YOLDIA, MYTILUS OR MODIOLUS 135 



stones, shells, piles, or even to sand grains, sometimes in 

 moderately deep water, but frequently between low- and 

 high-tide marks. The two forms may easily be distinguished 

 by the positions of their beaks. The beaks of Mytilus form 

 the anterior end of the shell. Those of Modiolus are placed 

 a short distance posteriorly. You should visit "mussel beds," 

 and see where and how they are attached and on what they 

 must depend for food. 



1. Place young specimens in dishes of sea water and see 

 if they will attach themselves by their byssal threads. (They 

 will generally require some hours.) If you can get them to 

 attach to slides, the attachment may be microscopically ex- 

 amined. 



2. Test the strength of the byssal threads of a rather old 

 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 adduc- 

 tor muscles (take note of their relative size), and examine 

 the arrangement of organs. 



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 attach- 

 ing byssal threads? 



8. See how the gills are attached to the body. The fila- 

 ments of the gills of this form are very loosely attached to 

 each other by modified clumps of cilia, that represent the 

 interfilamentar 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 



