220 



THE MECHANICAL FUNCTIONS. 



103 



decayed and been washed out, provided the car- 

 tilage and the ligament of the hinge are still 

 preserved.* 



The simple actions of opening and closing 

 the valves are capable of being converted into 

 a means of retreating from 

 danger, or of removing to a 

 more commodious situation, in 

 the case of those bivalves which 

 are not actually attached to 

 rocks or other fixed bodies. 

 Diquemare long ago observed 

 that even the oyster has some 

 power of locomotion, by sud- 

 denly closing its shell, and thereby expelling the 



* The Pholas is an exception to this rule ; for instead of its 

 valves being united, as usual, by an elastic ligament, they are 

 connected chiefly by means of muscles. This departure from 

 the ordinary structure is probably occasioned by a new condition 

 introduced into the economy of the animal in consequence of its 

 being fitted for excavating passages through hard rocks. It is 

 furnished, for this purpose, with a complicated boring apparatus 

 moved by many muscles, and requiring great freedom of action. 

 Fig. 103 represents the shell of the Pholas Candida extremely 

 expanded, in order to show the hinge, together with the liga- 

 ment, L ; the long and thin process of shell, P, to the ends of 

 which, on each side, a pair of fan-shaped muscles, more particu- 

 larly employed in boring, are attached ; and the two adductor 

 muscles, A A, which retain the valves in contact independently 

 of the ligaments. For a full description of this apparatus, I 

 must refer to a paper by Mr. Osier, on burrowing and boring 

 marine animals, contained in the Phil. Trans, for 1826, p. 342, 

 from which the above figure has been taken. 



