274 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



shell by separating the mantle from it along the pallial 

 line, with the handle of a scalpel, and then cutting the 

 two adductor muscles and the foot retractors. As soon as 

 these muscles are cut, notice that the elasticity of the 

 hinge ligament throws the ventral margins of the shells 

 apart. In the animal thus exposed, notice : — 



a. The semi-transparent rnantle (Fig. 142, a), which 

 lines the inner surface of the shell. 



b. The thickened muscular marofin. 



c. The band of small mantle muscles which were at- 

 tached to the shell along the pallial line. 



d. The adductor and retractor muscles. 



e. Notice that the two lobes of the mantle are united 

 along the region of the hinge. Above the anterior ad- 

 ductor they separate, and they are free from each other 

 along their anterior, ventral, and posterior margins as far 

 as the branchial siphon, which is simply an enlargement 

 of the narrow space between them. Above this opening 

 they unite to form the lower edge of the cloacal siphon, 

 and they are then separated as far as the posterior end of 

 the hinge ligament. 



/. On the dorsal surface of the body, a little anterior to 

 the posterior adductor, notice a region where' the mantle 

 is quite thin and transparent — the pericardial chamber. 

 Watch this chamber carefully, and notice through its wall 

 the pulsating heart. 



g. On the middle line of the upper posterior face of the 

 posterior adductor muscle, notice the small rectum (Figs. 

 143 and 144, m) , which opens by the aims into the cloacal 

 siphon. 



h. Raise up the loose portion of the mantle, and notice 

 the large branchial chamber into which the branchial siphon 

 ^pens. In this chamber notice : — 



