146 MARVELS OF FISH LIFE 



further, Dr. Dakin explained to me how the various 

 movements were performed. 



These movements are produced by the combined 

 action of the powerful muscle between the two valves 

 and the mantle edges. By the contraction of the muscle 

 referred to, the shell is closed and the contained water 

 is shot out. The mantle edges determine the position 

 of the exit of the water. If they prevent any water 

 from leaving in front, it is shot out in two streams 

 near the hinge behind, and the animal swims. If the 

 water is allowed to pass out in front the scallop jerks 

 itself back a few inches with the recoil. If a pecten 

 is placed on the " wrong " valve (the side marked by 

 the growth of zoophytes, sponges, and barnacles) it 

 soon opens, much wider than is usually the case, and 

 the internal organs can be seen between the two mantle 

 edges. The tentacles are fully extended, but as soon 

 as the opening movement of the valves ceases, they 

 are sharply withdrawn, and this is the signal for the 

 closing of the valves. Though the opening of the 

 valves has been a slow, steady movement, they close 

 with a sudden snap. The mantle edges now meet, but 

 leave a space for the water to pass out in front. As 

 the result, the animal is thrown back on to the hinge 

 line with the recoil caused by the water rushing out, 

 and rotating on its axis, turns over. 



Now that we have considered the scallop, let us 

 turn to the oyster, a bivalve with which most of us are 

 familiar. 



