224 THE MECHANICAL FUNCTIONS. 



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 backwards with its foot, as a boatman 

 impels his boat onwards, by pushing with his 

 pole against the ground, in a contrary 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 course of these locomotive bivalves may 

 readily be traced on the sand by the furrows 

 which they plough up in their progress. 



This, as well as many other of the bivalve 

 mollusca, are enabled by the great size and 

 flexibility of this organ to execute various other 

 movements, of which, from the habitual inacti- 

 vity of animals of this class we should scarcely 

 have supposed them capable. 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 with a loud snap. 



The Pinna, or Marine Muscle, when inhabiting 

 llie shores of tempestuous seas, is furnished. 



