Bull. U. S. B. F., 1919-20. 



Plate XXI. 



Uorsol musclcc 



Antarior 

 ratractor muse 

 Antarior 



adductor iriuacie' 

 Trotractor rnus 

 fosition 0/ niout 



labial paip^} 



Outer laft labial palp , 



Inner kjJ labial palp V.sc«.ral mttss 



Li^an^i 'twill 



Left niantU 



FaiUrior latractor musclii 



adductor r[^u6cl& 



upra-bran&nial 

 (ihainbar 



Anus 



ExWaltunt 

 s'lphonal 



Innal^nt 

 .^iphonixl 

 op&nin^ 



Outar right jill 

 inner right ^ill 

 liner Ujt Jill 

 Outer iaft Jill 



Fig. 



-The structure of a fresh-watcr mussel. (Based upon drawing of white heel-splitter, Sytnphynola coml'lanata, 



from Lefevre and Curtis, 1912.) 



Fig. 2. 



-A tool which, if employed with care, may be used for partially opening living mussels for examination of conditions 



within the shell. 



