The surface population is also exposed to the dangers of wave impact and dislodgment, and shells are again the chief form of protection. The heavy wear sometimes visible on the shells of shore molluscs indicates the severity of the abrasion to which they are subjected. To resist dislodgment, some of the surface forms have great powers of adhesion. The large algae are anchored to the rock by strong holdfasts. Barnacles and serpulid worms have shells firmly cemented to the rock. The common mussel, Mytilus edulis, attaches itself to rocks and stones by strong byssus threads. The remarkable adhesion of the foot of Patella has given rise to the expression 'sticking like a limpet'. Four different genera of fish found on British coasts, Gobius, Liparis, Cyclopterus and Lepadogaster, have the pelvic fins specialized to form a ventral sucker by which they can cling to a firm surface.
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