84 A GUIDE FOR THE STUDY OF ANIMALS 



Lines of growth, concentric lines around the umbones. 



Siphons, two openings at the posterior end of the clam, 

 the upper opening is the exciirrent opening and the lower 

 the incurrent. In the salt water clam the siphons form a 

 long tube, usually called the " neck." 



Hinge teeth, projections near the dorsal margin on the 

 inner surface of the shell. The anterior irregular structure 

 is the cardinal and the more posterior blade-like structure 

 is the lateral tooth. 



Muscle scars, scar-like markings on the inner surface of 

 the shell indicating the point where muscles were attached. 

 The large scar just in front of the cardinal tooth is the 

 anterior adductor muscle scar, and the one just back of the 

 lateral tooth is ih^ posterior adductor muscle scar. 



Pallial line, a line connecting the two muscle scars. 



Mantle, folds of skin covering the body of the clam and 

 lying close to the inner surface of the valves. 



Foot, a hatchet-shaped structure extending from the ven- 

 tral edge of the body. 



Gills, broad flap-like structures for respiration, situated 

 each side of the body in the mantle cavity. They consist 

 of a double fold of membrane through which run many 

 perforations lined with cilia. The waving of these cilia 

 cause the current of water needed for respiration. 



Palps, small flap-like structures near the anterior end of 

 the clam. They surround the mouth. On their surface 

 are cilia which cause currents of water toward the mouth. 



Adductor muscles, large muscles extending from valve to 

 valve. 



Observations. 



Identify anterior and posterior ends, dorsal and ventral 

 surfaces, right and left sides. 



