MOLLUSKS 



91 



mouth by means of the cilia on the great triangular lips or 

 palps which bound it on each side (Fig. 51, A, p). 



a.a. 



PIG. 51. Anatomy of fresh-water clam. A, right valve of shell removed ; B, dissec- 

 tion to show internal organs, a, external opening of kidney; a.a., the anterior 

 muscle for closing the shell ; >, opening of reproductive kidney ; c, brain ; ft., 

 foot ; g, gill ; h, heart ; i, intestine ; , kidney ; I, liver ; m, mantle (upper fig.), 

 mouth (lower fig.) ; p, palp (upper fig.), foot nerves (lower fig.) ; p.a., hinder 

 muscle for closing the shell; s, .space through which the water passes on 

 leaving the body ; st, stomach ; v, nerves supplying viscera. 



Between the halves of the shell in the hinge region is a 

 horny pad that acts like a spring, and without any muscu- 

 lar effort on the part of the clam keeps the shells open. 



