Page:Popular Science Monthly Volume 13.djvu/564

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design in the water; at that time they were from eight to fourteen days old, and, although very small, they resembled in every respect older teredos.

Teredos penetrate wood naturally by very small openings in a direction perpendicular to the surface (Figs. 12 and 15, C); then they generally turn about in order to follow the direction of the woody fibres, PSM V13 D564 Wood exposed to teredo depredation.jpgFig. 12.—Wood exposed from November, 1874, to September, 1876, in crib at Pier No. 1, New York, North River, twenty-five feet below mean low tide. usually upward, but sometimes downward. Although they do not enter into the earth or mud, one generally finds the first traces immediately above the line of the mud in which piles are driven; it is at this point that piles destroyed by the teredo generally break off.

When the teredos are lodged in a piece of wood, one recognizes them by very small holes on the surface, and the extremely delicate tubes which project from them (Fig. 12, e, d). These are the siphons, only one of which shows at first, the other appearing later. These siphons are generally kept outside the wood in the water, but the slightest touch causes the animal to retract them. One of them is shorter and larger than the other, but they both seem to serve for the expulsion of the fasces, which largely consist of particles of wood reduced to a very fine powder. It is known that the teredo does not perforate wood for nourishment, but only to procure a suitable abode; the woody substance, detached in the boring, passes through the intestinal canal, and then is expelled in the form of a very fine white substance by one of the siphons, generally, according to M. Vrolik, by the shorter, but sometimes by the longer. The long siphon appears to serve principally for the introduction of food, which consists of infusoria, diatoms, and other inferior animalcula, which the sea-water brings with it into the siphons. It is nevertheless still uncertain whether the matters expelled through the longer siphon come directly from the intestinal tube, or if they are first introduced from outside with the inflowing water to be expelled again after a short sojourn inside.

The teredo requires for respiration a clear, pure water. It has often