THE BAY OF BISCAY. 249 



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 vi- 

 bratile cilia disappear, but the little animal is not 

 on that account condemned to inactivity. In pro- 

 portion as the external cilia diminish, we observe 

 that another equally ciliated organ becomes deve- 

 loped, which widens and extends in such a manner 

 as to form a large ruff or collar margined with 

 fringes. This new organ of locomotion may be en- 

 tirely 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 con- 

 tinues 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, to rise 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 ex- 

 tended and contracted at will. The larva of the 

 Teredo possesses moreover organs of hearing similar 

 to those of several other Molluscs, and eyes analogous 

 to those of certain Annelids. 



During this period of its existence, our Mollusc 

 therefore enjoys to a high degree the characteristic 

 faculties of a living being. It moves, and is brought 

 in relation with the external world by organs of the 

 special senses. Now, however, comes a last meta- 

 morphosis, and this same Teredo loses its organs of 

 motion and sensation, becoming a kind of inert mass 



