THE OYSTER. 19 



and he said that, when free from embarrassment, he 

 could " shuck " thirty-six oysters a minute. 



The work of closing the shell is done by the muscle, 

 but we must go very much farther in the study of the 

 oyster in order to find why it closes. It is opened by 

 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 nerves 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 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 

 when 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 



