164 THE MECHANICAL FUNCTIONS. 



when clanger 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, expanding 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 habit- 

 ual inactivity 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 considera- 

 ble 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 the shores 

 of tempestuous seas, is furnished, in addition, with a singu- 

 lar apparatus for withstanding the fury of the surge, and se- 

 curing itself from dangerous collisions, which might easily 

 destroy the brittle texture of its shell. The object of this 

 apparatus is to prepare a great number 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 buffeting of the storm. The 

 foot of this bivalve is cylindrical, and has, connected with 

 its base, a round tendon of nearly the same length as itself, 

 the office of which is to retain all the threads in firm adhe- 

 sion with it, and concentrate their powxr on one point. The 

 threads themselves are composed of a glutinous matter, pre- 

 pared by a particular organ. They are not spun by being 

 drawn out of the body like the threads of the silk-worm, or 

 of the spider, but they are cast in a mould, when they hard- 

 en, and acquire a certain consistence before they are em- 



