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 ‘ valve (the side marked by 
the growth of zoophytes, sponges, and barnacles) it 
soon opens, much wider than is usually the case, and 
‘ wrong ” 
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are sharply withdrawn, and this is the signal for the 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, 
