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 effective 

 way of closing their shell by means of a hard 

 plate attached to the hind end of the muscular 

 "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 immediately withdrawn, and the animal is safe 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, because 

 it clings so firmly to the rocks by its sucker- 

 like foot that it is very difficult to dislodge. 

 Its shell is so thick that the water within it 



