PAMP-SHELIS AND THEIR ALLIES 17 
If you pick up a dead shell and look inside one of 
the valves you may see that it is marked somewhat 
like Figure 59, except that the letters will not be 
there to guide you. The line p. |. is called the pal- 
lial line, because the pallium, or mantle, or plain 
skin of the animal is attached to the shell along 
this line. If the creature lives deep in the mud 
and has long siphons there will be quite a notch 
at p. s., which stands for pallial sinus. A sinus 
is a gulf, or bay. If the creature simply lives 
on the surface, or is only partly buried in the 
sandy bottom, he will have no need for long pipes, 
and the p. s. will be omitted. 
The two big scars marked a and a show where 
the adductor muscles were grown to the shell. 
These muscles are strong affairs, and enable the 
creature to close his doors with a snap and to 
hold them closed against all intruders. Sometimes 
there is but one adductor, as in the oyster, and 
then it is situated near the center of the shell. But 
when the clam shuts his shell he is obliged to 
stretch a thick, heavy spring, which, when it is 
dry, resembles rubber, though it gets brittle when 
itis dry. This spring is marked ‘‘1”’ in the picture, 
which stands for ligament. In some species the 
ligament is internal instead of external, and in 
such cases it is compressed instead of lengthened, 
when the clam closes his shells. In either case 
the elastic ligament opens the valves as soon as the 
muscles become relaxed, either by the will of the 
clam or by his death, for dead shells are nearly 
always partly open. 
The ligament also serves to keep the two valves 
