EXTERNAL CHARACTERS 81 



II. DISSECTION OF THE MUSSEL. 



In dissecting the mussel, a small brush may with great 

 advamtage be used to remove the mucus and to clean the several 

 pa/rts. 



Lea/ve the animal in the left valve of the. shell, and fix it 

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

 readily be passed through the ligament. 



A. External Characters. 



1. The muscles. The anterior and posterior adductors are 



readily recognised, and form useful landmarks. Their 

 fibres run transversely from one valve to the other, 

 and by their contraction close the valves. 



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 

 sheU. 



The protractor is behind the lower border of the 

 anterior adductor, and the anterior retractor is be- 

 hind the middle 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 irmer surface 

 is ciHated. 



Dorsally it is continuous with the body- wall, of 

 which it is a prolongation. 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, extendiag from near 

 the dorsal border to the ventral edge of the shell. 



