142 INTRODUCTION TO ZOOLOGY CHAP. 



and the movements of the foot which carry the body forward. 1 

 Faint tracks can be seen diverging from the umbo to the 

 present position of these muscles, marking the paths along 

 which their attachments have shifted as the animal grew. 



Running parallel to the edge of the shell, and starting from 

 the outer side of these special muscle scars, is another long 

 thin scar where the muscles of the mantle were attached to 

 the shell (Fig. 90, pm). 



The pearly lining to the shell is deposited by the mantle 

 in thin overlapping films, causing the surface to be very 

 delicately ridged, and thus producing, by the play of light 

 on it, the " interference colours " which give the beautiful 

 iridescence characteristic of mother-of-pearl. This mother- 

 of-pearl layer is much more developed in some of the marine 

 mussels and in oysters. 



Formation If any foreign object, such as a grain of sand, 

 of Pearls, gets within the shell between it and the mantle, 

 the irritation of its presence causes a special secretion 

 of nacreous substance round the object, either merely cover- 

 ing it and cementing it to the shell, or in some genera 

 though not in Anodon forming complete concentric layers 

 round it, and so producing a pearl, or " shell-berry " as it used 

 to be called. 



In some instances it has been shown 2 that the pearl 

 formation in marine mussels is due to the presence in the 

 mantle tissues of a little parasitic worm, which whilst in its 

 resting stage is enclosed by the mussel in a pearly prison, 

 arid so is usually destroyed. Even if the worm manages to 

 make its escape, the formation of the pearl, having been begun, 

 is completed. It is probable that this is the cause of the 

 formation of the most perfect pearls a curious case of the 

 production of beauty resulting from a pathological stimulus ; 

 a more prosaic explanation, perhaps, but not less wonderful 

 and interesting, than the belief held by the ancients and 

 quoted by Pliny, that pearls were drops of rain which fell 

 into the shells when they were opened by the animal and 

 were then transformed into pearls ! 



1 See discussion on locomotion in Latter's Natural History, 1904, pp. 166 

 and 170-173. 



2 Dr. Lyster Jameson in Proc. Zool. Soc. vol. i. Lond. 1902. Professor 

 Herdmann, Nature, vol. Ixvii., April 30, 1903. 



