132 HARBOUR CONSTRUCTION. 



sneeze-wood (Ptceroxylon utile) have been largely used in Cape 

 Colony. 



All of these woods are, in a greater or less degree, subject 

 to attack from marine animals commonly spoken of as " sea- 

 worms" and their value for use in marine works depends 

 greatly upon their power to resist injury or destruction by them. 



The family of Pholadidee may perhaps be considered the 

 worst enemies of timber structures in the sea ; but the attacks 

 of their smaller relatives, Limnoria terebrans and Chelura 

 terebrans, are by no means to be despised, as will hereafter 

 appear. 



Of the Pholadidse, Pholas dactylus, although pre-eminently 

 a rock-borer, is often found in timber. It is, however, the 

 Teredo navalis, or common shipworm, of the same family, which 

 most frequently assails and is most destructive to timber 

 structures in the sea. 



These molluscs resemble worms, and they bore into the 

 hardest kinds of timber with apparently as much ease as they 

 do into the softer kinds. 



I have found Teredo navalis in the byssus or fibre of mussels, 

 on detaching bunches of them from concrete blocks, but I 

 could not perceive any indication of an attempt having been 

 made to perforate the concrete itself. 



Usually, soon after entering timber, these worms alter the 

 direction of their bore-holes until it corresponds with that of the 

 grain of the wood. These galleries, which are sometimes of great 

 length, are lined throughout with a hard, smooth, but very thin 

 shell ; and it is curious to see how, notwithstanding that a pile 

 may be completely riddled with them, they steer clear of each 

 other, although often approaching so close that not T J g inch of 

 timber divides them. 



Attempts have been made to explain how these worms can 

 bore into timber, by suggesting that they destroy or soften the 

 wood by secreting acids, or that they wear it away by the 

 continuous action of a water-jet, or by a rasping process caused 

 by a movement of the head, and so forth. 



If any one will take hold of one of these worms just behind 

 the head, pinching it firmly and compressing the back of the 

 head, such treatment which requires considerable pressure, 

 and, I fear, is not very pleasant to the worm will cause two 

 strong, sharp, black teeth to protrude, of such hardness that the 



