100 MOLLUSCA. 



each side is provided with a long appendage that may be pro- 

 truded from between the valves of the shell. This is the feeding 

 appendage. 



5. The gills. These are quite small and are composed of 

 parallel plates that are capable of being moved. They are 

 situated behind the palps, are attached dorsally by muscular 

 membranes to the body-wall, and posteriorly to the wall that 

 separates the siphons. They illustrate what is supposed to be the 

 most primitive type of lamellibranch gill. Watch their move- 

 ments and see if you can determine how they cause the jets of 

 water to leave the cloacal siphon. What reason is there for 

 forming such strong jets of water? 



6. The heart and ganglia are nicely shown in such a speci- 

 men. 



7. Remove one of the shell valves of an adult specimen and 

 examine the organs. An elongated sense tentacle occurs on one 

 or the other side of the base of the branchial siphon, between the 

 wall of the siphon and the corresponding mantle lobe. 



A drawing of the organs will prove profitable. 



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



