MOLLUSCA ACEPHALA. 



161 



103 



mize muscular power, whenever a substitute could be had, 

 and such a substitute she has here provided, by uniting with 

 the muscle an elastic ligament, of a peculiar construction. 

 It has a texture similar to that of the ligamentum nuchw^ 

 and being placed on the side of the muscle next to the hinge, 

 allows the valves to separate to the proper distance only.* 

 When the animal dies, the muscular force ceases, but the li- 

 gament with which the muscle is associated, retaining its elas- 

 ticity, allows the shell to open, but only to a certain extent; 

 and, accordingly, this is the state in which we find bivalve 

 shells that are cast upon the shore, after the soft flesh of the 

 animal has 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 arc 

 capable of being converted into a means of retreating from 



danger, or of removing to a more com- 

 modious situation, in the case of those 

 bivalves which are not actually attached 

 to rocks or other fixed bodies. Dique- 

 mare long; asro observed that even the 

 oyster has some power of locomotion, by 

 suddenly closing its shell, and thereby 

 expelling the contained water, with a de- 

 gree of force, which by the reaction of 

 the fluid in the opposite direction, gives 

 a sensible impulse to the heavy mass. He 



t 



* This remarkable structure was first described by Dr. 'Leach, in a paper 

 read before the Royal Academy of Paris. Bulletin des Sciences, 1818, p. 

 14. See also Gray, in Zoolog-ical Journal, I. 219. 



\ 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 occa- 

 sioned by a new condition introduced into the economy of the animal in con- 

 - sequence of its being- fitted for excavating passages through liai-d rocks. It 

 is furnished, for this purpose, with a complicated boring apparatus moved 

 by many muscles, and requiring great freedom of action. Fig. 103 repre- 

 sents the shell of the Fholas Candida extremely expanded, in order to show 

 the hinge, together with the ligament, l; the long and thin process of shell, 

 p, to the ends of which, on each side, a pair of fan-shaped muscles, more par- 

 ticularly employed in boring, are attached^ and the two adductor muscles. 

 Vol. I. 21 



