V -J THE FRESH-WATER MUSSEL. 315 



shell, and nearly straight. A curved ventral border, 

 opposite the dorsal. 



b. A wider anterior end. A narrower posterior end. 



c. A right and left side. 



2. Pin the animal down under water without removing 

 its remaining valve (insert the pins obliquely, so that 

 they embrace, and exercise a downward pressure 

 upon, the edges of the valve). Examine in order — • 



a. The mantle or pallium ; a bilobed semitransparent 

 membrane, one lobe lining each valve of the shell. 

 Examine its ventral border, it is yellowish in colour 

 and thickened to form a pallial muscle. 



b. The adductor muscles of the valves ; two immense 

 masses whose cut edges are visible at opposite ends 

 of the body. 



c. The retractor pedis muscles; smaller masses, one at 

 the upper end of each adductor. The anterior one 

 is posterior, and the posterior one anterior, to its 

 corresponding adductor. 



d. The protractor pedis muscle; uniform in diameter 

 with c. It lies a little below the anterior re- 

 tractor. 



e. The lesser retractors; insignificant bundles of fibres 

 arising from the extreme dorsal surface of the body 

 wall. 



3. Turn back the parallel lobe, and note under- 

 lying it — 



a. The ventral portion of the body. It projects for- 

 wards and downwards, immediately behind the 

 anterior adductor muscle : it is thickened ventrally 



