198 



THE MECHANICAL FUNCTIONS. 



elastic ligament, they are connected chiefly by 

 means of muscles. This departure from the ordi- 

 nary structure is probably occasioned by a new 

 condition introduced into the economy of the ani- 

 mal 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 P kolas ccmdida extremely 

 expanded, in order to show the 

 hinge, together with the liga- 

 ment (l); the long and thin pro- 

 cess of shell (p), to the ends of 

 which, on each side, a pair of 

 fan-shaped muscles, more parti- 

 cularly employed in boring, are attached ; and the 

 two adductor muscles (a, a), which retain the valves 

 in contact independently of the ligaments.* 



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, by 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 suddenly closing 

 its shell, and thereby expelling the contained 

 water, with a degree of force, which, by the re- 

 action of the fluid in the opposite direction, gives 



* 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. 



