IX GROWTH OF THE SHELL 259 



witli great rapidity, ami soon fills up the hole with solid matter. 

 For two consecutive luonths an animal, deprived of food, has been 

 known to reproduce this membrane daily after its removal every 

 morning.^ Prof. Schiedt has found that oysters, if deprived of 

 the right A'alve and exposed to the light, not only develop brown 

 pigment over the whole exposed surface of mantle and branchiae, 

 but actually succeed in part in reproducing the valve and hinge.^ 

 Deposit of Additionnl Layers of Shell. — Mollusca possess the 

 power of thickening the interior of the shell, l)y the deposit of 

 successive layers. This is frequently done in self-defence 

 against the attacks of Ijoring Mollusca, sponges, and w^orms. 

 Cases may often be noticed of Ostrea, Spondyhis,. and other 

 sedentary molluscs, which, unable to escape the gradual assaults 

 of their foes, have provided against them by the deposit of fresh 

 shelly matter. A somewhat similar plan is adopted to provide 

 against intrusion by way of the aperture. 

 Pearls are, in many cases, the result of 

 shell deposition upon the eggs or even the 

 body of some intrusive parasite (Bistoma, 

 Filar ia, etc.), and are, in some countries, 

 artificially produced by the introduction of 

 fragments of sand, metal, etc., into living 

 Uiiio and Anodonta. Little joss images '^^^•Jf^—^ srecimen of 



^ ® Helix rosacea Mull., 



are made in India and China, the nacre Cape of Good Hope, into 

 on which is produced hy thrusting them t^/jr\:l^^. 

 inside living Unionidae. come introduced. The 



Aty TT 7 ■ "4^1 animal has protected 



specimen ot Behx rosacea, m the .^^^j^ ,^^, ^^^^^.^g ^j^^ 



British Museum, into whose shell a piece gi-ass with a shelly layer. 



p , , • ; 1 1 1 ( From a specimen in the 



01 grass somehow became introduced, has British ]\iuseum. ) 

 partitioned it off by the formation of a 



sort of shelly tunnel extending throughout its entire length 

 (Fig. 167). 



Absorption of Internal Portions. — Certain genera have the 

 remarkaljle property of absorbing, when they become adult, the 

 internal portions of the whorls and the greater part of the 

 columellar axis. The effect of this is to make the shell, when 

 the process is complete, no longer a spiral but a more or less 

 produced cone, and it is found that in such cases the viscera of 



^ I\r. (le Yillepoix, Comptes Rendvs, cxiii. p. 317. 

 " Proc. Ac. Sat. Sc. FhiL 1892, ]). 350. 



