PECTEN IRRADIANS. 101 



specimen. Are they elastic? How would elasticity aid the 

 animal in remaining attached ? 



3. Leave specimens in sea-water for some hours, and see if 

 they change their positions. 



4. Notice the margins of the mantle. Are they fused? Why 

 are siphons not necessary? See if you can find where water 

 passes in and out. 



5. Wedge the valves of a specimen apart, cut the adductor 

 muscles (take note of their relative size), and examine the ar- 

 rangement of organs. 



6. Find where the hyssal threads are attached. 



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

 powerful foot muscles. Why are such powerful foot muscles 

 necessary ? 



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. 



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. 



