THE BORERS OF THE SEA. 



61 



de Quatrefages : " The larva, which is at first almost spherical and en- 

 tirely 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 



Fig. 1. 



Bock peeforatsd by Pholades. 



about a day and a half. Toward the end of this time the external 

 skin bursts, and, after being incrusted with calcareous salts, becomes 

 a shell, which is at first oval, then triangular, and at last very nearly 

 spherical. While the shell is being formed, the vibratile cilia disap- 

 pear, but the little animal is not on that account condemned to inac- 

 tivity. In proportion as the external cilia diminish, we observe that an- 

 other equally ciliated organ becomes developed, which widens and ex- 

 tends in such a manner as to form a lame 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, ac- 

 quires another organ, a sort of fleshy foot, which can be extended and 

 contracted at will. It has also organs of hearing similar to those of 

 several mollusks, and eyes analogous to those of certain annelids." 

 The last metamorphosis is when the Teredo takes its worm-shape, and 

 is ready to commence its boring operations. 



The Teredo is supposed to have been originally a native of tropical 



