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 watei 



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are sharply withdrawn, and this is the signal for the 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. 



