20 THE OYSTER. 



explanation of the movement which opens the shell is 

 found in the physical properties of the ligament, and 

 a piece of rubber in the same place would produce the 

 same effect; but while the closure of the shell is un- 

 doubtedly due to the physical properties of the muscle, 

 we must carry our investigations very much farther 

 in order to find the reason for its action, for we must 

 learn what was the change, external to the oyster, 

 which excited the sense organs, and we must ask how 

 the oyster has learned to associate such a sensation 

 with the presence of danger, and how it has learned 

 that the danger may be escaped by closing the shell. 



It is much more easy to ask this question than to 

 answer it. The oyster is by no means a simple animal, 

 and our efforts to study and understand its structure 

 bring us, at the first step, face to face with problems of 

 the most profound character ; problems which will tax 

 all the resources of investigators and philosophers for 

 many generations. We shall not enter into these deep 

 questions here, as we shall confine ourselves to simpler 

 matters. 



The muscle is attached to the shell at some distance 

 from the hinge, in order that it may have leverage and 

 work to advantage; and it must therefore be able to 

 move as the shell grows, for in an oyster three inches 

 long its area of attachment is outside what was the 

 extreme border of the shell when this was only an 

 inch long. The muscle travels by the addition of new 

 fibres on its outer surface, as those on its inner border 

 are absorbed and removed. As it is moved, the old 



