226 THE MECHANICAL FUNCTIONS. 



the object to which it is to be fixed; its extre- 

 mity being carefully attached to the solid sur- 

 face of that object. The canal of the foot is 

 then opened along its whole length, and the 

 thread, which adheres by its other extremity to 

 the large tendon at the base of the foot, is dis- 

 engaged from the canal. Lastly, the foot is re- 

 tracted, and the same operation is repeated. 



Thread after thread is thus formed, and ap- 

 plied in different directions around the shell. 

 Sometimes the attempt fails in consequence of 

 some imperfection in the thread ; but the ani- 

 mal, as if aware of the importance of ascer- 

 taining the strength of each thread, on which its 

 safety depends, tries every one of them as soon 

 as it has been fixed, by swinging itself round, so 

 as to put it fully on the stretch : an action which 

 probably also assists in elongating the thread. 

 When once the threads have been fixed, the 

 animal does not appear to have the power of 

 cutting or breaking them off. The liquid 

 matter out of which they are formed is so ex- 

 ceedingly glutinous as to attach itself firmly to 

 the smoothest bodies. It is but slowly produced, 

 for it appears that no Pinna is capable of forming 

 more than four, or at most five threads in the 

 course of a day and night. The threads that 

 are formed in haste, when the animal is dis- 

 turbed in its operations, are more slender than 

 those that are constructed at its leisure. Reau- 



