A Treatise On The Principles And Practice Of Harbour Engineering

66 HARBOUR ENGINEERING. young teredo attacks the wood in its immédiate vicinity by boring or tunnelling into it, principally in the direction of the grain. The boring implements are two strong, sharp black teeth, which can be disclosed for inspection by applying pressure at the back of the creature’s head. The holes, or galleries, increase in size with the growth of the animal, and they Fig. 58.— Teredo Navalis. are lined throughout with a chalky secretion forming a thin, hard, smooth shell. It is no uncomnion experience to find holes 1 inch or | inch in diameter, and the teredo has been known to attain a length of as much as 2 feet, though the average length is not more than 7 or 8 inches. Its operations seem to be chiefly confined within the tidal range: that is, between highest high water mark and lowest low water mark ; but it also attacks timber at any moderate depth. At times it works with extreme rapidity. Some of the Memel fir piles of the old pierhead at Southend showed signs of the teredo within six months after completion, and in twelve months’ time they were reported to be seriously injured. Fir and alder appear to furnish the most favoured fields for operation; oak and teak are less susceptible; greenheart and jarrah have a general reputation (not strictly maintainable) of being free from attack. Greenheart has been used for piles at the mouth of the Mersey without the slightest sign of deterioration of any part, even after the lapse of many years ; but at Bombay the same wood has been freely ravaged. The Pholas dactylus is another member of the Pholadidæ family. On the whole, it evinces a more pronounced taste for mineral substances, such as limestone and sandstone; but it also turns its attention to woodwork, which it honeycombs by boring a number of holes very closely together. Compared with the teredo, however, it is of small importance, so far as timber, at any- rate, is concerned. The animal attains a length of 4 or 5 inches. Another boring tribe of similar habits and tendencies is the Aylophaya. Its members are small in size, and they do not line their excavations. The Chelura terebrans (Amphipoda) is a small crustacean resembling a minute shrimp, both in shape and colour. It is very small (fig. 59), not more than | inch in length. In addition to feet, it is provided with a pair of limbs, near the tail, which it employs in leaping. The chelura destroys wood by cutting or tearing it away in thin flakes, working in- wards from the exposed surface. It manifests a decided partiality for pure, clear sea-water, and is con- sequently more often found along Fig. 59.— Chelura Terebrans. Fro. 60.— Limnoria Terebrans the open coast than in inclosed basins and harbours. The Limnoria terebrans (Asellidæ) is another lilliputian, whose length