TEREDO SHIP WORM. 129 



hemispherical valves anteriorly truncated, cover 

 ing the head of the animal, and two others of a 

 lanceolate shape near the extremity of the tail. 



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 the primary organs, and properly the 

 shell of the animal, as 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 mollusca 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 crea- 

 tures 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 speci- 

 mens, a soft pulpy mass of a chalky appearance, 

 which hardens when exposed to the air, has been 

 discovered at the opening of the primary valves, 

 and is no doubt the secretion with which they 

 lengthen the tube. The passage is open at both 

 ends, and the animal does not in any way adhere 



