﻿46 
  PROPAGATION 
  OF 
  FRESH-WATER 
  MUSSELS. 
  

  

  JUYENILE 
  STAGE. 
  

  

  The 
  period 
  in 
  the 
  life 
  of 
  the 
  mussel 
  following 
  the 
  parasitic 
  stage 
  

   has 
  been 
  given 
  comparatively 
  little 
  attention 
  by 
  investigators. 
  It 
  

   is 
  commonly 
  stated 
  that 
  the 
  mussel 
  upon 
  leaving 
  the 
  host 
  assumes 
  

   the 
  adult 
  form 
  and 
  manner 
  of 
  life. 
  The 
  investigations 
  made 
  upon 
  

   this 
  stage, 
  however, 
  indicate 
  that 
  the 
  differences 
  are 
  probably 
  of 
  

   practical 
  importance; 
  that 
  the 
  adult 
  form 
  is 
  not 
  completely 
  attained 
  

   for 
  some 
  time 
  (Schierholz, 
  1888), 
  and 
  that 
  the 
  manner 
  of 
  life 
  may 
  be 
  

   quite 
  different. 
  According 
  to 
  Schierholz 
  (1888, 
  writing 
  of 
  the 
  

   Unionidae) 
  : 
  

  

  The 
  following 
  organs 
  are 
  acquired 
  during 
  this 
  stage: 
  The 
  siphons, 
  "Lippentaster," 
  

   outer 
  gills, 
  and 
  sexual 
  glands. 
  In 
  Anodonta 
  the 
  outer 
  gills 
  are 
  acquired 
  the 
  second 
  

   summer, 
  in 
  Unio 
  the 
  third 
  and 
  foirrth 
  summer, 
  while 
  the 
  sexual 
  glands 
  are 
  developed 
  

   in 
  Anodonta 
  the 
  third 
  summer 
  and 
  in 
  Unio 
  the 
  fourth 
  and 
  fifth 
  summer. 
  

  

  The 
  acquirement 
  of 
  the 
  sexual 
  glands 
  would 
  mark 
  the 
  adult 
  condi- 
  

   tion 
  and 
  the 
  end 
  of 
  the 
  juvenile 
  period. 
  The 
  size 
  upon 
  first 
  attaining 
  

   the 
  adult 
  condition 
  would, 
  of 
  course, 
  vary 
  considerably 
  in 
  individuals 
  

   as 
  well 
  as 
  in 
  species. 
  The 
  smallest 
  gravid 
  mussels 
  I 
  have 
  secured 
  

   were 
  13 
  mm. 
  in 
  length. 
  This 
  was 
  in 
  the 
  very 
  small 
  species 
  Plagiola 
  

   donaciformis, 
  in 
  which 
  an 
  example 
  50 
  mm. 
  long 
  would 
  be 
  exceptionally 
  

   large. 
  

  

  The 
  washboard 
  (Quadrula 
  heros), 
  the 
  largest 
  of 
  American 
  mussels, 
  

   has 
  shell 
  markings 
  on 
  the 
  umbones 
  that 
  seem 
  to 
  be 
  characteristic 
  of 
  

   the 
  juvenile. 
  These 
  sculpturings 
  mark 
  the 
  shell 
  up 
  to 
  a 
  size 
  of 
  50 
  

   mm. 
  to 
  60 
  mm., 
  when 
  they 
  often 
  cease 
  abruptly. 
  The 
  size 
  at 
  

   maturity, 
  according 
  to 
  the 
  evidence 
  I 
  have, 
  would 
  be 
  much 
  greater, 
  

   as 
  the 
  smallest 
  breeding 
  individual 
  that 
  T 
  have 
  collected 
  was 
  91 
  mm. 
  

   long. 
  This 
  showed 
  a 
  growth 
  of 
  three 
  years 
  beyond 
  the 
  juvenile 
  shell 
  

   and 
  a 
  total 
  age 
  estimated 
  at 
  8 
  years. 
  

  

  In 
  statistical 
  estimates 
  in 
  ordinary 
  sized 
  species 
  I 
  have 
  taken 
  the 
  

   arbitrary 
  length 
  of 
  20 
  mm. 
  (Lefevre 
  and 
  Curtis, 
  1912) 
  as 
  a 
  limit 
  for 
  the 
  

   early 
  juvenile 
  stage. 
  For 
  large 
  and 
  small 
  species 
  it 
  is 
  evident 
  this 
  

   length 
  would 
  not 
  be 
  sufficiently 
  accurate. 
  

  

  The 
  sculpturings 
  mentioned 
  above 
  for 
  Q. 
  heros 
  are 
  characteristic 
  

   of 
  the 
  juveniles 
  of 
  most 
  species, 
  although 
  they 
  are 
  absent 
  in 
  some. 
  

   Commonly 
  they 
  are 
  limited 
  to 
  the 
  earlier 
  portions 
  of 
  the 
  shell. 
  

  

  The 
  presence 
  of 
  a 
  byssus 
  in 
  many 
  species, 
  as 
  reported 
  by 
  Sterki 
  

   (1891), 
  Frierson 
  (1903), 
  White 
  (1905), 
  and 
  Isely 
  (1911) 
  and 
  the 
  at- 
  

   tachment 
  of 
  the 
  young 
  mussels 
  to 
  stones 
  and 
  gravel 
  in 
  shallow 
  water 
  

   I 
  have 
  been 
  able 
  to 
  confirm 
  in 
  a 
  number 
  of 
  cases, 
  though, 
  excepting 
  

   Q. 
  'plicata, 
  not 
  among 
  the 
  Quodrulas. 
  This 
  habit 
  of 
  attachment 
  in 
  

   shallow 
  water 
  is 
  found 
  among 
  the 
  Lampsilis 
  group, 
  which 
  in 
  the 
  

   adult 
  stage 
  are 
  comparatively 
  active 
  mussels 
  commonly 
  in 
  deep 
  

  

  