknesses; yet, between each pair of the strongest lines, which are the lines of growth, properly so called, one discovers a multitude of others, much liner, which follow the same direction. Here (Fig. 6, e e) the lines of growth form partitions between as many rows of small, sharp,wedge-shaped teeth. Each of these teeth has two rectangular faces on either side of two small triangular faces inclined toward each other (Fig. 8); its cutting edge is placed in the direction of the axis of the animal.
The size of those small teeth varies according to the position they occupy: those which are in the vicinity of the hinge-border are the smallest, those which are on the outer edge are the largest. And, as the part of the shell which is nearest the hinge is the earliest formed, in fact the only portion which exists at a very early period, it follows that the average dimension of the teeth increases with the size of the shell, that is to say, with the growth of the animal. On a shell, for instance, of 72 millim. in its largest dimension, where the total number
|Fig. 7.||Fig. 8.|
of rows of teeth reaches 41, the width of each row near the hinge-part is 52 mmm. (19 millim.), and the size of each compartment occupied by a tooth is 28 mmm. (36 millim.), while the same measures, taken on the outer edge of the valve, give 145 and 45 mmm. (7 and 22 millim.). At this last point the small, wedge-shaped teeth rise to a height of 32 mmm. (33 millim.) above their common support. On an average, there are in each row about 100 teeth, and consequently more than 4,000 on each valve, and more than 8,000 on the two valves together.
The anterior part, in the form of a spoon, has a similar structure, but still more delicate. The lines of growth form an angle of a little more than 90° with those of the middle part, of which they are a continuation. They appear like small, projecting ribs, the outer edges of which are cut in the form of small teeth pressed one against the other (Fig. 6, c, and Fig. 9). These denticles are also in form of wedges; their cutting surfaces are perpendicular to the axis of the animal, and consequently form a right angle with the cutting surfaces of the teeth of the middle part of the shell. But they are much smaller than the latter; their width is only 10 to 15 mmm. (100 to 66 millim.). Their number is also more considerable, even although that part of the shell is less fully developed than the rest.
On the same shell of 72 millim. diameter, the number of these denticles is, on an average, 250 on each rib, which makes 10,250 on the 41 ribs, and 20,500 on the two valves.
We should also point out the fact that this spiral part of the shell is evidently composed of more solid matter than the rest of the shell. It has more lustre, and the look of porcelain, and its surface between the ribs is smooth and glossy.
The consideration of the structure which we have related led Mr. Harting to the conclusion that it would be difficult to imagine an instrument better adapted than this shell for boring galleries in wood.
In fact, each valve presents in a certain way a combined auger-bit, gouge, and file. The ordinary steel file is made with two rows of notches, in order that the tool may cut simultaneously in two different directions; in this shell the same end is attained by the two rows of denticles, the action of which is equally in two directions perpendicular to each other; and our shell has another advantage, that it does not foul so readily with the filings as does an ordinary file.
Nevertheless, the winding direction of the galleries, in which it is not unusual to find right angles, or even somewhat acute angles; the defective cylindricity in the form of the galleries, which often appear as if composed of rings piled up one upon another, some larger and some smaller; the form of the end of each gallery, which is always perfectly smooth and hemispherical without any projection in the centre—all these facts show, according to Harting, that the action brought to bear upon wood by the teredo is not that of an auger boring a hole by rotary motion, but rather that of a file; this is rendered more apparent from the results of the careful anatomical study given by Harting to the muscular system of the teredo.
Although confined during its entire life to the dwelling which it has itself constructed, the teredo still has a strongly-developed muscular system. It is evident, moreover, that he uses all his muscles, excepting only those which serve to move the siphons, more or less directly, in the perforation of his galleries.
The first system of muscles is that which one finds in the mantle. That organ is provided through its whole length with longitudinal and
|Fig. 9.||Fig. 10.|
transverse muscular fibres. These fibres give the teredo the power of elongating or shortening its body; and also, by the partial action of some bundles of fibres, to make a slight movement of torsion.
At the base of the palettes, at the posterior portion of the mantle, is a powerful muscular ring (Fig. 10, c); by means of this ring, when the posterior extremities are expanded, the siphons (Fig. 10, e and d) can be carried outside into the water; at the same time the access of the two siphons, and consequently the entrance and exit of water, can be more or less hindered. As we have seen, the mantle is prolonged in the direction of the shell in an appendage which extends over the two sides of the dorsal surface of the valves (Fig. 1, a, and Fig. 2, a), the central portion of which forms a swelling of considerable thickness, composed of various anatomical elements; beneath the epidermis the tissues are partly of vesicular and partly of membranous character, which, through inherent powers of swelling and hardening under the action of the blood, serve a purpose in operating the movement of the valves.
To explain the physiological rôle of this organ (a, Figs. 1 and 2), it is necessary to recall the fact that it receives on either side, in the arched folds of the mantle, the neck-portions of the valves of the shells. By the contraction of the bundles of muscular fibres, the two valves would separate slightly one from the other, a movement which is still better understood if it is proved that that organ can become hard by the afflux of the blood and thus furnish a better fulcrum for the action of the muscles. Up to a certain point this part is similar to the hinge-ligament of other bivalve mollusks; but only in this respect, that it serves to open the shell. For the true ligament, wherever it exists, is always composed of elastic tissue, and its action is purely passive, while with the teredo the opening of the shell is a muscular action and consequently active. Moreover, the hinge is wanting in this case, which allows the supposition that the animal has the power of modifying at will, by the partial contraction of its muscles, the direction in which the valves separate, so that it may be at one time the middle parts and at another the anterior parts of the valves which separate most from each other. Besides, the effort which this action demands is extremely feeble, and the movement of the valves themselves is very limited.
There are two adductor muscles. The first and largest is already well known; it has been described by all writers who have made the teredo an object of study. It extends (Fig. 11, m) between the two valves in the form of a muscular mass, relatively quite large, and occupies about two-thirds of the length of the shell and one-third of its width. It rests on either side on a sort of pad situated at the limit between the middle and neck parts of the valve. The second or small adductor muscle, which appears to have escaped the attention of most observers, is found near the dorsal side of the shell in the cavity between the anterior portions of the valves. One can see its exterior surface, clothed with a thin epidermis and slightly projecting immediately in front of the pallial prolongation, which extends over the dorsal face of the shell; in appearance it is only a continuation of this muscle, but in reality it is entirely distinct.
The principal mass of this muscle is implanted upon the sides, bent backward within the anterior, spiral parts of the valves (Fig. 11, p) below the line which passes by the two tuberculous extremities, coupled together, i. e., by the centre of rotation of the valves. From this principal mass some slender muscular fibres extend over the two thorny protuberances (Fig. 11, e e),
which may be compared to two arms of levers, whose common fulcrum is found in the centre of rotation of the valves. It is clear that, by this arrangement, the action of the muscle is considerably strengthened.
The two adductor muscles are composed of the same microscopic elements, that is, of fibres and fibro-cellular tissue, easily separated in form of a ribbon, six mmm. wide and one mmm. thick; their length is relatively considerable, and probably equal to that of the muscles themselves, inasmuch as one can nowhere discover any free extremities. These fibres are distinguished from the fibro-cellular tissue of the mantle not only by their greater length, but also by their darker outlines, which indicate thicker walls and, consequently, greater solidity and strength.
The effect produced by these muscles in contracting is very evident. The large adductor muscle, situated on a plane a little above the general centre of rotation of the valves, serves to draw together the rounded sides of the valves as well as all the other parts of the valves situated posteriorly on the same side of the centre of rotation. The small adductor muscle, placed in front of the centre of rotation, exercises a more limited action. When it contracts, the anterior extremities of the spiral part of the valves approach each other; simultaneously, all the parts of the shell situated behind experience a slight displacement forward, as if they tried to turn about an axis passing by the centre of rotation; but the one which they thus describe is necessarily very small, on account of the shortness of the muscle. It is evident that the direction of the movement made by one of these adductor muscles is nearly a right angle with that of the other; the anterior and middle parts of the valves, which are first acted upon by the action of the muscles, meet at an angle of 90°.
Finally, the teredo has also a muscular organ, without which it would be impossible for him to pierce his galleries. It is the part known as the foot, which has the power of projecting outside between the anterior opening of the valves (Fig. 5, b; Fig. 1, d—the dotted line indicating the outline of the foot in the state of extension). This foot has the power of extension and retraction, and terminates at the end with a suction-disk, by the aid of which the animal can attach itself to the wood.
From what precedes, it is evident that the teredo, far from being, as Deshayes has pretended, an animal having very few if any muscles, is, on the contrary, richly provided with those organs. There are the longitudinal and transverse muscles through the whole length of the mantle, a true sphincter (Fig. 10, c) at the base of the siphons, a muscular organ which receives and covers a part of the valves, two adductor muscles for the movement of bringing the valves together, and a foot provided with a suction-disk and susceptible of extension and retraction—truly a profusion of motive organs which one would not expect to find in an animal which passes its entire life in a narrow canal which it can never quit. Moreover, all these motive powers have only one essential end, namely, to endow the teredo with the power of boring his gallery—his home.
But all the muscles which we have enumerated do not coöperate to that end in an equally direct manner. When water has entered by the incurrent siphon, the animal can, by contracting the transverse muscles of the mantle, force the water through the whole length of its body up to the end of the gallery, and then drive it out by the excurrent siphon. The teredo undoubtedly makes use of this as a means of getting rid of the fine filings of wood which the valves of the shell have detached. He can then draw back a little the anterior part of his body by the action of the longitudinal muscular fibres, supporting himself by the two palettes pressed against the inner walls of the calcareous tube at a distance of two or three millimetres from the exterior opening, by which the siphons project outside the wood. It is probable that the teredo takes that position during his periods of repose, which come from time to time, and which he uses for repairing his tools.
The teredo possesses, on the other hand, the means of preventing or hindering the outflow of water at will, so that its body, distended by the liquid, occupies at that time the whole extent of the gallery, and his anterior portion touches with the valves of its shell the end of the gallery. In this position he can carry on his work of miner. He commences then by extending his foot (Fig. 1, d), which he fixes by suction against the side of the cavity. At the same time the valves separate a little; then, while the foot draws the shell to itself and thus presses its exterior surface against the wood, the valves close up again, and the denticles with which they are furnished cut into the wood.
In this labor there are still two peculiarities worthy of notice: First, the limited extent of movement with which the valves are endowed, their anterior extremities moving only a very short distance from each other. But this circumstance, in view of the narrow space in which the teredo works, gives him this advantage, that, by the rapid succession of movements of opening and closing the shell, he attains his end—namely, to reduce the wood to an impalpable powder—better than if each blow of the instrument had a wider range. In the second place, we should recall the fact that the directions of movement of the two adductor muscles are at right angles, as are also the directions of the cutting surfaces of the denticles on the two parts of the same valve. Hence it is clear, after the description which we have given above, that if the large adductor muscle contracts, the denticles of the anterior or spiral part of the valve cut the wood; if, on the contrary, the small adductor muscle is shortened, it is the middle part of the valve which undergoes a movement of rotation, and the teeth which it bears are set at work. Thus, then, whether the two muscles contract simultaneously or one by one, the woody cells are cut by successive incisions, which would divide them into small quadrangular pieces, if there be no rending of the fibres. It is evident that the hardest task is demanded from the spiral part of the valve, for it is that part which is first brought to bear upon the wood. This part also has a more solid structure and the denticles are much finer; it is moved by a muscle of considerable size; the power of this muscle is, moreover, sensibly increased by the fact of its being implanted upon the two middle parts of the valves, each of which can be considered as a long arm of a lever whose extremity passes over a space at least four times as large as the portion of the valve which, strictly speaking, does the work.
The foot remains fixed in the same spot a very short time only. The form of the end of the cavity, that of a regularly rounded basin, suffices to prove that the valves of the shell are placed every instant in contact with a different spot. The foot displaces itself, little by little, so as to give a rotating movement to the shell, and at the same time to all that part of the body beyond the shell, even as far as the palettes. When the torsion thus produced becomes excessive, the foot loosens its hold, and the body returns to its former position. Thus, then, the rotary movements remarked by some observers, far from being the cause, should be considered rather the effect; they are only the shifting of position of the animal, and nothing more.
The teredo does not bore out his galleries, but he hollows them out with an action analogous to that of a file, by means of the thousands of cutting teeth with which its valves are armed. If the teeth do not break away rapidly, it is due to their wedge-like form and to the oblique direction of the planes which bound each of these wedges. Moreover, as the animal grows, new rows of teeth are formed, so that the rows which have served in youth are no longer of any use in more advanced age; they are principally the outer rows of teeth, the last formed, which do the work.
The sense of touch exists in the teredo in the suction-disk. This is not only a muscular organ, but one rich in nerves. Quatrefages has already pointed out the two small ganglia, situated on the intestines, which furnish the nerves for that part of the body. The foot, when extended, commences by feeling the place before attaching itself to it and drawing the shell after it. Naturally, it avoids the places which seem to offer too much resistance; but he avoids with equal care the parts where there only remains a wall of wood too thin for sufficient resistance. In this case, in fact, the gallery is approaching either the surface of the wood or a neighboring gallery; a teredo is never known to destroy the work of another; that, moreover, would not serve him, for, even should he perforate the woody division between them, he would drive against the calcareous tubes, which, being scarcely less hard than the valves themselves, cannot be attacked by them. Whenever the teredo encounters an obstacle, he simply turns aside; he acts like the mole, which, excavating her trenches by preference in a rich loam, makes a détour around the stones which she meets in her way, and changes her direction when she comes near the breast of a ditch, to avoid the open air.
I will state, moreover, that the conclusions regarding the manner in which the teredo perforates his galleries, deduced at first by Harting from the anatomical examination of his organs, have since been fully confirmed by direct observation; Kater, having opened laterally one of the galleries, so as to partially expose the animal, has seen him at work, executing all the movements above mentioned.
- Extract from the Archives of Holland, vol. i., translated by Edward R. Andrews.