SECTION IV. 



THE CLASS OF ACEPHALA OR LAMELLIBRANCHIATA OR 



BIVALVES. 



THE mollusks of this Class seem to have no head, this 

 important part of the animal being but little developed, 

 and wholly concealed within the mantle, and the whole 

 covered with a shell composed of two pieces called valves. 

 The valves are joined together by a hinge, and held 



FIG. 17. 



Excurrent 



orifice. 



Foot or Locomotive 

 Organ. 



Respiratory 

 orifice. 



An AcepUal Fresh-water Mussel. Unio complanatus, Lea, U. S. 



FIG. 618. 



Internal Structure of a Fresh- water Mussel, Unio pic/orum, L., with the right valve 



and niautle lobe removed. 



a, a, adductor muscles; p, p. pedal muscles; x, accessory pedal muscles ; it, unibo ; 

 7, ligament ; b, branchial orifice ; u, anal opening ; /, loot ; o, mouth ; t, tentacles or 

 palpi. 



