136 MOLLUSCA 



bunches of cilia. Find 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 

 especially 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 it, 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 GIBBUS BOREALIS (Scallop) 



This species belongs in the order Pseudolamellibranchia 

 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 well developed. At rest on the bot- 

 tom 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 

 projection frequently called an "ear" or "wing." The pos- 

 terior wing slopes backward; the anterior, especially the one 

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

 the shell by a notch. 



3. Place specimens in dishes of sea water, and when they 

 have opened their shells notice: 



(a) Mantle. 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? What is 

 peculiar about the shape of the margin? What reason is 

 there for this structure? 



{b) The pallial eyes, bright specks along the margins of 

 the mantle. Are they placed in any order? 



(c) The arrangement of the tentacles on the margins of 



