58 Practical Zoology. 



\ dark-colored membrane bordering the edge of the shell. This is 

 the periostracum, an extension of the outer covering of the shell. 

 Serape off some of it to see its relation to the limy shell. Care- 

 fully study the relations of the periostracum to the mantle. Pinch 

 the edge of the right mantle lobe, and observe the effect on this 

 free border. Trace the right and left mantle lobes to their points 

 of union before and behind. 



4. Examine the thick, dark-colored, hinder edge of the mantle 

 lobes, and see how by their manner of meeting they form the two 

 short tubes, the siphons. Prove the great sensitiveness of the 

 margins of these siphon tubes. Are the margins of the two 

 openings alike ? 



5. Examine the ends of the anterior and posterior adductor 

 muscles where they were cut off in opening the shell ; scrape away 

 any part of these muscles that may remain attached to the left 

 valve, and note the marks or muscle scars which are shown. 



6. Turn the mantle lobe back as far as it will go, and observe 

 the soft central body ; its tough lower border is the foot ; prick it 

 with the dissecting needle, and observe what follows. 



7. Along each side of the body and extending back of it are 

 two thin membranes, the gills, showing vertical parallel markings. 

 Study closely the relations of the gills to each other, to the body, 

 and to the mantle. 



8. With a knife scrape off a little of the surface of the gill and 

 examine under the microscope to see the vibratory motion of the 

 hairlike projections, or cilia, borne on the cells thus obtained. 



9. In front of the gills, on each side of the body, are two thin 

 triangular flaps, much smaller than the gills, the labial palps. 



10. Raise the hind border of the left mantle lobe, and observe 

 that the gill next to the body unites with the corresponding gill of 

 the other side, thus forming a separate channel above the gills, 

 from which the upper siphon leads, while the lower siphon leads to 

 the lower cavity, outside of and below the gills. 



1 1 . With the thumb and all the fingers of the left hand seize 

 the left lobe of the mantle and pull it toward the ventral margin, 



