CLASS CONCHIFERA. 59 



water necessary for its nourishment and respiration; but, 

 as a security against danger, it was necessary to furnish 

 the animal with the means of rapidly closing the shell, 

 and retaining the valves in a closed state. These actions, 

 being only occasional, yet requiring considerable force, 

 are effected by a muscular power, for which purpose 

 sometimes one, sometimes two, or even a greater num- 

 ber of strong muscles are placed between the valves, 

 their fibres passing directly across from the inner surface 

 of the one to that of the other, and firmly attached to 

 both. They are named, from their office of bringing the 

 valves towards each other, the adductor muscles. 



Section of an Oyster, showing the situation of the Hinge, L., the 



Adductor Muscle, A, and the transverse du-ection of its Fibres, 



with respect to the Valves. 



The simple actions of opening and closing the valves, 

 are capable of being converted into a means of retreat- 

 ing 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. 



Diquemarc 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 offeree, which, by the reaction of the fluid in the 

 opposite direction, gives a sensible impulse to the heavy 



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