18 
LAND AND FRESH-WATER MOLLUSKS. 
shell is jerked forward; or the extremity of the 
protruded foot is made a lever, and the animal 
draws itself forward. By means of the same 
organ, the animal can bury itself in the sand or 
mud, in which it lives. The pedal muscles (p, p') 
retract the foot. 
The adductor muscles (a, a!) are two in number, 
they extend from one valve to the other, and 
by their contraction the shell is closed; they are 
hence named adductors . On their relaxation, or 
if they be cut, the elasticity of the ligament 
comes into play, and the valves are opened. The 
term dimyary bivalves is applied to such, as the 
Unio , with two adductor muscles; the monomyary 
bivalves, as the oyster, have only one. The fresh¬ 
water Conchifera are all dimyarian bivalves. 
Reproduction . — Each individual is male or 
female. The impregnated eggs, of which 200,000 
are computed to be produced by each indivi¬ 
dual in the year, pass into the external folds of 
the gills, which during the breeding season in 
early spring are broad and thick : here they are 
hatched, and the young remain for some time. 
The embryos are very unlike the parent, and 
have been described as parasites under the name 
of Glochidium . They have a triangular shell 
provided with serrated hooks, and have but one 
adductor muscle, and a long slender byssal fila¬ 
ment. In this condition they have been found 
