﻿404 
  I^- 
  T. 
  Young, 
  

  

  transition 
  in 
  various 
  species 
  of 
  Turbellaria 
  from 
  a 
  nucleated 
  to 
  a 
  

   non-nucleated 
  cuticula-like 
  epitlielium. 
  discussed 
  by 
  me 
  in 
  a 
  recent 
  

   paper 
  (Young, 
  1912 
  b), 
  also 
  supports 
  my 
  view. 
  

  

  In 
  the 
  other 
  ducts 
  a 
  cuticula 
  is 
  not 
  formed 
  and 
  the 
  epithelium 
  

   persists, 
  either 
  in 
  its 
  original 
  or 
  in 
  a 
  much 
  modified 
  form. 
  In 
  those 
  

   ducts 
  which 
  do 
  not 
  undergo 
  distension 
  by 
  the 
  sex 
  cells 
  or 
  embryos 
  

   (oviduct 
  and 
  yolk 
  duct) 
  the 
  epithelium 
  retains 
  its 
  original 
  structure, 
  

   but 
  in 
  those 
  (uterus, 
  vas 
  deferens) 
  which 
  are 
  greatly 
  distended 
  in 
  

   later 
  stages, 
  it 
  becomes 
  much 
  flattened 
  with 
  its 
  elements 
  widely 
  

   separated 
  from 
  one 
  another. 
  In 
  some 
  places 
  (oviduct 
  and 
  yolk 
  duct 
  

   branches) 
  there 
  is 
  some 
  loss 
  of 
  cells 
  which 
  are 
  cast 
  into 
  the 
  lumen 
  

   and 
  degenerate, 
  but 
  the 
  entire 
  epithelium 
  does 
  not 
  experience 
  this 
  

   fate. 
  Such 
  a 
  process 
  has 
  however 
  been 
  observed 
  in 
  larval 
  cestodes 
  

   by 
  Leuckaet 
  (1886), 
  Schauinsland 
  (1885), 
  etc., 
  and 
  in 
  trematodes 
  by 
  

   Thomas 
  (1883), 
  Looss 
  (1892, 
  1893, 
  1894) 
  etc. 
  

  

  The 
  difference 
  in 
  the 
  fate 
  of 
  the 
  epithelium 
  in 
  different 
  parts 
  

   of 
  the 
  reproductive 
  sj^stem 
  of 
  the 
  same 
  worm 
  may 
  throw 
  some 
  light 
  

   on 
  the 
  widely 
  divergent 
  views 
  regarding 
  this 
  tissue. 
  For 
  if 
  differ- 
  

   ences 
  exist 
  in 
  the 
  same 
  worm 
  in 
  different 
  ducts 
  it 
  is 
  not 
  impossible 
  

   that 
  as 
  great 
  differences 
  exist 
  in 
  the 
  development 
  of 
  different 
  forms. 
  

   The 
  absence 
  of 
  an 
  epithelium 
  in 
  the 
  uterus 
  and 
  oviduct 
  branches 
  

   in 
  Anoplocephala 
  magna 
  — 
  tide 
  Balss 
  (1908), 
  and 
  its 
  presence 
  in 
  

   these 
  ducts 
  in 
  Taenia 
  pisiformis, 
  for 
  example, 
  suggest 
  either 
  that 
  

   these 
  two 
  species 
  present 
  different 
  stages 
  of 
  tissue 
  change, 
  or 
  else 
  

   that 
  the 
  epithelium 
  of 
  the 
  reproductive 
  ducts 
  of 
  cestodes 
  is 
  not 
  a 
  

   structure 
  of 
  great 
  significance. 
  The 
  fact 
  that 
  an 
  epithelium 
  as 
  such 
  

   may 
  be 
  absent 
  in 
  places 
  where 
  one 
  would 
  ordinarily 
  expect 
  to 
  find 
  

   it, 
  strengthens 
  the 
  view 
  of 
  those 
  who 
  deny 
  its 
  presence 
  on 
  the 
  sur- 
  

   face 
  of 
  trematodes 
  and 
  cestodes. 
  

  

  To 
  what 
  extent 
  the 
  presence 
  or 
  absence 
  of 
  this 
  tissue 
  in 
  any 
  

   region 
  is 
  dependent 
  on 
  local 
  conditions 
  is 
  uncertain. 
  Peatt 
  (1909, 
  

   p. 
  722) 
  has 
  suggested 
  that 
  'Tt 
  is 
  probable 
  as 
  a 
  result 
  of 
  these 
  things 
  

   (parasitism), 
  that 
  the 
  changes 
  occurred 
  which 
  is 
  characterize 
  the 
  

   body 
  covering 
  of 
  these 
  worms. 
  The 
  integument 
  which 
  is 
  common 
  

   to 
  most 
  worms 
  apparently 
  would 
  not 
  furnish 
  a 
  sufficient 
  protection 
  

   to 
  animals 
  thus 
  situated, 
  and 
  it 
  consequently 
  came 
  about 
  in 
  the 
  

   €Ourse 
  of 
  their 
  evolution 
  that 
  the 
  outer 
  epithelium 
  with 
  its 
  cuticula 
  

   w^as 
  moulted 
  or 
  at 
  least 
  disappeared 
  and 
  the 
  parenchyma 
  acquired 
  

   the 
  property 
  of 
  forming 
  a 
  thick 
  cuticula-like 
  membrane 
  on 
  its 
  outer 
  

   surface 
  to 
  protect 
  the 
  animals 
  from 
  the 
  peculiar 
  dangers 
  of 
  their 
  

  

  