2oo THE SCIENCE OF LIFE. 



the symmetry of the valves, are the chief characters for 

 distinguishing genera and species of this class, which 



FIG. Q9. Anatomy of a bivalve Mollusk, (Mactra.) . Shell-muscles, b. Ganglia. 

 c. Heart, d. Liver, e. Mouth, f. Labial tentacles, g. Foot. h. Stomach. /.Intestine. 

 k. Anus. m. Mantle, n. Branchiae, o. Base of inhalent siphon. /. Base of exhalent 

 siphon. 



has been divided into groups, based on the possession 

 or non-possession of siphons, as follows : 



Section A. Asiphonidce. Without respiratory siphons, 

 so that the shell shows the pallial line simple, and not 

 indented. As in the families of Oysters, (Ostreid&l) 



