﻿18 
  GROWTH 
  AND 
  MIGRATION 
  OF 
  FRESH-WATER 
  MUSSELS. 
  

  

  underneath 
  older 
  layers 
  "- 
  of 
  the 
  same 
  structure, 
  and 
  sometimes 
  

   luiderneath 
  older 
  layers 
  of 
  nacre 
  as 
  well. 
  This 
  is 
  well 
  shown 
  in 
  

   figures 
  1-4, 
  showmg 
  sections 
  of 
  shells 
  made 
  through 
  the 
  growth 
  Imes. 
  

  

  In 
  watching 
  the 
  growth 
  of 
  tagged 
  specimens 
  it 
  was 
  noted 
  that 
  a 
  

   distmct 
  growth 
  luie 
  was 
  formed 
  at 
  the 
  time 
  of 
  tagging; 
  the 
  work 
  of 
  

   taggmg, 
  i. 
  e., 
  the 
  drillmg 
  of 
  the 
  hole 
  through 
  the 
  shell 
  and 
  the 
  placmg 
  

   of 
  the 
  wire, 
  caused 
  considerable 
  nritation, 
  probably 
  a 
  strong 
  con- 
  

   traction 
  and 
  partial 
  breaking 
  loose 
  of 
  the 
  mantle 
  from 
  the 
  edge 
  of 
  

   the 
  shell, 
  and, 
  as 
  a 
  result, 
  new 
  epidermal 
  and 
  prismatic 
  layers 
  were 
  

   put 
  down 
  underneath 
  those 
  already 
  formed, 
  as 
  the 
  mantle 
  worked 
  

   to 
  its 
  old 
  position, 
  and 
  formed 
  new 
  layers 
  over 
  the 
  wire 
  holdmg 
  the 
  

   tag 
  (pi. 
  Ill, 
  763). 
  Young 
  specimens, 
  especially, 
  show 
  a 
  conspicuous 
  

   and 
  well-defined 
  ring 
  passing 
  through 
  the 
  outer 
  side 
  of 
  the 
  wire 
  

   holdmg 
  the 
  tag. 
  

  

  The 
  so-called 
  annual 
  rings 
  had 
  better 
  be 
  called 
  "arrested 
  growth 
  

   or 
  rest 
  rings," 
  as 
  they 
  represent 
  retarded 
  growth, 
  which 
  may 
  be 
  

   very 
  temporary, 
  as 
  in 
  the 
  case 
  of 
  the 
  tagged 
  specimens, 
  and 
  still 
  

   leave 
  a 
  very 
  marked 
  rmg. 
  Ordinarily 
  the 
  prominent 
  rest 
  rings 
  are 
  

   presumably 
  winter 
  rings, 
  ^ 
  representing 
  delayed 
  growth, 
  due 
  to 
  

   inactivity, 
  a 
  withdrawal 
  of 
  the 
  mantle 
  from 
  the 
  extreme 
  edge 
  of 
  the 
  

   valves, 
  and 
  the 
  fornung 
  of 
  double 
  epidermal 
  and 
  prismatic 
  layers 
  

   as 
  a 
  result 
  of 
  renewed 
  active 
  growth 
  in 
  the 
  following 
  spring. 
  Other 
  

   rings 
  may 
  follow 
  arrested 
  growth, 
  due 
  to 
  various 
  unfavorable 
  con- 
  

   ditions 
  that 
  may 
  arise 
  in 
  the 
  life 
  of 
  the 
  mussel, 
  such 
  as 
  water 
  

   shrinkage, 
  temporary 
  strandmg 
  due 
  to 
  migration, 
  especially 
  at 
  flood 
  

   periods; 
  in 
  the 
  hghter 
  species 
  perhaps 
  washing 
  at 
  flood 
  times. 
  It 
  

   is 
  possible 
  that 
  in 
  certain 
  pond 
  forms, 
  as 
  Z7. 
  tetralasmus 
  and 
  others 
  

   that 
  hve 
  in 
  ponds 
  that 
  go 
  dry 
  for 
  short 
  periods 
  durmg 
  the 
  summer 
  

   season, 
  the 
  more 
  prominent 
  rings 
  are 
  summer 
  rings.*' 
  

  

  That 
  the 
  concentric 
  rings 
  are 
  by 
  no 
  means 
  dependable 
  as 
  abso- 
  

   lute 
  annual 
  rings 
  is 
  well 
  shown 
  in 
  many 
  specimens 
  under 
  observation 
  

   in 
  this 
  investigation. 
  

  

  A 
  few 
  specimens 
  shown 
  photographically 
  (pi. 
  i, 
  ii, 
  iii) 
  will 
  clear 
  

   up 
  some 
  pomts 
  in 
  regard 
  to 
  growth. 
  Rest 
  ring-s 
  are 
  not 
  always 
  

   brought 
  out 
  clearly 
  by 
  photographic 
  methods 
  on 
  account 
  of 
  the 
  

   unequal 
  or 
  convex 
  surface 
  of 
  the 
  shell. 
  A 
  sHght 
  ridge 
  that 
  may 
  have 
  

   no 
  connection 
  with 
  a 
  rest 
  ring 
  will 
  show 
  as 
  a 
  shadow 
  Hne 
  beyond 
  

   the 
  ridge. 
  This 
  is 
  shown 
  in 
  photograph 
  H, 
  plate 
  i, 
  in 
  the 
  line 
  

   marked 
  "o." 
  Additional 
  explanations 
  are 
  given 
  with 
  the 
  plate 
  

   figures. 
  

  

  a 
  This 
  explanation 
  was 
  first 
  suggested 
  to 
  me 
  by 
  Dr. 
  Cokcr, 
  and 
  later 
  verified 
  in 
  connection 
  with 
  my 
  

   study 
  of 
  shell 
  sections 
  through 
  arrested 
  growth 
  lines. 
  

  

  b 
  ^Vhile 
  in 
  this 
  paper 
  wo 
  have 
  emphasized 
  the 
  point 
  that 
  " 
  annual 
  rings 
  " 
  are 
  not 
  annual 
  rings 
  absolutely, 
  

   the 
  statement 
  that 
  the 
  prominent 
  rest 
  rings 
  are 
  usually, 
  under 
  stable 
  environmental 
  conditions, 
  winter 
  

   rings 
  is 
  clearly 
  within 
  the 
  evidence 
  of 
  this 
  investigation. 
  

  

  c 
  Live 
  specimens 
  of 
  this 
  species 
  were 
  plowed 
  up 
  in 
  the 
  Browne 
  pond 
  (p. 
  7) 
  three 
  months 
  after 
  the 
  

   pond 
  had 
  gone 
  dry. 
  

  

  