80 Beautiful Shells. 
morsel has been extracted be taken in the hand, it 
may be noticed that one is much thinner, smoother, 
and flatter than the other: this is the side most 
exposed to the action of the water; the rougher 
and rounder side is that which is attached to the 
rock or other substance to which the animal forms 
an attachment that is usually life-long. The two 
portions of the shell are joined together by a hinge 
of curious workmanship, which is formed of the 
inner layer of the shell, and strengthened by a 
ligament which is wonderfully elastic; when the 
shell is drawn together by the abductor muscles, the 
ligament is at full stretch, and as soon as they relax 
at all, it contracts, and causes the shell to gape. 
This process 1s repeated as often as may be neces- 
sary for the safety and sustenance of the animal 
within, whose mouth is situated at the narrowest 
part of its habitation—namely, near the joint of the 
hinge which connects its upper and under shell. The 
anatomical structure of the Oyster is more perfect 
than would be supposed, from its apparently low 
state of organization; it has a heart, liver, and in- 
testinal canal, and a bag near the mouth, which 
answers the purpose of a stomach. Its breathing 
organs are gills, closely resembling those of most 
other fish; it has little vessels which convey the 
