114 THE BIOLOGY OF THE SEA-SHORE 



which may assist an animal to escape its pursuers, or may 

 less frequently be aggressive in character, enabling a car- 

 nivorous form to stalk its prey unseen. 



Some form of protective armour (spicules, plates, spines) 

 is of particularly frequent occurrence on the shore, since the 

 necessity for retaining a firm hold on the substratum makes 

 for sluggish movement and renders many shore forms very 

 liable to attack. Shells and tubes have the double advantage 

 of protecting shore animals both from enemies and from 

 desiccation at low tide. Offensive weapons may act purely 

 mechanically as, for example, the chelae of crabs and lobsters, 

 or both mechanically and chemically, as the nematocysts of 

 Coelentera. 



Very interesting is the suggestion that certain features 

 of structure and behaviour have their explanation in the 

 reciprocal relation of attack and defence between inveterate 

 enemies. 



