122 MOLLUSCA. 



places where the cilia have pulled apart. Notice the size and 

 shape of the ostia and find the two kinds of movable cilia. 



9. This form usually shows the way food is gathered espe- 

 cially well. Place powdered carmine on the surface of a gill and 

 see what becomes of it. 



10. Notice the thickened condition of the mantle. The gonads 

 extend into them, and the thickening is due to sexual products. 



Drawings of the arrangement of the organs, and especially of 

 the microscopic structure of the gill, will prove profitable. 



Meisenheimer : Entwicklungsgeschichte von Dreissensia polymorpha. 

 Zeit. f. Wiss. Zool., 69, 1900. 



PECTEN IRRADIANS. (Scallop.) 



This species belongs in the order Pseudo-lamellibranchia 

 and lives on muddy or sandy bottoms, generally where the water 

 is from a few inches to several fathoms deep. It has the power 

 of swimming pretty well developed. At rest on the bottom it 

 always lies on the right valve of the shell. 



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 mantle. See if it is sensitive. How far can it be drawn 

 back into the shell? What muscles are used in withdrawing it? 

 Why is it necessary to withdraw it? What is peculiar about the 

 shape of the margin? What reason is there for this structure? 



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

 the mantle. Are they placed in any order? 



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

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



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 



