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 ap- 

 pears 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 ap- 

 pearance, which hardens when exposed to the air, 

 has been discovered at the opening of the hemis- 

 pherical valves, and is no doubt the secretion 

 with which they lengthen the tube. The tube 

 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 con- 

 tract the aperture, and render it very small ; 

 K 2 



