270 MOLLUSCA. 
whole body is completely enclosed n a pair of large oval 
shells which, unlike the shells of Brachiopoda, are lateral. 
The animal is found half buried in the mud of ponds and 
streams. The shells are of a dark brownish-black colour 
and composed of the same three layers as in 
the snail. On the dorsal side they move against 
each other by a Aimge, and they can be opened 
by the contraction of an elastic /igament, just outside the 
hinge Just above the hinge is a small first-formed part 
External 
Features. 
5 > yee 
Fig. 189.—LATERAL ViEW (LEFT) OF ANODONTA IN NATURAL 
POSITION AND FEEDING. 
(Mainly after Howes.) 
Exhalent Current. 
Exhalent Tube. 
‘Water current 
passing into 
Inhalent 
-— Aperture. 
Umbo. 
Lines of Growth. 
Level of Mud. 
called the wmébo. On the inside, the dried shells have 
several scars caused by the attachment of parts of the 
body. A little way inside and parallel to the ventral 
edge of the shell is a line called the pallial line, caused 
by the edge of the mantle, or pallium.. At the anterior 
end of the pallial line is a large oval scar produced by 
the anterior adductor muscle; and at the posterior end is 
