yo The Shipworm. [Sess. 



by which it enters is very small, and the bore for a short 

 distance goes across the grain. As it grows rapidly, it enlarges 

 the diameter of the bore, at the same time lining it with a 

 smooth calcareous incrustation, which forms a tube apparently 

 to protect its soft body. This lining appears to be much 

 thicker in some species than in others, and by the very young 

 animal is deposited in transverse ridges. In the Teredo 

 gigantea the calcareous tube, said to exceed 6 feet in length, 

 has walls estimated to be from 4 to 6 lines in thickness. 

 The excavation formed by the rock -boring Pholas, on the 

 other hand, is of a short elliptic form, broader at the bottom 

 than the top, and is designed only for protection. As seen in 

 the blue shale of Port Seton harbour, where they are in 

 hundreds, their burrow is beautiful in its regularity. 



Various and contradictory views have been brought forward 

 from time to time regarding the actual boring agent of the 

 Teredo. It has been held that it bores by means of its shells, 

 fixing itself by its sucker-like foot and rasping the wood by 

 the toothed edges of the valves. A second view is, that the 

 excavations are due to the action of a solvent secreted by the 

 animal, which thus acts as a pioneer in mining the passage, 

 afterwards increased to its final dimensions by the valves. A 

 third theory is, that the excavating power is due to siliceous 

 particles imbedded in the anterior portion of the disc ; while a 

 fourth is, that the " foot " is the organ alone by which it is 

 accomplished. Arguing from what can be seen in the case of 

 the Pholas, it is certainly the shell, and the shell only, which 

 gives the bore its greatest diameter ; and although a fragile 

 substance, it can, by ceaseless action, coupled with the constant 

 renewal of its teeth, make its way into very hard rock. If 

 this result be accomplished by the Pholas, we may safely 

 attribute the boring in the case of the Teredo to its shell, 

 assisted by the sucker-like foot, from which, used as a pivot 

 or fulcrum, the necessary purchase is derived. 



Coming now to instances of destruction, these might be 

 multiplied indefinitely, but one or two historical examples will 

 suffice. Along the west coast of the Netherlands, the low- 

 lying lands are protected by a line of sandhills or dunes ; and 

 where this natural defence is wanting, strong dykes have been 

 constructed, and are kept up at great expense to prevent the 



