42 THE SEAS 



surface of the shell of sea urchins are little clawed spines, 

 of various patterns, but usually with three teeth which 

 can open and snap together. These are used to clean the 

 shell, removing any fragment of waste matter, but also for 

 protection, for if an enemy attacks an urchin the long spines 

 on that side turn away exposing the smaller clawed spines 

 beneath, which snap at any part of the enemy touching 

 them, and also produce a poison which passes into the 

 wound. 



INIore universal are methods of passive defence. The 

 thick shell of the crustaceans and of the univalve and 

 bivalve molluscs are examples. The last-named are 

 frequently safe so long as their shell can be firmly closed and 

 the two muscles concerned with this — the adductor muscles 

 of the shell — are extremely powerful, though very variably 

 so in different beascs, those of the cockle for example having 

 less than a quarter the power of those of Venus. How the 

 bivalves may be successfully attacked and eaten by whelks 

 and starfish we shall see in Chapter IX. The hermit crabs 

 have found an excellent means of protection by using 

 empty mollusc shells. Then there are the various devices 

 whereby an animal escapes attention by toning with the 

 background. This may be done by having a colour similar 

 to the surroundings — one particular set of surroundings 

 or any surroundings, the animal in the latter case changing 

 its colour to tone with the background. This is particularly 

 common in small crustaceans and flat-fish on the shore. 

 The vivid colours of many sea slugs, which are unable, 

 however, to change colour, usually tone with those of the 

 rouks or weed on which they live. Many spider crabs 

 " mask " themselves by deliberately decking their shells 

 with pieces of weed or sponge which continue to grow 

 there. The dahlia anemone and the smaller sea urchin 

 bach cover themselves with stones and pieces of shell. 



