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 and the foot are immediately withdrawn, and 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. Its shell is so thick that the water within 



