62 CLASS CONCHIFERA. 



nothing but the extremity of the tube can be perceived 

 above the sand. 



By a process exactly the inverse of this, that is, by 

 doubhng up the foot, and pushing with it ^downwards 

 against the sand below, the shell may be again made to 

 rise by the same kind of efforts which before protruded 

 the foot. By this process of burrowing, the animal is 

 enabled quickly to retreat when danger presses, and 

 when this is past, it can, with equal facility, emerge 

 from its hiding-place. 



The Cardium can also advance at the bottom of the 

 sea, along the surface of the soft earth, pressing back- 

 wards with its foot, as a boatman impels his boat onwards 

 by pushing with his pole against the ground in a con- 

 trary direction. It is, likewise, by a similar expedient, 

 that the Solen forces its way through the sand, expand- 

 ing the end of its foot into the form of a club. 



The Tellina is remarkable for the quickness and 

 agility with which it can spring to considerable distances, 

 by first folding the foot into a small compass, and then 

 suddenly extending it, while the shell is, at the same 

 time, closed wnth a loud snap. 



The Pinna, or marine mussel, when inhabiting the 

 shores of tempestuous seas, is furnished, in addition, 

 with a singular apparatus for withstanding the fury of 

 the surge, and securing itself from dangerous collisions, 

 which might easily destroy the brittle texture of its shell. 

 The object of this apparatus is, to prepare a great num- 

 ber of threads, which are fastened at various points to the 

 adjacent rocks, and then tightly drawn by the animal, 

 just as a ship is moored in a convenient station, to avoid 

 the buifeting of the storm. The foot of this bivalve is 

 cylindrical, and has, connected with its base, a round 



