CLASS CONCHIFERA. 63 



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 composed 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 ; tl'.ere is a deep groove which passes along 

 the foot, from the root of the tendon to its other extre- 

 mity, and the sides of this groove are formed so as to 

 fold and close over it, thereby converting it into a canal. 

 The glutinous secretion, which is poured into this canal, 

 dries into a solid thread ; and, when it has acquired 

 sufficient tenacity, the foot is protruded, and the thread 

 it contains is applied to the object to which it is to be 

 fixed, its extremity being carefully attached to the solid 

 surface 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 disengaged from the canal. Lastly, 

 the foot is retracted, and the same operation is repeated. 

 Thread after thread is thus formed, and applied in 

 different directions around the shell. Sometimes the 

 attempt fails, in consequence of some imperfection in 

 the thread: but the animal, as if aware of the importance 

 of ascertaining 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 



