PECTEN GIBBUS BOREALIS. 125 



6. Find where the byssal threads are attached. Where 

 secreted. 



7. Notice the relatively small foot, and compare it with the 

 powerful foot muscles. Why are such powerful foot muscles 

 necessary? How does the foot function in attaching byssal 

 threads? 



8. See how the gills are attached to the body. The filaments 

 of the gills of this form are very loosely attached to each other by 

 modified clumps of cilia, that represent the inter-filamentar 

 junctions. Cut off a piece of a gill, mount it in sea-water under 

 a cover, and examine with low and high powers. Find places 

 where filaments are attached by the 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 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 GffiBUS BOREALIS. (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. 



