114 



BOYS AND GIRLS IN BIOLOGY. 



and so it is called an exo-skeleton. You, too, have a 

 skeleton, or framework, but yours lies inside your body, 

 with the soft parts outside, and so yours is called an 

 endo-skeleton, or inside skeleton. In describing the 

 animal himself, I pointed out the straight dorsal and 

 the curved ventral edges, also the large anterior and 

 the pointed or smaller posterior ends. Along the 

 lower ventral edge of each valve, or half of the shell, 

 you will notice a rim that is softer than the rest of the 

 shell. When we study how the shell is formed, we may 

 find out why this part is soft. On the upper, or dorsal, 



Fig. 107. 

 Umbo, or Knob. 



Lines of Growth. 



edge of each half-shell, or valve, is a clear, shining lit- 

 tle knob called the umbo (Fig. 107), because it stands 

 out like the boss, or central part, of a shield. Behind 



