130 FRESH- WATER MUSSEL -COMMON MARINE MUSSEL 



anterior end of the shell. [In Mytilus there is no anterior 



adductor muscle.] 

 (ii) The anterior retractor muscle, a small rounded area, contiguous 



with the posterior dorsal edge of the anterior adductor, 

 (iii) The protractor muscle, a rounded area behind the lower part 



of the anterior adductor muscle but quite separate from it. 



[In Mytilus there is no protractor muscle.'] 

 (iv) The posterior adductor muscle, a large oval area, larger than 



the anterior adductor, near the posterior dorsal end of the 



shell, 

 (v) The posterior retractor muscle, a small rounded area, continuous 



with the antero-dorsal edge of the posterior adductor. 



Anterior, retractor 



Dorsal retractors 



JPosterior 

 retractor 



--Posterior 

 adductor 



Pallial line 



Fio. 4.3.— Inner side of the Shell (right valve) of the Marine Mussel {Mytilus), 

 showing the marks of the muscle insertions and the pallial. line. 



[(vi) In Mytilus there are three dorsal retractor muscles, the impres- 

 sions ofrchich lie along the mid-dorsal part of the shell.] 

 Note that the adductor muscles close the shell, and the other muscles work 

 the "foot." 



(b) The pallial line, a curved line marking the attachment of the mantle to 



the shell, and running parallel to the ventral border of the shell. It 

 connects the posterior border of the posterior adductor area mth the 

 ventral border of the anterior adductor. 



(c) The hinge, a smooth longitudinal ridge extending backwards from the 



umbo along the dorsal edge of the shell. 



Notice the inner pearly or nacreous layer of the shell. This layer is made by 

 the surface of the mantle which lines the shell. The median prismatic limy 

 layer and the outer horny periostracum are formed by the thickened edges of 

 the mantle. 



2. Note the mantle closely lining the inside of the shell and covering the 

 sides of the body. It is attached to the shell along the pallial line. Just behind 

 the posterior adductor muscle the two lobes or flaps of the mantle fuse together, 

 and this fused portion extends forward along the dorsal surface of the posterior 



