SPECIAL PROBLEMS OF SHORE LIFE 73 



others are as evidently ill-adapted. One has only to com- 

 pare the compressed and hatchet-like form of Psam?tiobia 

 tellinella with that of the comparatively globular Cardium. 

 In a form like the latter, however, the ridges and other 

 outgrowths, when present, serve to give the shell firm 

 anchorage. In particular the long spines of Cardium acule- 

 atum are of service in increasing its holding powers in the 

 fine sand. The excrescences on the shell of C. tuberculatum 

 are of similar value while C. echinatum owes its safety to its 

 powers of burrowing and to its spines which, recurved in the 

 direction of its tubes, offer the minimum of resistance to 

 progress, but the maximum of resistance to dislodgment. 

 The long wing-like processes of Aporrhais pes-pelicani are, 

 according to Hunt {he. eit.), " self-acting pieces of mechanism 

 that will in the majority of cases ensure the mollusc being 

 ultimately left in its normal posture should it encounter 

 wave-currents sufficiently strong to upset it." An individual 

 of this species when lying on its back will rock freely back- 

 wards and forwards along a line regarded as joining the end 

 of the middle wing-like process and one of the ridges on 

 the body of the shell, so that a very moderate alternate 

 current will cause the shell to resume its normal position. 



The snails Natica catena, Buccinum undatum, Nassa 

 reticulata, Bulla hydates, Philine aperta have no special means 

 of withstanding wave currents on the surface of the sea-floor, 

 but there is no need of them as they all burrow freely. 



The method by which starfishes deal with wave currents 

 is interesting. The common Asterias lies rigid on the bottom 

 with extended arms, its flattened form offering the minimum 

 of resistance to the current. Added to this it has the power 

 of sinking vertically into the sand. The swimming crab 

 {Portunus) can also escape beneath the surface, but in this 

 case the sand is shovelled away actively by the swimming 

 feet. 



The habits of the sand-eel (Ammodytes) are well known 

 to fishermen, to whom it affords valuable bait. Frequenting 

 sandy shores between tide-marks, it escapes wave action by 

 burrowing rapidly beneath the surface. 



