462 DEPTHS OF THE OCEAN chap. 



the water in more rapid motion, and so bring nourishment to 



their mouths inside their shells, but when exposed at ebb-tide the 



shells are closed and 

 the animals remain 

 concealed within 

 them. Immediately 

 below the barnacle- 

 belt we frequently 

 find a belt consist- 

 ing of dense masses 

 of mussels {Mytilus 

 edit lis, see Fig. 3 1 6), 

 though the individ- 

 uals in such locali- 

 ties never attain 

 any considerable 

 size. On the rocks 

 we find everywhere 



four species of gasteropods, which are very characteristic of this 



area, namely, the limpet {Patella vulgafa, 



see Fig. 317), two periwinkles {Littorina 



Fig. 316. 



Mytilus edulis, L. 



Fig. 317. 

 Patella vulgata, L. a. From the side ; 



from beneath. 



Fig. 318. 

 Littorina littorea, L. 



littorea, see Fig. 318, and L. rudis), and the purple snail 

 (Purpura lapillus, see Fig. 319), this last being 

 often plentiful in the barnacle -belt, where it 

 feeds on these crustaceans. These forms live 

 chiefly on the naked rock, but, except the limpets, 

 also often on the algae in the tidal area. But 

 when the belt of fucoids is exposed at ebb- 

 tide, especially in sheltered places where a 

 good current runs, we see that the algae, the 

 species of Fucus in particular, have their special 

 Fig. 319. fauna, consisting chiefly of attached forms. 



Purpura iapiiius,\.. -j^j^^ majority of them are hydroids, the com- 

 monest species being Dynamena puniila (see Fig. 320), 



