272 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



h. In some specimens the large white fleshy foot (Fig. 

 142, h) will be seen to project from between the anterior 

 ventral edges of the shell. 



i. At the posterior end of the body the margins of the 

 mantle are so modified as to form two imperfect tubes 

 which project beyond the shell, the siphon tubes. 



1. One of these, the cloacal siphon (Fig. 142, c?), is on 

 the dorsal surface of the shell, between the ligament and 

 the posterior end. 



2. The other or hranchidl siphon (Fig. 142, br) is at 

 the posterior end of the body, and its edges are set with a 

 fring-e of small sensitive tentacles. 



3. Fill a long slender glass tube with some colored sub- 

 stance, such as finely powdered carmine in water. Place 

 the lower end of the tube near the cloacal siphon, and 

 allow a few drops of the fluid to escape. It is violently 

 driven away from the mouth of the siphon. When the 

 coloring matter is allowed to escape near the lower or 

 branchial siphon it is sucked in, and in a short time some 

 of it is driven out through the upper or cloacal opening ; 

 thus showing that a continuous current of water, the bran- 

 chial current, is passing in at the one and out at the other 

 opening. 



II. Before opening the specimen it is best to study the 

 inner surface of another shell, noticing: — 



a. The large kidney-shaped scars for the attachment of 

 the two adductor muscles. 



b. A line w^hich runs from the lower edge of the scar of 

 the anterior adductor along the ventral edge of the valve 

 to the scar of the posterior adductor. This palUal line is 

 the area of attachment of the edge of the mantle to the 

 margin of the shell. 



c. Above and in front of the scar of the posterior ad- 



I 



