VENUS MERCENARIA. 115 



ings of the siphons and determine the direction of the current 

 of water for each. Touch portions of the animal and find what 

 parts are most sensitive. 



Shell. — Note its general shape, and that it is composed of 

 two symmetrical parts, the valves. For each valve notice: 



1. The outline. 



2. A swelling, the umbo, ending in a point, the beak, from 

 which growth has proceeded. 



3. The lines of growth. Were the valve cut off along one of 

 these lines, the shape would not be changed. Why are the lines 

 arranged in this manner? How were they formed? 



The two valves are joined by the ligament. The margin 

 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- 



