124 MOLLUSC A. 



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 mil prove profitable. 



Drew: The Anatomy, Habits, and Embryology of Yoldia limatula. Mem. 



Biol. Lab. Johns Hopkins Univ., 4, 1899, 

 : The Life-History of Nucula dclphinodonta. Quart. Jour. Mic. Sci., 



44, 1901. 

 Mitsukuri: On the Structure and Significance of some Aberrant Forms of 



Lamellibranchiate Gills. Quart. Jour. Mic. Sci., 21, 1881. 



MYTILUS OR MODIOLUS. (Massels.) 



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 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 hyssal 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 

 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. 



