v.] THE FRESH-WATER MUSSEL. 315 



shell, and nearly straight. A curved ventral border, 

 opposite the dorsal. 



h. 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 ox palliuin ; 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 ?i. pall ial muscle. 



b. The adductor muscles of the valves ; two immense 

 masses whose cut edges are visible at opposite ends 

 of the body. 



c. The 7'etractor 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 



