86 THE FRESH-WATER MUSSEL. 



Leave the animal in the left valve of the shell, and fix it 

 down firmly to the dissecting -board under water. Pins can 

 he passed through the ligament. 



A. External Characters. 



1. The muscles. The anterior and posterior adductors are 



easily recognised, and form useful landmarks. Their 

 fibres run transversely from one valve to the other, 

 and by their contraction close the shell. 



Close to the adductors are the smaller protractor 

 and retractor muscles of the foot, the origins of 

 which have already been seen on the inner surface 

 of the shell. 



The protractor is behind the lower border of the 

 anterior adductor, and the anterior retractor is 

 behind the upper part of the same muscle. The 

 posterior retractor is continuous with the upper and 

 anterior border of the posterior adductor. 



2. The right mantle-lobe is the flap of integument which 



covers the side of the body : it is in contact during 

 life with the inner surface of the shell, to which it 

 is attached along the pallial line. Its inner surface 

 is ciliated. 



Dorsally it is continuous with the body-wall, of 

 which it is an outgrowth. Its ventral border is free 

 and grooved, this free border secreting the outer and 

 middle layers of the shell. 



The mantle-lobe begins in front above the middle 

 of the anterior adductor muscle, and runs round its 

 anterior border as a narrow fringe. Opposite the 

 part of the body between the two adductor muscles 

 the mantle-lobe is very wide, extending from near 

 the dorsal border to the ventral edge of the shell. 



At the hinder end of the animal, below the 

 posterior adductor, the margin of the mantle which 

 here bounds the inhalant aperture is very greatly 



