MOLLUSC A. 13 



Fig. 46). Between this and the body of the animal 

 the mantle is striped with bands of muscles running 

 across the intervening space, and easily detected by 

 their opaque aspect. In the midst of the mantle is 

 seen the large muscle ; in the free valve the depression 

 and peculiar marks it makes in the shell ; and these are 

 still more distinct in the burnt shells. 



The outer layer of the mantle really runs between the 

 muscle and the shell ; and it is the surface of this which 

 builds the thin layer of shell secreted over the scar or track 

 of the muscle, and not the muscle itself. This is not 

 organically attached to the shell, but is held in place by 

 the pressure of the surrounding water and air. The move- 

 ments of the muscle along its track are due, in all proba- 

 bility, partly to the growth of the animal and of the muscle, 

 and partly to the mechanical effect of the swelling and con- 

 tractions of its transverse dimensions, occasioned by the 

 closing and opening of the valves. These actions would 

 tend to make the muscle move in its bed ; and this motion 

 could only take place in one direction, — that of least resist- 

 ance, — away from the body. Thus, though the oyster pre- 

 fers to grow to the left, if its bed is not favorable for this 

 direction it grows to the right, and reverses the spiral of 

 the shell. 



The horny hinge is formed by the area of the soft mantle 

 attached to it. This area is similar in shape, minutely 

 ridged, and, when drawn away, shows the deep brown color 

 of the layer of horn which it is about to deposit. (Fig. 

 12,//'.) 



The manner in which the ligament advances is evidently 

 by the building of new layers on the inside, and the wear- 

 ing off of the old, useless layers on the outside. The broad 

 furrows leading from the hinge to the apex of the valve (///") 

 is the track of the ligament ; and often the pieces or strips 



