﻿46 
  THE 
  ZOOLOGIST. 
  

  

  artery. 
  There 
  — 
  that 
  is, 
  in 
  the 
  mantle 
  — 
  they 
  form 
  sporocysts. 
  

   The 
  same 
  occurs 
  in 
  the 
  Cockle, 
  only 
  they 
  are 
  found 
  along 
  the 
  

   anterior 
  pallial 
  artery. 
  Within 
  the 
  sporocysts 
  cercarise 
  are 
  de- 
  

   veloped, 
  with 
  a 
  pair 
  of 
  eye-spots, 
  but 
  without 
  the 
  appendage 
  or 
  

   tail 
  so 
  often 
  seen 
  in 
  such 
  forms. 
  These 
  minute 
  parasites 
  escape 
  

   from 
  the 
  sporocysts, 
  and 
  probably 
  by 
  currents 
  or 
  by 
  creeping 
  

   find 
  their 
  way 
  into 
  the 
  Mussels 
  between 
  the 
  mantle 
  and 
  the 
  

   shell. 
  After 
  creeping 
  about 
  on 
  the 
  inner 
  surface 
  of 
  the 
  shell 
  

   (Jameson) 
  they 
  come 
  to 
  rest, 
  assuming 
  a 
  spherical 
  form, 
  and 
  are 
  

   visible 
  to 
  the 
  naked 
  eye 
  as 
  little 
  yellowish 
  spots 
  in 
  the 
  con- 
  

   nective-tissue 
  of 
  the 
  mantle. 
  By 
  and 
  by 
  the 
  tissues 
  of 
  the 
  host 
  

   give 
  rise 
  to 
  an 
  investment 
  of 
  epithelium 
  (simple 
  columnar) 
  f 
  

   which 
  ultimately 
  becomes 
  the 
  sac 
  of 
  the 
  pearl 
  (Jameson). 
  The 
  

   parasite 
  may 
  migrate 
  from 
  its 
  original 
  site, 
  and 
  thus 
  form 
  a 
  

   double 
  pearl 
  or 
  even 
  three. 
  In 
  any 
  case, 
  the 
  presence 
  of 
  this 
  

   parasite 
  acts 
  specifically 
  on 
  the 
  Mussel, 
  and 
  the 
  sac 
  around 
  the 
  

   Distomid 
  soon 
  secretes 
  conchyolin, 
  followed 
  by 
  the 
  pearly 
  sub- 
  

   stance, 
  "and 
  from 
  this 
  point 
  the 
  growth 
  of 
  the 
  pearl 
  probably 
  

   takes 
  place 
  on 
  the 
  same 
  lines 
  and 
  at 
  the 
  same 
  rate 
  as 
  the 
  thick- 
  

   ening 
  of 
  the 
  shell" 
  (Jameson). 
  The 
  latter 
  author 
  points 
  out 
  

   that 
  other 
  parasites 
  are 
  not 
  surrounded 
  by 
  such 
  a 
  sac 
  of 
  pro- 
  

   liferating 
  cells. 
  If 
  the 
  larva 
  dies, 
  its 
  tissues 
  become 
  a 
  struc- 
  

   tureless 
  mass, 
  which 
  retains 
  the 
  form 
  of 
  the 
  parasite 
  owing 
  to 
  

   the 
  rigid 
  cuticle. 
  Calcification 
  occurs 
  in 
  many 
  cases 
  in 
  the 
  

   tissues 
  of 
  the 
  parasite 
  before 
  it 
  is 
  surrounded 
  by 
  the 
  sac* 
  

   Whether 
  in 
  any 
  the 
  sac 
  is 
  formed 
  by 
  an 
  infolding 
  of 
  the 
  surface 
  

   of 
  the 
  mantle 
  has 
  yet 
  to 
  be 
  determined. 
  

  

  For 
  the 
  completion 
  of 
  the 
  life-cycle 
  of 
  the 
  parasite 
  it 
  is 
  

   necessary 
  that 
  either 
  the 
  Eider-Duck 
  or 
  the 
  Common 
  Scoter 
  

   should 
  eat 
  the 
  Mussels 
  before 
  the 
  parasite 
  is 
  enclosed 
  by 
  the 
  

   secretions 
  of 
  the 
  mantle. 
  Like 
  Kelaart, 
  Dr. 
  Jameson 
  broaches 
  

   the 
  idea 
  of 
  infecting 
  the 
  molluscs 
  with 
  pearl-producing 
  parasites, 
  

   just 
  as 
  Antonio 
  Villa* 
  did 
  in 
  relation 
  to 
  De 
  Filippi's 
  discovery. 
  

  

  If 
  the 
  origin 
  of 
  the 
  Ceylonese, 
  Australian, 
  and 
  other 
  pearls 
  

   is 
  similar 
  to 
  those 
  of 
  the 
  Common 
  Mussel, 
  the 
  final 
  hosts 
  will 
  

   probably 
  be 
  found 
  in 
  fishes 
  which 
  prey 
  on 
  the 
  shell-fishes, 
  such 
  

   as 
  the 
  Great 
  Kay, 
  with 
  a 
  powerful 
  dental 
  apparatus, 
  mentioned 
  

  

  :;; 
  " 
  Sull' 
  Origine 
  delle 
  Perle, 
  e 
  sulla 
  possibility 
  di 
  producte 
  artificial, 
  

   mente," 
  'Est. 
  dal 
  Politecnico,' 
  fasc. 
  xlviii., 
  Giugno, 
  1860. 
  Milano. 
  

  

  