MOLLUSCA ACEPHALA. 219 



tioii of the hinge l, the adductor muscle a, and 

 the transverse direction of its fibres, with respect 



(to the valves. When these muscles are not in 

 ? action, the elasticity of the cartilage attached to 

 i the hinge is sufficient to separate the valves ; 

 but as they were not intended to open beyond a 

 certain extent, it was necessary to provide some 

 limitation to the action of the cartilage. The ad- 

 ductor muscle might, it is evident, be called into 

 play to counteract that action ; but this would re- 

 quire a constant muscular exertion, and a great 

 expenditure, therefore, of vital force. Nature 

 has always shown a solicitude to economize mus- 

 cular 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 nuchce, and being placed 

 on the side of the muscle next to the hinge, 

 allows the valves to separate to the proper dis- 

 tance only.* When the animal dies, the mus- 

 cular force ceases, but the ligament, with which 

 the muscle is associated, retaining its elasticity, 

 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 



* 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 Zoolos^ical Journal, 

 1.219. 



