MOLLUSKS 



117 



to tlie other. In the oysters aad 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/ Mussel-s exposed at low tide. Annisquain, 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 hy 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 tliickened and are usually more or less 

 fused, thus binding the two valves together along their edges. Def- 

 initely placed o]3enings 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- 



