128 FRESH-WATER MUSSEL-COMMON MARINE MUSSEL 



(0) The dorsal exhalent siphon (if protruded), lying just above the inhalent 

 siphon. The lips are smooth. [In Mytilus the inhalent and exhalent 

 siphons do not protrude beyond the margin of the shell] 



Fm. 41.— The Common Marine Mussel [Mytilun edn/i«). x 1|. 



Internal Structure. 



1. Place the handle of a scalpel between the ventral edges of the valves of 

 the shell and separate them. The muscles connecting the valves may then be 

 seen. Cut through these as close as possible to their points of attachment to 



Hinge 



Anterior retractor 



Posterior retractor 



Posterior 

 adductor ■ 



Pallial line 



Fig. 42. — Inner side of the shell (right valve) of the Fresh-water Mussel (Anotlonta), showing 

 the marks of the muscle insertions and the pallial line. 



the left valve of the shell. Cut through the ligament along the hinge-line, and 

 remove the left shell. 



Make a sketch of the inside of this shell showing : — 



(a) The marks of attachment of the following muscles : — 



(* The anterior adductor muscle, a large oval area, close to the 



