VENUS 125 



portions of the animal and find what parts are most sensi- 

 tive. 



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 

 margins, and more firmly attached a half inch or more from 

 the margins. 



Separate the mantle from one valve, and cut the adduc- 

 tors 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 

 adductor 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 

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

 The posterior end of this line is indented to form the pallial 



