102 MOLLUSCA. 



1. Do the valves of the shell differ in color or shape? 



2. On each side of the beak of each valve is a flattened pro- 

 jection frequently called an "ear" or "wing," the posterior 

 of which slopes backward, while the anterior, especially the one 

 on the right valve, is somewhat separated from the body of the 

 shell by a notch. 



Place specimens in dishes of sea-water, and when they have 

 opened their shells notice : 



3. The bright specks, the pallial eyes, along the margins of 

 the mantle. Are they placed in any order? 



4. The arrangement of the tentacles on the margins of the 

 mantle. Why should sense organs be placed in this position? 



5. The mantle and see if it is sensitive. How far can it be 

 drawn back into the shell? What muscles are used in with- 

 drawing it? Why is it necessary to withdraw it? 



6. Specimens in aquaria will often swim. If possible, notice 

 how this is accomplished. 



Wedge the valves of a specimen apart and notice the single 

 large adductor muscle. What need has Pecten for such a large 

 adductor? Notice the foot and compare it with the foot of 

 Venus. 



How are the gills attached to the body? What would be 

 the effect on the gills if they were attached to the mantle and to 

 each other, as in most forms, when water is ejected in swimming. 



Examine the structure of the gill and notice how much 

 larger the inter-filamentar junctions are near the inter-lamellar 

 junctions than elsewhere. Near the margins of the gills the 

 junctions are frequently simple bunches of cilia, as in Mytilus. 

 Observe the muscular movements of the gills. The gills of this 

 form need to be muscular so they can be drawn together when 

 the animal swims. 



Drawings to show the arrangement of the organs and the struc- 

 ture of the gill are desirable. 



