The Canadian Oyster. 147 



water. The only resource left to the Oyster when it 

 finds itself on a muddy bottom, and the mud is being 

 deposited slowly, is to maintain upward growth of the 

 edges of the mantle and shell, so that the gaping slit be- 

 tween them is kept above the mud level. In this way is 

 to be explained the curious crimped and curved edges of 

 the so-called Mud-oysters, which form a considerable pro- 

 portion of the Malpeque Oysters, and which since they 

 are very difficult to open do not fetch so high a price 

 as those of more regular shape grown on clean hard 

 ground. 



But other dangers besides starvation and suffocation 

 threaten the Oyster. The starfish is a deadly enemy. 

 This animal has the power of turning the stomach 

 inside out, and of digesting alive anything with which 

 this organ comes into contact. When the starfish gets 

 the chance of introducing his out-turned stomach be- 

 tween the valves of an Oyster's shell Tt^s all over with 

 the Oyster. 



To guard against the danger the Oyster has the 

 power of closing the .valves of the shell by means of a 

 powerful muscle, the adductor, which runs straight 

 across from one valve to the other. Closing the valves 

 is equivalent to what in Man is holding the breath, but 

 the Oyster can hold its breath for several days without 

 suffering damage. This, nevertheless, requires a con- 

 tinuous effort on the part of the Oyster, for the " hinge " 

 which unites the two valves is so placed with reference 

 to the teeth with which the one valve articulates on the 

 other, that it is compressed when the valves are closed, 

 and its elastic recoil tends to open the valves as soon as 

 the pull of the adductor muscle is relaxed. 



In most Lamellibranchiata there are two adductors, 

 but the anterior one has been lost in the Oyster; the 

 hinge, too, is usually so placed that it is stretched and 

 not compressed when the valves are closed, but of 

 course the effect is the same. 



Everyone knows that it is necessary to cut the adduc- 

 tor in order to " open " a live oyster. When, however, 

 the Oyster, placed under unfavourable circumstances, 



