﻿PROPAGATION 
  OF 
  FRESH-WATER 
  MUSSELS. 
  43 
  

  

  provision 
  giving 
  them 
  the 
  size 
  of 
  other 
  juveniles 
  when 
  taking 
  up 
  the 
  

   post-parasitic 
  Hfe. 
  It 
  would 
  not 
  be 
  surprising 
  if 
  in 
  the 
  case 
  of 
  the 
  

   latter 
  the 
  metamorphosis 
  were 
  carried 
  further 
  than 
  in 
  those 
  forms 
  

   which 
  have 
  no 
  growth 
  beyond 
  the 
  glochidial 
  shell. 
  

  

  Duration 
  of 
  the 
  'parasitic 
  period. 
  — 
  Periods 
  of 
  parasitism 
  have 
  been 
  

   reported 
  (Lefevre 
  and 
  Curtis, 
  1912, 
  p. 
  168) 
  as 
  short 
  as 
  12 
  days, 
  and 
  

   I 
  have 
  observed 
  a 
  period 
  as 
  long 
  as 
  six 
  months 
  in 
  the 
  case 
  of 
  Q. 
  lieros 
  

   carried 
  through 
  the 
  winter. 
  Tiiese 
  cases 
  were 
  observed 
  in 
  artificial 
  

   infections. 
  In 
  natural 
  infections 
  I 
  have 
  seen 
  evidence 
  of 
  a 
  long 
  

   period 
  in 
  mussels 
  carried 
  by 
  the 
  sturgeon, 
  Scaphorhynchus 
  plato- 
  

   rhyncTius; 
  matured 
  larvae 
  were 
  seen 
  in 
  late 
  season 
  catches, 
  while 
  

   none 
  were 
  seen 
  in 
  those 
  taken 
  through 
  April 
  and 
  most 
  of 
  May 
  or 
  in 
  

   October. 
  Lefevre 
  and 
  Curtis 
  have 
  called 
  attention 
  to 
  the 
  lack 
  of 
  

   correspondence 
  between 
  their 
  results 
  and 
  those 
  of 
  other 
  observers 
  

   (Harms, 
  1907, 
  1909; 
  Schierholz, 
  1888) 
  as 
  to 
  the 
  effect 
  of 
  temperature 
  

   on 
  the 
  length 
  of 
  parasitism. 
  Considering 
  the 
  great 
  variation 
  in 
  the 
  

   period 
  of 
  metamorphosis 
  for 
  different 
  species, 
  I 
  think 
  it 
  is 
  obvious 
  

   that 
  the 
  influence 
  of 
  temperature 
  could 
  be 
  determined 
  accurately 
  

   only 
  in 
  a 
  single 
  species 
  or 
  in 
  species 
  having 
  the 
  same 
  period. 
  Indi- 
  

   vidual 
  variation 
  in 
  the 
  length 
  of 
  period 
  for 
  mussels 
  which 
  were 
  

   implanted 
  at 
  the 
  same 
  time 
  on 
  the 
  same 
  fish 
  Lefevre 
  and 
  Curtis 
  

   explain 
  as 
  due 
  to 
  differences 
  in 
  nutrition 
  of 
  the 
  larvae. 
  

  

  Larval 
  thread. 
  — 
  The 
  presence 
  of 
  a 
  thread 
  gland 
  and 
  larval 
  thread 
  

   in 
  the 
  mature 
  glochidium 
  I 
  have 
  reported 
  and 
  described 
  under 
  

   Quadrula 
  heros 
  and 
  Quadrula 
  plicata. 
  Lefevre 
  and 
  Curtis 
  (1912, 
  p. 
  

   151, 
  152), 
  in 
  a 
  thorough 
  discussion 
  of 
  this 
  interesting 
  organ, 
  in 
  which 
  

   they 
  state 
  that 
  they 
  have 
  found 
  it 
  only 
  among 
  the 
  Anodontas 
  and 
  

   Unios, 
  agree 
  with 
  Lillie 
  (1895) 
  that 
  it 
  is 
  an 
  excretory 
  organ 
  and 
  the 
  

   thread 
  is 
  an 
  excretion 
  primarily. 
  Thej^ 
  do 
  not 
  agree 
  with 
  Schierholz 
  

   (1888) 
  in 
  considering 
  the 
  thread 
  an 
  efficient 
  organ 
  for 
  aiding 
  in 
  

   attachment 
  to 
  fishes. 
  In 
  this 
  connection 
  it 
  is 
  interesting 
  to 
  note 
  that 
  

   in 
  this 
  form, 
  which 
  is 
  the 
  only 
  one 
  of 
  its 
  kind 
  known 
  to 
  take 
  the 
  

   habit 
  of 
  external 
  or 
  fin 
  infection, 
  we 
  have 
  an 
  organ 
  that 
  is 
  charac- 
  

   teristic 
  of 
  fin-infecting 
  groups, 
  Anodontas 
  and 
  the 
  European 
  Unios. 
  

   Since 
  making 
  this 
  observation 
  on 
  Q. 
  heros 
  1 
  have 
  noted 
  the 
  thread 
  

   gland 
  m 
  Unio 
  gihhosus 
  and 
  also 
  observed 
  this 
  glochidium 
  in 
  natural 
  

   infection 
  upon 
  the 
  fins. 
  

  

  From 
  these 
  observations 
  it 
  looks 
  as 
  if 
  this 
  habit 
  might 
  have 
  some 
  

   bearing 
  on 
  the 
  function 
  of 
  the 
  thread 
  gland. 
  Here 
  we 
  have 
  among 
  

   the 
  bookless 
  glochidia 
  a 
  form 
  showing 
  the 
  thread 
  gland 
  and 
  thread 
  

   more 
  higlily 
  developed 
  apparently 
  than 
  in 
  the 
  Anodontas. 
  The 
  

   inference 
  is 
  natural 
  that 
  the 
  thread 
  has 
  some 
  function 
  other 
  than 
  

   excretion 
  alone. 
  If 
  a 
  fundamental 
  embryonic 
  (Lillie, 
  1895) 
  organ 
  

   present 
  at 
  some 
  time 
  in 
  the 
  development 
  of 
  all 
  glochidia, 
  its 
  per- 
  

   sistence 
  to 
  the 
  mature 
  state 
  in 
  some 
  cases 
  and 
  not 
  in 
  others 
  would 
  

  

  