THE SWAN MUSSEL 



313 



It will have been noticed that the shell and mantle of 

 the mussel are bilaterally symmetrical. The 

 same symmetry is found in all the other organs 

 of the body, both internal and external. Above 

 the attachment of the mantle, at its lowest point 

 near each end, may be seen on each side the cut surface 

 of the great adductor muscles, anterior and posterior, which 



External 

 Features: 

 Locomotion 

 and Feeding 



v p. r. 



Fig. 222. — A swan mussel removed from its shell and lying 

 on its right side with the greater part of the left lobe of 

 the mantle cut away. 



a. ad., Anterior adductor muscle; a.r., anterior retractor; d.s., dorsal 

 siphon ; _/", foot; l.i.g., left inner gill ; LiuL, remains of left mantle 

 lobe turned back; l.o.g., left outer gill; /./., labial palps ; p. ad., 

 posterior adductor muscle \p.r., posterior retractor \pro., protractor ; 

 r.tnL, right mantle lobe ; r.ml' ., thickened edge of the same ; v.s., 

 ventral siphon with papilla;. 



pass through the body from side to side and draw together 

 the valves of the shell. To the upper and inner sides 

 of these lie the anterior and posterior retractor muscles, 

 which draw the body forwards upon the foot when the 

 latter has been thrust out. Behind the lower end of 

 each anterior adductor is a protractor muscle, which draws 

 the body backward upon the foot. On turning back 

 the mantle the rest of the external organs are laid bare. 

 The most conspicuous of these are the foot and two pairs 



