94 MUSCULAR POWER. 



on the part of the inclosed moUusk, but that its closure 

 is effected by means of a muscle passing from valve to 

 valve. In some bivalve mollusks, this muscle, as in 

 the oyster, though composed of two portions of different 

 texture, is single ; but in other bivalves, as in the 

 mussel, there are two distinct muscles, separated from 

 each other, but acting, at the same time, as adductors. 



This is a structure well deserving attention. While 

 the animal is alive, the common and natural state of its 

 shell is that of being very partially kept open, so that 

 the water required for respiration and nourishment may 

 freely enter and pass out. But to secure the creature 

 from danger, it was necessary to provide it with the 

 means of rapidly closing its shell, and of keeping the 

 valves together. These efforts, though only occasional, 

 require considerable force, and are made by a muscular 

 power, to obtain which sometimes one, at others two, or 

 even a greater number of strong muscles are placed 

 between the valves. These are called, from their office, 

 adductor muscles. 



When these muscles are not in action, the elasticity 

 of the cartilage attached to the hinge is sufficient to 

 separate the valves ; but as they were designed to open 

 only to a certain extent, it was necessary to limit by 

 some means the action of the cartilage. Were this 



