THE SCHOOL OF THE SHORE 21 
keeps a big bubble of water in the cavity of 
its body, and this serves to mediate between 
the living cells and the dry air. The bivalves, 
like mussels and oysters, keep the two halves 
of the shell firmly closed, and imprison enough 
of sea-water to keep the delicate gills and skin 
moist for many hours. While the mussels are 
uncovered at low tide the shells are never 
opened, and the animal remains quiet, not 
feeding, scarcely breathing, simply waiting 
until the sea returns. Periwinkles, buckies, 
and many other sea-snails have a very effec- 
tive way of closing their shell by means of a 
hard plate attached to the hind end of the mus- 
cular “foot.” When the animal withdraws 
into its shell, this lid (operculum) closes the 
mouth of the shell and fits very neatly. If we 
watch a periwinkle walking about on the floor 
of a pool we see that it glides along on its 
“foot,” and that the head with its horns is 
also protruded. But if we pick it up the head 
and the foot are immediately withdrawn, and 
the animal is safe behind its closed door. 
The limpet needs no door to its shell, be- 
cause it clings so firmly to the rocks by its 
sucker-like foot that it is very difficult to dis- 
lodge. Itsshell isso thick that the water within 
