MYTILUS OB MODIOLA. 109 



MYTILUS OR MODIOLA. (Mussels.) 



These animals belong to the order Filibranchia, and show 

 comparatively simple gills, as well as interesting modifications 

 for their manner of living. They live attached to 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 Modiola 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 examined. 



2 v 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 adductor 

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

 rangement of organs. 



6. Find where the byssal 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 



