﻿NO. 
  1950. 
  CRUSTACEAN 
  PARASITES 
  OF 
  FISH 
  AND 
  CRABS— 
  WILSON. 
  267 
  

  

  are 
  certain 
  facts 
  already 
  definitely 
  known, 
  and 
  from 
  these 
  we 
  may 
  

   draw 
  some 
  conclusions 
  : 
  

  

  1. 
  The 
  host 
  is 
  a 
  land 
  crab 
  which 
  lives 
  for 
  IH 
  months 
  out 
  of 
  the 
  

   water, 
  and 
  only 
  resorts 
  to 
  the 
  ocean 
  once 
  a 
  year 
  for 
  a 
  week 
  or 
  10 
  days, 
  

   in 
  order 
  that 
  its 
  eggs 
  may 
  hatch 
  into 
  the 
  customary 
  free-swimming 
  

   zcEas. 
  Hence 
  while 
  its 
  gills 
  are 
  always 
  necessarily 
  moist 
  enough 
  to 
  

   keep 
  the 
  copepod 
  alive, 
  there 
  is 
  no 
  chance 
  for 
  the 
  latter 
  to 
  do 
  any 
  

   swimmmg 
  or 
  to 
  lay 
  its 
  own 
  eggs 
  except 
  during 
  this 
  week 
  or 
  10 
  days. 
  

   This 
  copepod 
  therefore 
  must 
  be 
  content 
  with 
  one 
  breeding 
  season 
  a 
  

   year, 
  instead 
  of 
  the 
  two 
  or 
  three 
  which 
  its 
  kind 
  ordinarily 
  enjoy. 
  

  

  2. 
  The 
  copepod's 
  eggs 
  must 
  be 
  ripened 
  and 
  ready 
  to 
  push 
  out 
  into 
  

   the 
  external 
  sac 
  at 
  the 
  exact 
  time 
  that 
  the 
  crab's 
  eggs 
  are 
  ready 
  to 
  

   hatch. 
  Hence 
  the 
  extrusion 
  of 
  the 
  copepod's 
  eggs, 
  their 
  develop- 
  

   ment 
  into 
  larvae, 
  the 
  escape 
  of 
  the 
  latter 
  in 
  the 
  form 
  of 
  nauplii, 
  their 
  

   development 
  through 
  the 
  various 
  metanauplius 
  and 
  copepodid 
  or 
  

   Cyclops 
  stages 
  to 
  the 
  adult 
  form, 
  and 
  the 
  fastening 
  of 
  the 
  latter 
  on 
  

   the 
  gills 
  of 
  a 
  crab 
  host 
  must 
  all 
  be 
  accomplished 
  within 
  the 
  short 
  

   period 
  that 
  the 
  crabs 
  remain 
  in 
  the 
  ocean. 
  If 
  this 
  be 
  true, 
  such 
  a 
  

   development 
  is 
  exceptionally 
  rapid 
  and 
  is 
  followed 
  by 
  a 
  whole 
  year 
  

   of 
  rest 
  before 
  the 
  female 
  can 
  lay 
  her 
  first 
  batch 
  of 
  eggs. 
  If 
  it 
  is 
  not 
  

   true, 
  the 
  copepod 
  remains 
  a 
  free 
  swimmer 
  during 
  the 
  first 
  year 
  of 
  its 
  

   existence 
  and 
  then 
  becomes 
  parasitic. 
  It 
  must 
  thus 
  lay 
  its 
  first 
  

   batch 
  of 
  eggs 
  while 
  swimming 
  about 
  freely 
  or 
  wait 
  two 
  years 
  for 
  that 
  

   occasion. 
  In 
  all 
  the 
  crabs 
  captured 
  before 
  going 
  into 
  the 
  ocean 
  the 
  

   female 
  copepods 
  were 
  found 
  to 
  have 
  their 
  oviducts 
  crowded 
  with 
  ripe 
  

   eggs 
  all 
  ready 
  for 
  extrusion. 
  On 
  the 
  contrary, 
  not 
  a 
  single 
  egg 
  could 
  

   be 
  found 
  in 
  the 
  oviducts 
  of 
  copepods 
  taken 
  from 
  crabs 
  that 
  had 
  come 
  

   out 
  of 
  the 
  ocean. 
  

  

  3. 
  This 
  copepod 
  can 
  not 
  move 
  around 
  much 
  on 
  the 
  crab's 
  gills 
  

   when 
  the 
  latter 
  is 
  out 
  of 
  the 
  water, 
  and 
  no 
  food 
  can 
  possibly 
  be 
  brought 
  

   to 
  it. 
  Hence 
  it 
  must 
  feed 
  upon 
  the 
  blood 
  in 
  the 
  gills, 
  the 
  slime 
  that 
  

   covers 
  them, 
  or 
  something 
  of 
  the 
  sort 
  that 
  can 
  be 
  obtained 
  in 
  sufficient 
  

   abundance 
  in 
  situ. 
  This 
  means 
  that 
  it 
  is 
  parasitic 
  and 
  not 
  commensal. 
  

   The 
  peculiar 
  modification 
  of 
  the 
  mandibles 
  into 
  a 
  sort 
  of 
  proboscis 
  

   or 
  mouth 
  tube 
  and 
  their 
  sharp, 
  toothed 
  tips 
  would 
  corroborate 
  such 
  

   an 
  inference. 
  

  

  4. 
  It 
  would 
  naturally 
  be 
  supposed 
  that 
  such 
  rigorous 
  conditions 
  

   could 
  only 
  be 
  met 
  by 
  a 
  copepod 
  belonging 
  to 
  some 
  thoroughly 
  para- 
  

   sitic 
  family, 
  and 
  we 
  should 
  look 
  for 
  considerable 
  modification 
  as 
  the 
  

   result 
  of 
  its 
  degenerate 
  habits. 
  But 
  what 
  we 
  really 
  do 
  find 
  is 
  as 
  far 
  

   removed 
  from 
  this 
  as 
  possible. 
  Cancrincola 
  belongs 
  to 
  a 
  group, 
  the 
  

   Harpacticoida, 
  which 
  has 
  always 
  been 
  considered 
  to 
  be 
  typical 
  free 
  

   swimm^ers, 
  and 
  no 
  member 
  of 
  the 
  group 
  has 
  hitherto 
  been 
  known 
  

   to 
  live 
  under 
  any 
  other 
  conditions. 
  

  

  