FEB8H-WATEB MUSSEL 196 



What is the texture of the extreme margin of the 

 mantle ? Color ? Structure ? Notice on the dor- 

 sal surface, in front of the posterior adductor 

 muscle, a thin portion of the mantle. This por- 

 tion covers the pericardial chamber. 



c. The adductor muscles of the valves. — How many 



are there ? How are they situated ? How are 

 they distinguished from the rest of the body? 

 What is the shape and size of each? Do they 

 differ in shape and size ? How are they attached 

 to the shell? Are they covered by the mantle? 

 How do you account for their number ? In what 

 direction do their fibres run ? Why do they run 

 in this direction ? Why are these muscles called 

 " adductors " ? What advantage in having them 

 in the position in which you find them ? 



At the upper end of each adductor muscle look 

 for 



d. The retractor pedis muscles. — Compare with c in 



every respect. Do these muscles form a " scar " 

 on the shell ? 

 A httle below the anterior retractor muscle find 



e. The protractor pedis muscle. — What is its shape? 



Size ? Compare with the adductor and retractor 

 pedis muscles in every respect. 

 At the posterior end of the body find 



/. The siphons. — How many are there? How are 

 they formed ? What is their relative position to 

 each other % How are they separated ? How 

 long are they ? How wide ? With a pipette in- 

 ject water into each siphonal opening. Where does 

 the cavity of each lead? ISTote the tentacles — 

 their number, shape, size, color, and structure. 



