92 MOLLUSCA. 



bearing the ligament is dorsal, and that toward which the beaks 

 point is anterior. Which valve is right and which is left? 



Draw a valve, showing the points observed. 



Pry the two valves apart and insert a knife-blade between 

 the mantle and one valve of the shell. Notice that the lobes 

 of the mantle are loosely attached to the shell along their mar- 

 gins, and more firmly attached a half inch or more from the 

 margins. 



Separate the mantle from one valve, and cut the adductors 

 where they are attached to this valve. Why do the valves gape 

 now? Press them together, and notice that they stay closed 

 only while held. Remove a valve and study its interior. 



1. Find the large scars where the anterior and posterior ad- 

 ductor muscles were attached. 



2. Find smaller scars where the anterior and posterior foot 

 muscles were attached. The anterior scar is dorsal and a little 

 posterior to the corresponding adductor muscle scar. (Not the 

 position for Unio.) The posterior scar connects with the dorsal 

 portion of the corresponding adductor muscle scar. 



3. The ventral borders of the adductor muscle scars are con- 

 nected by a distinct line, the pallial line. What forms it? The 

 posterior end of this line is indented to form the pallial sinus. 

 (Not true for Unio.) What is the meaning of this sinus? 



4. Along the dorsal margin of the valve notice prominences, 

 the teeth. There are two kinds of teeth. The anterior, cardi- 

 nal, consist of short elevations. The posterior, lateral, are not 

 very prominent, but are comparatively long and extend along 

 the dorsal margin. Notice that the teeth on the two valves 

 interlock. What is their function? 



Draw a valve as seen from the inside. 



5. By examining the inside of a shell of Unio or Mytilus near 

 its margin, the tj'pical three la^'ers of which it is composed can 

 be seen. How is it possible for all three layers to be secreted 

 by the mantle, which lines the inside of the shell? Can you find 

 any reason for more than one layer? 



Mantle. — This consists of two lobes (one of w^hich is normally 



