VENUS MERCENARIA LINN/EUS 217 



of the retractor of the siphon, and the serration of the valve 

 edge. 



4. Break the shell and examine the broken edge through a lens. 

 Make a drawing showing the outer layer or periostracum, the 

 median or prismatic layer, and the inner or mother-of-pearl layer. 



5. Cut off a piece of the left mantle fold near the anterior 

 adductor, examine it under the dissecting microscope and make 

 a drawing showing the fringe of papillae. 



6. Cut off a larger piece of the mantle in the region of the foot. 

 Place it on a slide and examine with the naked eye. Make a 

 drawing showing the free edge of the mantle and the portion 

 normally adhering to the shell. In the former label fringe, 

 thickened portion and muscular portion. 



7. Remove the entire left mantle fold by cutting it with 

 scissors around the retractor siphons, adductors and along its 

 dorsal attachment. This exposes that portion of the mantle 

 cavity which is called the branchial chamber. Make a full size 

 drawing showing the outline of the right valve, edge of right 

 mantle fold, anterior adductor, anterior retractor of the foot, 

 posterior adductor, posterior retractor of the foot, the keel- 

 shaped foot forming a continuation of the visceral complex, the 

 two triangular labial palps just above the anterior adductor; 

 the two left ctenidia or gills partly covering the visceral complex; 

 the transparent upper branchial chamber limited dorsally by a 

 white line the attachment place of the gills to the body; above 

 this line in front, dark organs visible through the body wall 

 (dark brown liver, yellow gonad, dark Keber's organ); the 

 large pericard and behind it the dark kidney. 



8. Take the right valve with the animal in it into your left 

 hand and hold it so as to obtain a front view of the siphons (left 

 siphonal retractor on your left). Notice the pigmented siphons. 

 The upper one is the cloacal or excurrent siphon. Make an 

 incision in its wall and expose the anus. Make a full size draw- 

 ing showing left retractor of the siphon, left and right mantle 

 edge, anus, cloacal siphon, branchial siphon. 



