﻿[11] 
  DECAPODA 
  FfeOM 
  ALDATROSS 
  DfcEDGlNilS. 
  615 
  

  

  tainly 
  indicates 
  some 
  difference 
  in 
  the 
  conditions 
  as 
  to 
  light 
  in 
  caverns 
  

   and 
  in 
  the 
  abysses 
  of 
  the 
  ocean, 
  and 
  make 
  it 
  appear 
  probable, 
  in 
  spite 
  of 
  

   the 
  objections 
  of 
  the 
  physicists, 
  that 
  some 
  kinds 
  of 
  luminous 
  vibrations 
  

   do 
  penetrate 
  to 
  depths 
  exceeding 
  even 
  2,000 
  fathoms. 
  The 
  fact 
  that, 
  ex- 
  

   cluding 
  shallow-water 
  species, 
  there 
  is 
  no 
  very 
  definite 
  relation 
  between 
  

   the 
  amount 
  of 
  the 
  modification 
  of 
  the 
  eyes 
  and 
  the 
  depth 
  which 
  the 
  

   species 
  inhabit, 
  many 
  of 
  the 
  species 
  with 
  the 
  most 
  highly 
  modified 
  eyes 
  

   being 
  inhabitants 
  of 
  much 
  less 
  than 
  1,000 
  fathoms, 
  might 
  at 
  first 
  be 
  

   thought 
  antagonistic 
  to 
  this 
  view. 
  But 
  when 
  we 
  consider 
  how 
  vastly 
  

   greater 
  the 
  purity 
  of 
  the 
  water 
  must 
  be 
  in 
  the 
  deep 
  ocean, 
  far 
  from 
  

   land, 
  than 
  in 
  the 
  comparatively 
  shallow 
  waters 
  near 
  the 
  borders 
  of 
  the 
  

   continents, 
  and 
  how 
  much 
  more 
  transparent 
  the 
  waters 
  of 
  the 
  ocean 
  

   abysses 
  than 
  the 
  surface 
  waters 
  above, 
  we 
  can 
  readily 
  understand 
  that 
  

   there 
  may 
  usually 
  be 
  as 
  much 
  light 
  at 
  2,000 
  fathoms 
  in 
  mid 
  ocean 
  as 
  at 
  

   500 
  or 
  even 
  at 
  200, 
  near 
  a 
  continental 
  border. 
  These 
  considerations 
  

   also 
  explain 
  how 
  the 
  eyes 
  of 
  specimens 
  of 
  species 
  like 
  Parapagurm 
  

   pilosimanus, 
  coming 
  from 
  2,220 
  fathoms, 
  are 
  not 
  perceptibly 
  different 
  

   from 
  the 
  eyes 
  of 
  specimens 
  from 
  250 
  fathoms. 
  

  

  Although 
  some 
  abyssal 
  species 
  do 
  have 
  well-developed 
  black 
  eyes, 
  

   there 
  can 
  be 
  no 
  question 
  that 
  there 
  is 
  a 
  tendency 
  toward 
  very 
  radical 
  

   modification 
  or 
  obliteration 
  of 
  the 
  normal 
  visual 
  organs 
  in 
  species 
  in- 
  

   habiting 
  deep 
  water. 
  The 
  simplest 
  and 
  most 
  direct 
  form 
  of 
  this 
  tend- 
  

   ency 
  is 
  shown 
  in 
  the 
  gradual 
  reduction 
  in 
  the 
  number 
  of 
  the 
  visual 
  

   elements, 
  resulting 
  in 
  the 
  obsolescence 
  and, 
  in 
  some 
  cases, 
  in 
  final 
  ob- 
  

   literation 
  of 
  the 
  eye. 
  The 
  stages 
  of 
  such 
  a 
  process 
  are 
  well 
  represented, 
  

   even 
  among 
  the 
  adults 
  of 
  living 
  species. 
  The 
  abyssal 
  species 
  with 
  

   black 
  eyes, 
  referred 
  to 
  in 
  a 
  previous 
  paragraph, 
  contains 
  the 
  first 
  part 
  

   of 
  such 
  a 
  series, 
  beginning 
  with 
  species 
  like 
  Gcryon 
  quinquedens 
  and 
  

   Liihodes 
  Agassizii 
  and 
  ending 
  with 
  Ethusina 
  abyssicola, 
  in 
  which 
  there 
  

   are 
  ouly 
  a 
  very 
  few 
  visual 
  elements 
  at 
  the 
  tips 
  of 
  the 
  immobile 
  eye- 
  

   stalks. 
  A 
  still 
  later 
  stage 
  is 
  represented 
  by 
  A. 
  M.-Edwards's 
  genus 
  

   Cymonomus, 
  in 
  which 
  the 
  eye- 
  stalks 
  are 
  immobile, 
  spiny 
  rods, 
  taper- 
  

   ing 
  to 
  obtuse 
  points, 
  without 
  visual 
  elements, 
  or 
  even 
  (according 
  to 
  the 
  

   description) 
  a 
  cornea. 
  Cymonomus 
  is 
  not 
  known 
  to 
  be 
  an 
  abyssal 
  genus, 
  

   neither 
  of 
  the 
  species 
  having 
  been 
  recorded 
  from 
  much 
  below 
  700 
  fath- 
  

   oms, 
  and 
  is 
  a 
  good 
  example 
  of 
  the 
  fact 
  already 
  mentioned, 
  that 
  many 
  

   of 
  the 
  species 
  with 
  the 
  most 
  highly 
  modified 
  eyes 
  are 
  inhabitants 
  of 
  

   comparatively 
  shallow 
  water. 
  There 
  are, 
  however, 
  several 
  cases 
  of 
  

   very 
  closely 
  allied 
  species 
  inhabiting 
  different 
  depths, 
  where 
  the 
  eyes 
  

   of 
  the 
  deeper-water 
  species 
  are 
  much 
  the 
  smaller, 
  for 
  example 
  : 
  Symj)it- 
  

   gufus 
  pictus, 
  164 
  to 
  2G4, 
  and 
  Parapagurus 
  pilosimanus, 
  250 
  to 
  2,221 
  

   fathoms; 
  Pontophilus 
  gracilis, 
  225 
  to 
  458, 
  and 
  P. 
  abyssi, 
  1,017 
  to 
  2,221 
  

   fathoms 
  ; 
  and 
  Nematocarcinus 
  cursor, 
  384 
  to 
  838, 
  and 
  N. 
  ensiferus, 
  588 
  

   to 
  2,033 
  fathoms. 
  

  

  In 
  a 
  large 
  number 
  of 
  deep-water 
  and 
  abyssal 
  species 
  (lie 
  ocular 
  pig- 
  

   ment 
  is 
  deep 
  purplish, 
  brownish, 
  reddish, 
  light 
  purplish, 
  light 
  reddish, 
  

  

  