FRESHWATER MUSSEL 85 



8. Below the hinge line of the shell is found a slowly pul- 

 sating organ, the heart. Carefully cut through the surrounding 

 pericardium, and expose the heart. Distinguish the median ven- 

 tricle and the lateral auricles, the left one being uppermost. 



9. Examine the left valve of the shell and distinguish the 

 scars on the valve where the anterior and posterior adductor 

 muscles were attached. 



10. Examine the pallial line and note the corresponding part 

 of the mantle. The mantle muscle and its mode of attachment 

 may be seen on the right side of the animal. Compress the mar- 

 gin of the right mantle. What is the effect? What is the func- 

 tion of the mantle muscle? Notes required. 



11. Distinguish the umbo near the anterior part of hinge 

 ligament. This is the older part of the shell, and concentric with 

 this are the lines of growth. How are these formed? What do 

 they indicate ? Where is the newest part of the shell ? 



II. THE EXTERNAL CHARACTERS OF THE SOFT PARTS 



i. Examine the soft parts of the mussel on* the "half shell." 

 Compare the cut muscles on the body with the scars on the shell, 

 and also with those yet in place on the right side. Distinguish 

 the following muscles and their scars : 



(a) The large anterior adductor, near the dorsal an- 

 terior end. Posterior to this is a pair of muscles : 



(b) The anterior retractor is the upper or dorsal one ; 



(c) The protractor is the lower or ventral one. The 

 protractor compresses the visceral mass and forces the foot 

 from the shell. Determine the functions of the other muscles. 

 At the posterior end are: 



(d) The large posterior adductor. 



(e) Dorsal and anterior to the preceding is the pos- 

 terior retractor. 



