1 8 THE OYSTER, 



the mechanical properties of the ligament, but the 

 cause of its closure cannot be the mechanical proper- 

 ties of the muscle, for these are just the same whether 

 it is open or at rest. Careful investigation shows the 

 existence of a wonderful apparatus, consisting of the 

 muscle which does the work, of nerv:es which connect 

 the muscle with the brain, of other nerves which run 

 to the more exposed parts of the oyster's body, and of 

 sense organs which are connected with the ends of these 

 sensory nerves, and these serve to put the animal into 

 communication with the external world. Though 

 very much simpler, the mechanism is essentially like 

 that of our own bodies. The oyster's shell is lined by 

 a fleshy mantle, which is fringed by a border of dark- 

 colored sensory tentacles, which are partially exposed 

 w^hen the shell is opened. The approach of danger is 

 perceived by these organs, which transmit a sensation 

 of danger along the sensory nerves to the brain, and 

 this in turn sends a nervous discharge along another 

 set of nerves to the muscle, and this shortens under 

 the stimulus and pulls the shells together and holds 

 them fast. 



The contrast between the opening and the shutting of 

 the oyster's shell is an excellent illustration of the dif- 

 ference between vital activity and non-vital action. The 

 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, 

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

 our investigation very much farther, and must learn 



