100 THE BIOLOGY OF THE SEA-SHORE 



Mimicry. — The term " mimicry " is applied to those 

 cases in which forms themselves quite innocuous bear a 

 marked resemblance to other forms in the same habitat 

 which have some particularly efficient means of protection, 

 and, consequently acquiring a false reputation, are able to live 

 undisturbed. Cases of so-called " mimicry " are frequently 

 recorded from the shore which should really be placed under 

 the heading of convergent resemblance or under that of 

 protective coloration. Only a very few examples of the 

 phenomenon of " mimicry " (in the strict sense of the term) 

 have been recorded from the shore. Mortensen {op. cit.) 

 mentions the little crab Zebrida adamsii from the Pacific 

 which devours the spines, tube feet and pedicellariae of 

 the sea-urchins Salmacis hicolor and Toxopneustes pileolus, 

 generally following one area from the top downwards. The 

 crab is conspicuously coloured, with brown and white 

 longitudinal stripes, and has curious projections on the legs 

 and body resembling the close short spines of Toxopneustes, 

 the colour of which (brown and white) corresponds to that 

 of the crab. The close resemblance of crab and sea-urchin 

 probably renders the former immune from the attacks of 

 fish, and may therefore be a genuine case of mimicry. 



Protective Armour. — Many shore animals have become 

 specialised for defence. Apart from other considerations, 

 the necessity, in many cases, for retaining a firm hold on 

 the substratum in order to resist dislodgment by wave- 

 shock makes for sluggish progression, and would render 

 the existence of such forms very precarious if it were not 

 for the presence of some special form of protection. We 

 may distinguish three chief means of defence. 



I. Skeletal Structures forming an Integral Part of the 

 Animal. — The armour in this case consists of limy or flinty 

 spicules which are embedded in the tissues. Under this 

 heading may also be included the hard external skeleton of 

 Crustacea and the shells of molluscs which, though com- 

 pletely external to the animal, yet form an integral part of 

 its structure. In addition to their protective function such 

 structures obviously serve to support the body, and to give 



