9 8 



ANIMAL BIOLOGY 



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FIG. 188. 



ANODON, or fresh-water 

 mussel. 



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

 side away from yon and tJie blunt end to tJie /<*/?( Fig. 1 88). 



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 the bottom ? 

 Our word " valve " is derived 

 from a word meaning shell, 

 because the Romans used 

 shells for valves in pumps. 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 coveiing, 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 

 plows 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 

 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. 



