398 THE MUSSEL chap 



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

 and they can than 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 

 \he.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 muscles of the foot, 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 

 muscle posteriorly. Just behind the posterior adductor muscle the mar- 

 gins of the mantle are much thickened ; and in life, the two approxim- 

 ated mantle-edges here separate to form the exhalant 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; — a. t\ie foot and visceral mass j b. the left pair, of ^7& 

 at the sides of the visceral mass; and anteriorly i. the left pair of 

 small, triangular, labial palps. (Note that there is no distinct head). 

 These parts are situated between the two mantle-folds in the larg 



