ANODONTA. 271 
a similar scar of the posterior adductor. The adductors 
run across from shell to shell, and their contraction draws 
the shells together. Inside each adductor scar is a smaller 
round scar, caused by the anterior and posterior re¢ractors, 
which serve to draw the foot into the shell. Lastly, near 
the anterior adductor scar is a small protractor scar, the 
muscle serving to draw the foot forward. The attachments. 
of the muscles shift outwards and downwards as the shells 
grow. 
Fig. 190.—INTERNAL VIEW OF RIGHT SHELL OF ANODONTA. 
(Ad nat.) . 
Umbo. 
Hinge. 
Anterior Retractor. f 
Posterior 
Retractor. 
Anterior 
Adductor. 
Posterior 
Adductor. 
Pallial Line. 
Protractor. 
When the shells are forced open they expose a large 
mantle-cavity. This is bounded dorsally by the body of the 
animal and laterally by the lateral mantle folds ; ventrally 
it is widely open to the exterior, except when the shells are 
shut. The mantle-flaps line the inner surface of the shells, 
which they secrete. The free edges are pressed together, 
except at the posterior end, where they diverge to form a 
large znhalent opening, then meet, and again diverge to form 
the smaller exhalent opening. 
In the centre of the mantle-cavity a large muscular foot 
depends downwards and on occasion it can be protruded 
aenweiy outwards between the shells. Embedded in the 
* foot, near the pedal ganglia, are the otocysts, but 
Anodonta has no eyes. There.is a pair of osphradia or 
sense-organs of an olfactory nature at the base of the gills, 
innervated from the visceral ganglia. 
