FRESHWATER MUSSELS 59 



and are so found in dead specimens. A definite muscular effort 

 is required to close the valves. 



Upon the inner surface of each valve is a number of marks 

 where, in life, muscles are attached. These muscular impressions 

 are more conspicuous in shells of Unio, but can be seen without 

 difficulty in Anodonta. In the anterior region is a large shallow 

 pit caused by the anterior of the two powerful muscles which 

 run across the body, and which by their contraction bring the 

 margins of the two valves together ; this pit is therefore termed 

 the impression of the anterior adductor muscle. In the posterior 

 region is a similar depression due to the attachment of the posterior 

 adductor muscle. Just behind the ventral end of the anterior 

 adductor impression is a smaller pit, where is inserted a muscle 

 by whose action the animal can move backwards or, if the shell 

 be fixed, draw the foot forwards. As a matter of fact, the former 

 movement is the normal, but the latter has served to give the 

 name of " protractor of the foot " to this muscle. A similar in- 

 verted view of the fixed and free ends has unfortunately influenced 

 the names given to all the muscles concerned with the locomotion 

 of these animals. Confluent with the dorsal part of the anterior 

 adductor impression is the mark of one of the muscles which 

 pull the shell forward (the anterior retractor of the foot) ; and 

 just dorsal and anterior to the posterior adductor impression is 

 the scar of the other (posterior retractor of the foot) employed 

 for the same purpose. Towards all these impressions there lead 

 from the region of the umbo faint tracks marking the path 

 along which the several muscles have shifted as the shell has 

 increased in size. From the anterior to the posterior adductor 

 impression there runs, parallel with the margin of the shell and 

 at a slight distance from it, the " pallial line." This is caused 

 by the insertion of numerous muscle fibres (pallial muscles) lying 

 in the mantle ; their use will be explained later. Besides these 

 larger there are two groups of smaller impressions in the dorsal 

 region, one near the anterior and the other near the posterior 

 end of the ligament. The small muscles which cause these marks 

 run down into the mantle and pull the shell in a ventral direction. 

 If the shell be broken and the fractured edge examined it will be 

 seen, even with the unaided eye, to be composed of three layers. 



