MOLLUSKS IU) 
to the other. In the oysters and scallops there is but one adductor 
muscle, while in clams and mussels there are two. When the ad- 
ductor muscles relax, the hinge-cushion causes the valves to gape; 

Fig. 79; Mussels exposed at low tide. Annisquam, Mass. 
and this is the normal condition while the animal is immersed and 
feeding. When the tide goes out, however, or the creature is sud- 
denly disturbed the adductors contract, thus closing the valves. 
The shell is secreted by a membrane called the mantle, that 
arises from the right and left sides of the mid-dorsal line and 
extends downward as a sheet on both sides of the body; the form 
of the sheet being similar to that of the shell it secretes. The outer 
edges of the mantle are thickened and are usually more or less 
fused, thus binding the two valves together along their edges. Def- 
initely placed openings are, however, always found where the man- 
tle edges do not fuse. One or two of these are opposite the posterior 
end of the body of the animal, and allow water to pass in and out 
of the mantle-cavity, while another on the ventral side allows the 
foot to be thrust out. In the clams the openings for the admission 
and expulsion of water are separated and the mantle is here ex- 
