﻿THE 
  ANATOMY 
  OF 
  PCEOILOCH^TUS, 
  CLAPAREDE. 
  115 
  

  

  Jargei* 
  pair 
  on 
  the 
  ventral 
  surface. 
  All 
  four 
  ejcs 
  have 
  pi-ac- 
  

   tically 
  the 
  same 
  structure, 
  aud 
  are 
  of 
  a 
  very 
  simple 
  type. 
  

   Fig. 
  83 
  (PI. 
  10) 
  represents 
  a 
  section 
  through 
  one 
  of 
  the 
  ventral 
  

   eyes 
  cut 
  in 
  the 
  longitudinal 
  vertical 
  plane. 
  The 
  eye 
  consists 
  

   of 
  a 
  single 
  large 
  optic 
  cell 
  with 
  one 
  nucleus. 
  The 
  proto- 
  

   plasm 
  of 
  the 
  swollen, 
  rounded 
  end 
  of 
  the 
  cell 
  is 
  slightly 
  

   modified, 
  being 
  more 
  transparent 
  than 
  that 
  found 
  in 
  the 
  rest 
  

   of 
  the 
  cell, 
  and 
  showing 
  indications 
  of 
  a 
  radial 
  structure. 
  

   This 
  end 
  of 
  the 
  cell 
  is 
  surrounded 
  by 
  a 
  large 
  cup-shaped 
  mass 
  

   of 
  black 
  pigment, 
  made 
  up 
  of 
  numerous 
  spherical 
  drops 
  of 
  

   black 
  substance. 
  A 
  nucleus 
  can 
  often 
  be 
  detected 
  at 
  the 
  

   outer 
  margin 
  of 
  this 
  mass 
  of 
  pigment, 
  but 
  I 
  have 
  been 
  unable 
  

   to 
  satisfy 
  myself 
  as 
  to 
  whether 
  more 
  than 
  one 
  nucleus 
  belongs 
  

   to 
  each 
  ; 
  that 
  is 
  to 
  say, 
  whether 
  the 
  pigment 
  cup 
  is 
  unicellular, 
  

   or 
  whether 
  it 
  consists 
  of 
  several 
  cells. 
  

  

  Such 
  simple 
  eyes 
  are 
  now 
  well 
  known 
  amongst 
  the 
  Platy- 
  

   helminths, 
  for 
  instance, 
  in 
  Pi 
  an 
  aria 
  torva, 
  and 
  also 
  in 
  

   certain 
  Polychietes, 
  as, 
  for 
  example, 
  Spio 
  fuliginosus 
  and 
  

   Polyophthalmus 
  pictus. 
  A 
  full 
  account 
  of 
  the 
  literature 
  

   of 
  the 
  subject, 
  together 
  with 
  a 
  wealth 
  of 
  new 
  observations, 
  

   will 
  be 
  found 
  in 
  the 
  series 
  of 
  papers 
  "Untersuchuugen 
  iiber 
  

   die 
  Organe 
  der 
  Lichtempfindung 
  bei 
  niederen 
  Thieren,^^ 
  by 
  

   Kichard 
  Hesse 
  (see 
  especially 
  Hesse, 
  1897 
  and 
  1899). 
  

  

  Alimentary 
  Canal. 
  

  

  The 
  Divisions 
  of 
  the 
  Alimentary 
  Canal. 
  — 
  The 
  ex- 
  

   ternal 
  features 
  of 
  the 
  mouth 
  are 
  seen 
  in 
  fig. 
  8 
  (PI. 
  8), 
  and 
  

   have 
  already 
  been 
  described 
  (see 
  p. 
  88). 
  The 
  animal 
  

   possesses 
  a 
  short 
  proboscis 
  with 
  thick 
  walls, 
  which 
  in 
  pre- 
  

   served 
  specimens 
  is 
  always 
  almost 
  if 
  not 
  entirely 
  withdrawn 
  

   into 
  the 
  mouth. 
  Pig. 
  42 
  (PI. 
  11) 
  represents 
  a 
  section 
  through 
  

   these 
  parts. 
  The 
  external 
  folds 
  surrounding 
  the 
  mouth 
  are 
  

   seen, 
  as 
  well 
  as 
  the 
  median 
  tentacle 
  {m. 
  tent.), 
  which 
  has 
  its 
  

   point 
  of 
  insertion 
  close 
  to 
  the 
  anterior 
  edge 
  of 
  the 
  mouth, 
  

   into 
  which 
  its 
  basal 
  portion 
  can 
  be 
  withdrawn. 
  

  

  The 
  portion 
  of 
  the 
  alimentary 
  canal 
  extending 
  from 
  the 
  

  

  