PHYLUM MOLLUSCA 



Many peculiarities of organization set off the animals of the 

 phylum Mollusca from those previously studied. Molluscs 

 possess bilateral symmetry, but a pronounced regional differ- 

 entiation is lacking. 



Class Pelecypoda Unio or Anodonia (or Any of the Bivalves 

 Commoyihj Used). — Animals of these genera wander along the 

 muddy bottoms of creeks, ponds, and rivers, with the anterior 

 end buried in the mud. The food consists of minute plants and 

 animals gathered from the respiratory current. 



Exterior of Shell. — The shell comprises right and left halves 

 (right and left valves) which retain their continuity along the 

 dorsal side in the elastic hornlike hinge. At other points the 

 two halves touch upon one another only. Note the concentric 

 lines on the shell indicating stages in its growth. The knob 

 on each shell near the hinge about which the growth lines are 

 concentrically arranged is the beaPc or umbo. Its apex is directed 

 toward the anterior end of the animal. Taking the position 

 of the hinge and umbo as a basis, determine the toponomy of the 

 clam body. 



Structure of the Shell. — The shell is composed of three layers: 

 the thin outer dark-colored layer of conchiolin (hornlike in 

 appearance) ; the middle or limestone layer of perpendicularly 

 arranged prisms; and the inner or mother-of-pearl layer formed 

 of limestone arranged in sheets. Break a fragment from the edge 

 of one valve and examine these layers. 



Inner Surface of the Shell. — Lay back the left half of the shell 

 and study the internal surface. This can be done by first 

 opening the shell slightly. Insert a scalpel at each end of the 

 hinge and cut the muscles which draw the halves together. The 

 left valve must also be carefully freed from the soft parts within 

 before the two halves are widely separated. 



In Unio the valves articulate with each other by means of 

 conical and ridged projections called hinge teeth. These dove- 

 tailing teeth hold the valves in their proper relations when moving 

 upon one another. Hinge teeth are almost entirely wanting in 



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