﻿THE 
  STORY 
  OF 
  A 
  PEARL. 
  45 
  

  

  viz. 
  their 
  origin 
  from 
  the 
  cells 
  of 
  the 
  mantle, 
  the 
  soft 
  outer 
  layer 
  

   or 
  envelopment 
  of 
  the 
  shell-fish, 
  and 
  the 
  structure 
  which 
  secretes 
  

   the 
  shell 
  itself. 
  The 
  latter 
  is 
  composed 
  of 
  three 
  layers 
  — 
  an 
  

   outer 
  chitinous 
  or 
  horny 
  layer 
  (periostracum), 
  a 
  calcareous 
  

   middle 
  or 
  prismatic 
  layer, 
  and 
  a 
  calcareous 
  inner 
  or 
  nacreous 
  

   layer. 
  The 
  middle 
  layer 
  is 
  formed 
  of 
  prisms 
  in 
  vertical 
  section, 
  

   and 
  in 
  horizontal 
  section 
  the 
  areas 
  are 
  hexagonal 
  or 
  polygonal. 
  

   It 
  is 
  chiefly 
  composed 
  of 
  calcium 
  carbonate 
  and 
  calcium 
  phos- 
  

   phate. 
  The 
  inner 
  or 
  nacreous 
  layer 
  is 
  similar 
  in 
  chemical 
  

   composition, 
  and 
  consists 
  of 
  thin 
  layers 
  or 
  plates 
  of 
  the 
  secretion, 
  

   which, 
  as 
  the 
  illustrious 
  Sir 
  David 
  Brewster 
  showed, 
  are 
  so 
  

   arranged 
  as 
  to 
  cause 
  interference 
  with 
  the 
  rays 
  of 
  light, 
  and 
  

   thus 
  give 
  rise 
  to 
  the 
  iridescent 
  pearly 
  lustre 
  of 
  the 
  surface. 
  

   Eecent 
  observers, 
  such 
  as 
  Biedermann, 
  insist 
  that 
  the 
  calcareous 
  

   substance 
  of 
  the 
  shell 
  can 
  only 
  be 
  deposited 
  on 
  the 
  organic 
  

   basis 
  (conchyolin), 
  which 
  will 
  be 
  alluded 
  to 
  subsequently, 
  and 
  

   the 
  same 
  holds 
  with 
  regard 
  to 
  pearls. 
  Indeed, 
  some, 
  such 
  as 
  

   Mobius, 
  consider 
  that 
  a 
  pearl 
  represents 
  the 
  shell 
  with 
  its 
  coats 
  

   reversed 
  — 
  having 
  in 
  the 
  centre 
  the 
  nucleus, 
  then 
  the 
  perio- 
  

   stracum 
  (horny 
  layer) 
  — 
  outside 
  which 
  is 
  the 
  layer 
  of 
  hexagonal 
  

   prisms, 
  whilst 
  externally 
  is 
  the 
  nacreous 
  layer. 
  Such 
  a 
  view, 
  

   however, 
  is 
  not, 
  as 
  a 
  rule, 
  in 
  accordance 
  with 
  the 
  structure. 
  

  

  The 
  most 
  recent 
  observer, 
  Dr. 
  Lyster 
  Jameson, 
  following 
  

   De 
  Filippi, 
  Garner, 
  and 
  others, 
  holds 
  that 
  pearls 
  are 
  due 
  to 
  the 
  

   presence 
  of 
  a 
  minute 
  parasite, 
  which, 
  in 
  the 
  case 
  of 
  the 
  Common 
  

   Mussel 
  — 
  a 
  species 
  he 
  has 
  worked 
  out 
  most 
  minutely 
  — 
  is 
  a 
  fluke 
  

   or 
  Distomid 
  of 
  the 
  subgenus 
  Leucithodendriimi 
  (Loos), 
  and 
  very 
  

   closely 
  resembling 
  L. 
  somaterice 
  (Levinsen), 
  which 
  occurs 
  in 
  the 
  

   mature 
  condition 
  in 
  the 
  intestine 
  of 
  the 
  Eider-Duck. 
  Like 
  many 
  

   other 
  flukes 
  — 
  for 
  example, 
  that 
  which 
  causes 
  the 
  well-known 
  

   liver-rot 
  in 
  Sheep 
  — 
  the 
  life-history 
  of 
  the 
  Distomid 
  of 
  the 
  Mussel 
  

   is 
  complex. 
  So 
  far 
  as 
  is 
  known 
  at 
  present, 
  it 
  occurs 
  in 
  its 
  adult 
  

   condition 
  in 
  the 
  intestine 
  of 
  the 
  Eider-Duck 
  and 
  Scoter 
  {(Eclemia 
  

   nigra), 
  both 
  of 
  which 
  feed 
  on 
  Mussels. 
  The 
  eggs 
  from 
  these 
  

   give 
  rise 
  to 
  young 
  forms 
  (larvae, 
  but 
  whether 
  by 
  free- 
  swimming 
  

   Miracidium 
  forms 
  is 
  unknown). 
  Jameson 
  thinks 
  the 
  eggs 
  are 
  

   carried 
  by 
  currents 
  into 
  the 
  mouth 
  of 
  a 
  common 
  shell-fish 
  

   (Tapes 
  decussatus), 
  hatch 
  out 
  in 
  the 
  alimentary 
  canal, 
  enter 
  the 
  

   circulatory 
  system, 
  and 
  reach 
  the 
  course 
  of 
  the 
  posterior 
  pallial 
  

  

  