II.] INTRODUCTION. xU 



that time as to the means by which the ship-worm bores 

 into wood. He showed conclusively that their shells 

 could not be the instruments of perforation ; and he 

 asked how it was possible that the extremely tender 

 shell of the young Teredo (in fact a mere film) could 

 make a hole in solid oak, a material ten times harder 

 than itself. Besides, as he justly remarked, the form of 

 the tube is e\ddently not the result of an auger-like in- 

 strument, because it is broader at the bottom than at 

 the top and sides. The conclusion he formed, after a 

 most laborious and impartial investigation, was that the 

 Teredo perforates by suction, aided by continual mace- 

 ration and softening of the wood. One of his numerous 

 quotations from the ancient poets, in support of his 

 argument, may be here appropriately repeated. It is 

 from his favourite, Ovid : — 



" Quid magis est durum saxo? quid mollius unda? 

 Dura tamen molli saxa cavantur aqua." 



I profess myself to be a follower of Sellius ; and I 



am convinced that the sole instiniment of perforation by 



the Mollusca of stone, wood, and other substances, is in 



every case their foot or muscular disk, which is closely 



applied to the concave end of the hole and is constantly 



supplied with moisture through the glandular tissues of 



the body. The strength of this organ may be easily 



tested by any one who tries to remove a limpet from its 



native rock, after having touched it and thus given it 



due notice of his intention. By this simple, yet gradual, 



process the fibres of wood or grains of sandstone may 



easily be detached or disintegrated, time and patience 



being allowed for the operation. When it is considered 



that the hole made by an adult Pholas or Saxicava is 



only a few inches deep, and that an aged Patella scarcely 



penetrates a quarter of an inch into a limestone rock. 



