32 THE SEAS 



marine snails, of which the common limpet (Patella) and 

 the periwinkles (Littorina) (Plate 10) and the top-shells 

 (Gibbula and Calliostoma) are the commonest ; they browse 

 upon the weeds and corallines which cover the rocks. The 

 limpets (Plate 8) have definite " homes " on the rock face 

 to the surface of which their shells are exactly fitted so that, 

 when disturbed, they can, by pulling down the shell, fix 

 themselves so firmly that very great force is needed to 

 disturb them ; it has been estimated that limpets with a 

 basal area about one square inch need a pull of seventy 

 pounds to remove them. In all directions around can 

 occasionally be seen radiating paths cleared of weed showing 

 where the limpet has foraged for its food. There are a 

 variety of periwinkles which live in different regions of the 

 shore, one (L. rudis) always living very high on the shore, 

 often where it is untouched by sea water for weeks together, 

 the common winkle (L. littorea) lives lower on the shore 

 always on rocks, while the smaller and less pointed species 

 (L. obtusata) which varies greatly in colour from black to 

 white, is always found on the fronds of fucus. Resembling 

 the winkles but rather larger and with a thicker shell is the 

 dog- whelk (Purpura), which inhabits the upper half of 

 the shore (Plate 13), preying upon the limpets or smaller 

 top-shells. Other peculiar animals allied to the snails are 

 the chitons of which our largest species are about an inch 

 long and about half as wide ; they are flattened, with the 

 shell arranged in eight parts each slightly overlapping the 

 one behind, and when pulled away from the rocks on which 

 they browse, they curl up like little armadillos. There is a 

 numerous population growing on the weeds, notably many 

 kinds of hydroids, sea mats and small snails. 



Nearer low-water mark we find the rocks covered with 

 sponges of which the " crumb-of -bread sponge " [Halt- 

 chondria) is the commonest (Plate 11), forming a dense mass 



