270 CONCHIFERA. 



for themselves the dens in which they reside, in stone, 

 clay, wood, or other substances, and in these exca- 

 vations they live a sedentary life. Their shell is 

 generally thin and brittle, but it is extremely hard, 

 and covered externally with file-like teeth, that 

 seem to constitute the tools employed in their boring 

 operations. 



The Ship-worm (Teredo)* well characterized by 

 Linnaeus as the " calamitas navium," seems to have 

 been specially appointed by Providence for the re- 

 moval of floating timber, which otherwise, by its 

 accumulation, might impede the navigation of the 

 sea. The mantle is excessively lengthened into a 

 sort of tube, while the valves are minute, so that 

 the appearance of these bivalves is rather that of 

 a Worm than of a Mollusk. It bores holes in all 

 directions through wood lying in the sea, lining 



FlG. 207. SHIP- WORM AND ITS SHELL. 



the interior of its excavations with a shelly crust. 

 The piles of piers and wharves, the gates of docks, 

 and the bottoms of ships are soon riddled and pierced 

 by these' animals, insomuch that serious fears have 

 been more than once entertained for the safety of 



* repecy, tereo, to bore. 



