56 FIRST LESSONS IN ZOOLOGY. 



that in outline they correspond to the concentric lines of 

 growth near the beaks. 



We also notice in shells which are not too much beach- 

 worn that the outside is covered with a thin skin or scurf, 

 which easily peels off when dry ; this is called the " epi- 

 dermis." 



Now, turning the valves over, we see that the inside is 

 smoother and more shiny than the outside ; it is lined with 

 a thin layer of pearl, while the outside is dull chalk- 

 white. 



We also see near the upper or hinge side of the shell, 

 near each end, two rounded spots which shine or glisten 

 more than the others ; these are called the "muscular im- 

 pressions," because the two great thick muscles which open 

 and close the shell are attached to the valves at these 

 points. The muscular impressions are connected by a nar- 

 row, rather irregular line, which sends a great sharp loop 

 along and near the lower edge to a point opposite the front 

 muscular impression; this is the "pallial line." 



Having learned something about the clam's shell, we will 

 now look at the clam itself, for the shell is to the clam what 

 scales are to the fish or our finger-nails are to our bodies ; 

 they are dead parts, produced by certain organs of the body 

 to protect it from harm, and are not essential, vital, living 

 parts. If the shell is broken, the animal repairs it by se- 

 creting* from the surface of its mantle limy matter to 

 mend the hole or crack. 



We will now look at the clam. To do this we should re- 

 move one valve, and then immerse the clam in a dish or 

 small tin-pan of water. The clam should be carefully 

 opened so as not to injure the animal or to hurt it, though 

 the sense of pain, slight as it is, is only felt in the black 

 " head," or siphon, as it is called. If a knife be thrust into 

 the opening of the shell at each end and close to one valve, 

 so as to sever the two "adductor" muscles, one shell can be 



*i.e., separating from the Wood sent to the mantle. 



