144 MOLLUSKS. 



ency of its inner part, and the sensitiveness and mobility 

 of its border. 



Observe that it consists of two lobes corresponding in 

 size to the two valves of the shell, and that its border is 

 continuous from one to the other. 



Observe how loosely it infolds the body, and covers it 

 above and on the sides. 



2. The siphons. Observe that the siphon orifices are 

 formed from the edge of the mantle, and that the adjacent 

 mantle margins are grown together between the upper and 

 lower orifices, entirely separating them, and that below 

 the lower orifice the mantle margins are free. The upper 

 tube leads out from the cloacal chamber, and is therefore 

 called the cloacal siphon. The lower opening leads into 

 the branchial (or gill) chamber, and is therefore called the 

 branchial siphon. 



3. The principal muscles. Observe the severed ends 

 of the two stout, white adductor muscles projecting ver- 

 tically through the mantle. 



Close beside each of these find the end of a much 

 smaller, oblique muscle, which was also severed in open- 

 ing the shell. These two are the retractor muscles. They 

 retract the foot. Prick the point of the foot, and watch 

 the posterior adductor muscle. Prick the back part of 

 the foot, and watch the anterior adductor muscle. If 

 these were fast to the shell, instead of being drawn down, 

 they would draw the foot up, as may be seen later by 

 pricking the retractors of the other side, near their origin, 

 before severing them from the valve. 



In the thickened border of the mantle are many muscle 

 fibers, to which its mobility is due, and by which it was 

 attached to the shell along the pallial line. 



4. The gills. Turn the loose part of the mantle back 

 upon the dorsal surface, and expose the gills. They hang 

 suspended in the branchial chamber at the posterior part 



