270 



MOLLUSC A, 



whole body is completely enclosed m 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 



Features. ^^^ other by a hingCy and they can be opened 



by the contraction of an elastic ligament, just outside the 



hinge Just above the hinge is a small first-formed part 



Fig. iS 



-Lateral View^ (Left) of Anodonta in Natural 

 Position and Feeding. 



(Mainly after Howes.) 



Exhalent Current. 

 Exhalent Tube. 



Umbo. 



Water current 



/passing- into 

 Inhalant 

 Aperture. 



Lines of Growth. 



Level of Mud. 



called the umbo. 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 



