THE EUROPEAN SEAS. 



95 



limpets {Patella vulgata). Where the shore shelves 

 a little, and rocky ledges decline gradually into the 

 sea, numerous creatures are found living in this 

 sub-region. In such a locality the common mussel 

 delights to live, moored by its byssal cable in the 

 crevices of rocks or, still more numerously, often 

 in great companies, anchored among masses of gra- 

 vel, the pebbles of which are tied together by its 

 silky filaments. The rocksides and the floors of 

 transparent pools are here often thickly coated with 

 a hard pale red crust. This is a nullipore, in re- 

 ality a seaweed, though putting on the aspect of a 

 coral. Not very long ago it was regarded even by 

 naturalists as a zoophyte, and is fairly believed to 

 be a coral even at present, by fishermen who draw 

 up branching varieties of it in their nets, from 

 depths that are never uncovered by the tide. The 

 region of half-tide forms a third subdivision of the 

 littoral zone, one exceedingly prolific in marine ani- 

 mals and plants. Here we find Fucus articulatus, 

 with its graceful even-edged rich brown fronds 

 growing in profusion, mingled occasionally with 

 the less elegant Fucus nodosus. Here limpets 

 throng, and dog-periwinkles (Purpura lapillus), 

 crawl observantly, seeking to bore more passive 

 mollusks, and extract their juicy substance. This 

 is the home of the best of periwinkles, the large 

 black Littorina littorea, gathered in thousands for 

 the London market. On our western coasts we find 

 it in company with the purple-striped top shell 



