vin PRACTICAL DIRECTIONS 413 



valves of the shell. Note the lines of growth, and the umbo near the 

 hinge-line. The anterior end is rounded, the posterior end more 

 pointed. In dead specimens the valves gape, owing to the elasticity of 

 the ligament and to the relaxation of the adductor muscles (see below), 

 and they can then be seen to be lined by a fold of the integument, the 

 mantle ot pallium. By wedging the valves open still further, the anterior 

 and posterior adductor muscles are seen connecting the two valves ; also 

 the/00/, visceral mass, and^'//j, between the two mantle folds. 



B. Dissection. 



I. Carefully insert a scalpel between the left valve and mantle-edge, 

 and separate them all round the margin of the valve. Then cut through 

 the anterior and posterior adductor muscles close to the shell, and 

 remove the left valve, cutting through the ligament. 



1. Compare the positions of the cut muscles on the body with the 

 muscular impressions on the shell : a. the large anterior adductor near 

 the dorsal and anterior end ; and close behind it b. the protractor and 

 c. the anterior retractor, the latter dorsal to the former ; d. the large 

 posterior adductor near the dorsal and posterior end, on the upper and 

 anterior margin of which is e. the posterior retractor. Note also the 

 thickened ventral edge of the mantle, the corresponding pallial line 

 on the valve, as well as the lines from the muscular impressions con- 

 verging towards the umbo, the smooth longitudinal hinge (with hinge- 

 teeth in Unio), the ligament, and the iridescent inner layer of the shell. 

 Sketch the inner surface of the valve. 



2. Break the shell across, and examine the edge with a lens, noting 

 the three layers of which it is composed : a. the thin outer uncalcified 

 periostracum ; b. the thicker middle prismatic layer ; and c. the inner 

 pearly or nacreotis layer. 



Pin down the animal firmly under water, leaving it in the right valve : 

 insert the pins through the ligament and muscular part of the foot, and 

 obliquely against the edges of the shell. Note : 



3. The left mantle-lobe uniting with the right above the middle of the 

 anterior adductor muscle anteriorly, and behind the posterior adductor 

 posteriorly. Just behind the latter muscle the margins of the mantle 

 are much thickened ; and in life, the two approximated mantle-edges 

 here separate to form the exhalaut and inhalant apertures, the latter 

 of which is provided with short tentacles. 



4. Turn back the left mantle-lobe and note the mantle-cavity and its 

 contents (Fig. 101) : a. the foot and visceral mass ; b. the left pair of 



