IX GROWTH OF THE SHELL 



259 



great rapidity, and soon fills up the hole with solid matter. For 

 two consecutive months 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 valve 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 Additional Layers of Shell. — MoUusca possess 

 the power of thickening the interior of the shell, by the deposit 

 of successive layers. This is frequently done in self-defence 

 against the attacks of boring Mollusca, sponges, and worms. 

 Cases may often be noticed of Ostrea, Spondylus^ 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 (^Distorna, 

 Filajna, etc.), and are, in some countries, 

 artificially produced by the introduction of 

 fragments of sand, metal, etc., into living 

 Unio and Anodonta. Little joss images Fig. 167.— A specimen of 

 are nmde in India and China, the nacre ^^^Z^^^ 

 on which is produced by thrusting them which a piece of grass 



• I T • ^ XI ' -j^^ " lias by some means be- 



msidelivnig Unionidae.^ ^ come introduced. The 



A specimen of Helix rosacea^ in the animal has protected 

 British Museum, into whose shell a piece gra1s^viL^s^en"4yer! 



of grass somehow became introduced, has (From a specimen in the 



, • , • ^ •!_ nc ^ l^ £ j.' £ British Museum.) 



partitioned it on by the lormation 01 a 



sort of shelly tunnel extending throughout its entire length 



(Fig. 167). 



Absorption of Internal Portions. — Certain genera have 

 the remarkable 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 



1 M. de Villepoix, Comptes Bendns, cxiii. p. 317. 



2 Proc. Ac. Nat. Sc. Phil., 1892, p. 350. 



