locomotion. Touch portions of the animal and find what parts are 

 the most sensitive. 



II. SHELL ; EXTERIOR. 



Note its general shape, and that it is composed of two symmetrical 

 parts, the valves. For each valve notice: 



1 i ) The outline. 



(2) A swelling, the \imbo, ending in a point, the beak, from 

 which growth has proceeded. 



( 3 ) The lines of growth. Were the valves 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 beak points is an- 

 terior. Which valve is right and which is left? 



Drazv 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 at their margins, and more 

 firmly attached at a point a half inch or more back from the margin. 



III. SHELL; INTERIOR. 



Separate the mantle from one valve and cut the adductor 

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

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

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



( i ) 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. 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 pallal line. What forms it? The 

 posterior end of this line is indented to form the pa-Hal sinus. 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, cardinal, 

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

 prominent, but extend for some distance along the dorsal margin. 



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