TEREDO SHIP WORM. 131 



OBSERVATIONS ON THE SHELL AND ITS 

 INHABITANT. 



Many doubts have arisen whether this genus 

 ought to be retained among the univalve shells. 

 Some naturalists consider the two hemispherical 

 valves as properly the shell of the animal since 

 it adheres to them by a strong muscle, and has 

 its head lodged in them, and they regard the 

 tube with the posterior valves as only accessory. 

 The use of the tube is to secure to the mollusk a 

 lubricated passage and a protection against the 

 rough surfaces of the wood in which it resides. 

 It is formed by a calcareous secretion, which 

 appears to have a solvent power over the resin, 

 and even over the fibre of the wood. Unless 

 this be the case it is difficult to account for the 

 fact, that these creatures pierce through the 

 stoutest oak timber, as they do not seem to 

 possess any organ sufficiently strong to effect 

 such perforations by a simple mechanical power. 

 On examining fresh specimens, a soft pulpy 

 mass of a chalky appearance, which hardens 

 when exposed to the air, has been discovered at 

 the opening of the hemispherical valves, and is 

 no doubt the secretion with which they lengthen 

 the tube. This is open at both ends, and 

 the animal does not in any way adhere to 

 it ; the posterior end is thickened and provided 

 with plates which contract the aperture, and 

 render it very small ; this part is always even 

 K 2 



