vin PRACTICAL DIRECTIONS 413 



except along the dorsal hinge-line, where it forms an elastic 

 ligament connecting the two lateral 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 t]^ relaxation of the adductor muscles 

 (see below), and they can then be seen to be lined by a fold 

 of the integument, the mantle or pallium. By wedging 

 the valves open still further, the anterior and posterior 

 adductor muscles are seen connecting the two valves ; also 

 the foot, visceral mass, and gills, 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 converging 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 nacreous 

 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 



