THE SHIP-WORM. 



437 



the creature forms is either wholly or partially lined with shell, and it is worthy of notice 

 that the Ship-worm and its mode of burrowing was the object that gave Sir I. Brunei 

 the idea of the Thames Tunnel. 







SHIP.WORM.-Temfo aavalls. 



The Teredo did not always lead this fixed and darkling life, but at one time of its 

 existence it swam freely through the ocean, having organs of sight and hearing for the 

 purpose of guarding itself against the dangers of the deep. The following description 

 of the successive stages of this mollusc is from the pen of the able naturalist whom we 

 have already mentioned, and who has devoted much time to the study of the Ship- 

 worm and its habits. 



" The larva, which is at first almost spherical and entirely covered with vibratile cilia, 

 may be compared to a very minute hedgehog, in which every spine acts as a natatory 

 organ. It swims in all directions with extreme agility, and this first state continues 

 about a day and a half. Towards the end of this time the external skin bursts, and after 

 being encrusted with calcareous salts, becomes a shell, which is at first oval, then tri- 

 angular, and at last very nearly spherical. While the shell is being formed the vibratile 

 cilia disappear, but the little animal is not on that account condemned to inactivity. 

 In proportion as the external cilia diminish we observe that another equally ciliated 

 organ becomes developed, which widens and extends in such a manner as to form a large 

 collar or ruff margined with fringes. This new organ of locomotion may be entirely 

 concealed within the shell, or may be extended from it, and acts in the manner of the 

 paddle-wheel of a steamboat. 



By means of this apparatus the young larva continues to swim with as much facility as 

 in its earlier age, but it now moreover requires another organ, which enables it to advance 

 upon a resisting plane, as, for instance, along the side of a glass vessel. This is a sort 

 of fleshy foot, very similar to a long and very mobile tongue, which can be extended 

 and contracted at will. The larva of the Teredo possesses, moreover, organs of hear- 

 ing similar to those of several molluscs, and eyes analogous to those of certain annelids. 



