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BEGINNERS' ZOOLOGY 



Fig. 188. 



• Anodon, or fresh-water 

 mussel. 



shells for valves in pumps. 



Lay the shells, fitted together, in your hand with the hinge 

 side away from yon and the blunt end to the left (Fig. 188). 



Is the right or the left shell 

 uppermost ? Which is the 

 top, or dorsal, side ? Which 

 is the front, or anterior, 

 end ? Is the straight edge 

 at the top or at the bottom ? 

 Our word " valve " is derived 

 from a word meaning shell, 

 because the Romans used 

 Is the mussel a univalve or a 

 bivalve ? Which kind is the oyster ? The snail ? 



Does the mussel have bilateral symmetry ? Can you 

 find a horny coveting, or epidermis, over the limy shell 

 of a fresh specimen ? Why is it necessary ? Does water 

 dissolve lime . ? Horn ? Find a bare spot. Does any of 

 the shell appear to be missing there ? 



The bare projection on each shell is called the umbo. 

 Is the umbo near the ventral or the dorsal line? The 

 posterior or anterior end ? Is 

 the surface of the umbones 

 worn ? Do the umbones rub 

 against the sand as the mussel 

 ploughs its way along? How are 

 the shells held together ? Where 

 is the ligament attached ? (Fig. 

 189.) Is it opposite the um- 

 bones or more to the front or 

 to the rear? Fig.189.) Is the liga- 

 ment of the same material as the shell? Is the ligament 

 in a compressed condition when the shell is open or when 

 it is closed? (Fig. 189.) When is the muscle relaxed? 



Fig. 189. — Diagram of Shell 

 open and closed, showing mus- 

 cle, m, and ligament, b. 



