﻿Crustacea 
  of 
  Dominica. 
  21 
  

  

  length 
  of 
  the 
  rostrum 
  in 
  the 
  three 
  forms, 
  unhesitatingly 
  

   regard 
  them 
  as 
  belonging 
  to 
  one 
  species, 
  in 
  which, 
  while 
  

   passing 
  from 
  the 
  young 
  to 
  the 
  adult 
  condition, 
  the 
  body 
  

   increases 
  in 
  thickness 
  more 
  than 
  it 
  does 
  in 
  length, 
  and 
  the 
  

   eye-stalks 
  grow 
  less 
  rapidly 
  than 
  do 
  the 
  other 
  appendages. 
  

  

  But 
  when 
  the 
  rostrum 
  is 
  taken 
  into 
  consideration 
  the 
  

   problem 
  becomes 
  far 
  more 
  complex. 
  It 
  must 
  be 
  borne 
  in 
  

   mind, 
  however, 
  that 
  there 
  is 
  a 
  gradation, 
  though 
  not 
  a 
  

   perfect 
  one, 
  from 
  the 
  long 
  rostrum 
  of 
  the 
  young 
  elongata, 
  

   through 
  intermedia 
  and 
  gladiator, 
  to 
  the 
  short 
  rostrum 
  of 
  

   the 
  adult 
  brevirostris 
  ; 
  but, 
  to 
  the 
  best 
  of 
  my 
  belief, 
  it 
  is 
  

   contrary 
  to 
  all 
  experience 
  that 
  this 
  organ 
  should 
  progressively 
  

   decrease 
  in 
  length 
  as 
  the 
  body 
  increases 
  in 
  bulk. 
  Conse- 
  

   quently 
  I 
  do 
  not 
  for 
  a 
  moment 
  imagine 
  it 
  likely 
  that 
  the 
  

   differences 
  are 
  due 
  directly 
  to 
  age. 
  But 
  it 
  seems 
  to 
  me 
  quite 
  

   possible 
  that 
  in 
  the 
  specimens 
  named 
  brevirostris, 
  gladiator, 
  

   and 
  intermedia 
  the 
  rostrum 
  at 
  first 
  resembled 
  that 
  of 
  elon- 
  

   gata, 
  but 
  has 
  been 
  broken 
  off 
  and 
  is 
  reacquiring 
  the 
  form 
  

   characteristic 
  of 
  that 
  species. 
  In 
  support 
  of 
  this 
  hypothesis 
  

   may 
  be 
  urged 
  the 
  following 
  considerations: 
  — 
  (I) 
  It 
  may 
  be 
  

   inferred 
  a 
  priori 
  (a) 
  from 
  the 
  armature 
  and 
  length 
  of 
  the 
  ros- 
  

   trum 
  that 
  it 
  is 
  a 
  weapon 
  of 
  offence, 
  and 
  (b) 
  that 
  as 
  such, 
  

   owing 
  to 
  its 
  thinness, 
  it 
  is 
  exceedingly 
  liable 
  to 
  breakage 
  ; 
  

   (2) 
  from 
  the 
  fact 
  that 
  in 
  40 
  per 
  cent, 
  of 
  the 
  specimens 
  of 
  

   elongata 
  obtained 
  this 
  organ 
  is 
  imperfect 
  we 
  know 
  that 
  it 
  is 
  

   subject 
  to 
  damage, 
  be 
  the 
  cause 
  what 
  it 
  may 
  ; 
  (3) 
  the 
  power 
  

   possessed 
  by 
  the 
  Crustacea 
  of 
  reproducing 
  lost 
  parts 
  is 
  well 
  

   known 
  ; 
  and 
  (4) 
  if 
  the 
  rostrum 
  of 
  the 
  above-mentioned 
  species 
  

   is 
  undergoing 
  a 
  process 
  of 
  regrowth 
  its 
  present 
  form 
  in 
  each 
  

   case 
  is 
  exactly 
  what 
  might 
  have 
  been 
  predicted 
  for 
  it 
  ; 
  for 
  the 
  

   shorter 
  rostra 
  are 
  copies 
  in 
  miniature 
  of 
  the 
  longer, 
  the 
  only 
  

   important 
  difference 
  being 
  the 
  difference 
  of 
  absolute 
  thickness 
  

   and 
  length. 
  And, 
  further, 
  assuming 
  the 
  specimens 
  to 
  be 
  

   different 
  species, 
  I 
  would 
  point 
  out 
  that 
  it 
  is 
  a 
  thing 
  of 
  no 
  

   common 
  occurrence 
  in 
  the 
  Caridea 
  for 
  the 
  teeth 
  to 
  be 
  so 
  con- 
  

   stant 
  in 
  position 
  as 
  they 
  are 
  in 
  the 
  present 
  instance, 
  when 
  

   the 
  rostrum 
  is 
  so 
  variable 
  in 
  size 
  ; 
  for 
  in 
  all 
  the 
  specimens 
  of 
  

   Xiphocaris, 
  whether 
  the 
  rostrum 
  be 
  long 
  or 
  short, 
  provided 
  

   only 
  that 
  its 
  form 
  be 
  perfect, 
  there 
  is 
  a 
  series 
  of 
  teeth 
  in 
  the 
  

   proximal 
  half 
  above 
  separated 
  by 
  a 
  space 
  from 
  a 
  few 
  teeth 
  at 
  

   the 
  apex 
  and 
  a 
  series 
  of 
  teeth 
  below 
  running 
  from 
  the 
  apex 
  

   to 
  the 
  orbit 
  ; 
  so 
  that 
  the 
  position 
  presented 
  by 
  these 
  teeth 
  

   becomes 
  a 
  character 
  of 
  generic 
  significance. 
  

  

  But 
  supposing 
  a 
  specimen 
  of 
  elongata 
  had 
  lost 
  the 
  anterior 
  

   third 
  or 
  anterior 
  half 
  of 
  its 
  rostrum, 
  I 
  presume 
  that 
  when 
  

   growth 
  set 
  in, 
  this 
  organ 
  would 
  present 
  the 
  appearance 
  seen 
  

  

  