202 THE MECHANICAL FUNCTIONS. 



meats, of which, from the habitual inactivity of 

 animals of this class we should scarcely have sup- 

 posed 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 ex- 

 tending 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 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 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 adhesion with it, and concentrate their power 

 on one point. The threads themselves are com- 

 posed of a glutinous matter, prepared 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, where 

 they harden, and acquire a certain consistence 

 before they are employed. This mould is curiously 

 constructed ; there is a deep groove which passes 

 along the foot from the root of the tendon to its 

 other extremity ; and the sides of this groove are 

 formed so as to fold and close over it, thereby 



