64 THE WONDER OF LIFE 



probably in part physiological — though as yet beyond 

 statement — for instance that the deposition of conchin and 

 carbonate of hme by the skin or mantle may be an organized 

 way of dealing with the waste products of the animal's 

 body, and perhaps also with by-products of digestion. 



This must remain vague in the meantime, and therefore we 

 turn with pleasure to a secondary or oecological explanation 

 which has been suggested by Mr. Cyril Crossland — that the 

 thickness of the shell and the outgrowths on it must be 

 credited with protective value. The shell- eating fish 

 Balistes prefers the bivalves with weaker shells. Another 

 enemy, the boring gastropod Murex, kiUs more of those 

 with smoother shells. It kills large numbers of Margariti- 

 fera mauritii, which has small and weak outgrowths on its 

 shell ; it kills few of another species, Margaritifera mar- 

 garitifera, which has large strong processes remaining for 

 at least six years. It seems that the strong processes on 

 the surface of the shell prevent the Murex from readily 

 getting a firm hold with its foot, and without this it cannot 

 work the drill in its mouth that it uses to bore through 

 the bivalve's defences. In some species of bivalve the 

 outgrowths of the shell are larger in the young forms, and 

 they are of the greater value therefore during the relatively 

 more active period when the young pearl oyster, or hammer- 

 shell (Avicula), or Tridacna, is crawling about and seeking 

 a suitable place for setthng down on. Mr. Crossland' s 

 suggestion may require modification, but he backs it up 

 with definite facts showing the actual hfe-saving value 

 not of the armour merely, but also of the decorations which 

 it bears. 



When the Murex gets a good grip on a relatively smooth 

 shell it drills a hole through, and allows some paralysing 



