﻿1874.] 
  and 
  Skull 
  of 
  Amphioxus 
  lanceolatus. 
  129 
  

  

  frioged 
  by 
  tentacles, 
  external 
  to 
  which 
  lies 
  a 
  lip, 
  which 
  is 
  continuous 
  

   behind 
  with 
  the 
  ventro-lateral 
  ridge 
  of 
  the 
  body. 
  The 
  oral 
  chamber 
  is 
  

   spacious, 
  and 
  extends 
  back 
  to 
  the 
  level 
  of 
  the 
  junction 
  between 
  the 
  sixth 
  

   and 
  seventh 
  myotomes 
  (fig. 
  A). 
  Here 
  it 
  is 
  divided 
  from 
  the 
  branchial 
  

   cavity 
  by 
  a 
  peculiarly 
  constructed, 
  muscular 
  velum 
  palati, 
  the 
  upper 
  

   attachment 
  of 
  which 
  to 
  the 
  ventral 
  aspect 
  of 
  the 
  sheath 
  of 
  the 
  notochord 
  

   lies 
  vertically 
  below 
  the 
  anterior 
  angle 
  of 
  the 
  seventh 
  myotome. 
  

  

  Eight 
  pairs 
  of 
  nerves 
  are 
  given 
  off: 
  from 
  the 
  cerebro-spinal 
  axis 
  as 
  

   far 
  as 
  this 
  point. 
  The 
  eighth, 
  or 
  most 
  posterior, 
  of 
  these, 
  which, 
  for 
  

   convenience, 
  may 
  be 
  called 
  h, 
  passes 
  out 
  between 
  the 
  sixth 
  and 
  seventh 
  

   myotomes, 
  and 
  runs 
  down 
  parallel 
  with 
  the 
  lateral 
  attachment 
  of 
  the 
  

   velum. 
  The 
  next 
  five 
  (</,/, 
  e, 
  d, 
  c) 
  pass 
  out 
  between 
  the 
  first 
  six 
  myo- 
  

   tomes, 
  and 
  are 
  distributed 
  by 
  their 
  dorsal 
  and 
  ventral 
  branches 
  to 
  those 
  

   myotomes, 
  to 
  the 
  integument, 
  and 
  to 
  the 
  walls 
  of 
  the 
  buccal 
  cavity. 
  

   The 
  foremost 
  two 
  nerves 
  (6 
  and 
  a) 
  pass 
  in 
  front 
  of 
  the 
  first 
  myo- 
  

   tome, 
  and 
  the 
  nerve 
  a 
  runs 
  parallel 
  with 
  the 
  upperside 
  of 
  the 
  noto- 
  

   chord 
  to 
  the 
  end 
  of 
  the 
  snout, 
  giving 
  off 
  branches 
  to 
  that 
  region 
  

   of 
  the 
  body 
  which 
  lies 
  in 
  front 
  of 
  the 
  mouth. 
  This 
  nerve 
  lies 
  above 
  the 
  

   eye-spot. 
  

  

  In 
  the 
  Marsipobranch 
  fishes 
  Myxine 
  and 
  Ammoccetes 
  (now 
  known 
  to 
  

   be 
  a 
  young 
  condition 
  of 
  Petromyzon) 
  a 
  velum 
  also 
  separates 
  the 
  buccal 
  

   from 
  the 
  branchial 
  cavity 
  (figs. 
  B, 
  C, 
  D). 
  But 
  this 
  velum 
  is 
  in 
  connexion 
  

   with 
  the 
  hyoidean 
  arch. 
  The 
  resemblance 
  of 
  the 
  buccal 
  cavity, 
  with 
  its 
  

   tentacles, 
  in 
  Ammoccetes 
  to 
  the 
  corresponding 
  cavity 
  in 
  Amphioxus 
  is 
  so 
  

   close, 
  that 
  there 
  can 
  be 
  no 
  doubt 
  that 
  the 
  two 
  are 
  homologous. 
  In 
  the 
  

   Ammoccetes 
  there 
  is 
  a 
  hyoidean 
  cleft 
  which 
  has 
  hitherto 
  been 
  overlooked. 
  

   The 
  auditory 
  sac 
  lies 
  at 
  the 
  dorsal 
  end 
  of 
  the 
  arch 
  and 
  above 
  the 
  dorsal 
  

   attachment 
  of 
  the 
  velum. 
  The 
  latter, 
  therefore, 
  corresponds 
  with 
  the 
  

   auditory 
  region 
  of 
  the 
  skull, 
  and 
  the 
  nerve 
  h 
  should 
  answer 
  to 
  the 
  last 
  

   of 
  the 
  prseauditory 
  cranial 
  nerves, 
  which 
  is 
  the 
  portio 
  dura. 
  Assuming 
  

   this 
  to 
  be 
  the 
  case, 
  though 
  the 
  detailed 
  homologies 
  of 
  the 
  cranial 
  nerves 
  of 
  

   the 
  higher 
  Vertebrata 
  are 
  yet 
  to 
  be 
  worked 
  out, 
  it 
  follows 
  that 
  the 
  segment 
  

   of 
  the 
  cerebro-spinal 
  axis 
  which 
  in 
  Amphioxus 
  lies 
  between 
  the 
  origin 
  

   of 
  the 
  nerve 
  h 
  and 
  the 
  eye, 
  answers 
  to 
  all 
  that 
  part 
  of 
  the 
  brain 
  which 
  

   lies 
  between 
  the 
  origin 
  of 
  the 
  seventh 
  nerve 
  of 
  Petromyzon 
  and 
  the 
  optic 
  

   nerve. 
  Consequently, 
  the 
  lateral 
  walls 
  of 
  the 
  neural 
  canal 
  in 
  the 
  same 
  

   region 
  answer 
  to 
  that 
  region 
  of 
  the 
  skull 
  in 
  Petromyzon 
  which 
  lies 
  

   between 
  the 
  origin 
  of 
  the 
  seventh 
  and 
  the 
  origin 
  of 
  the 
  optic 
  nerve. 
  

   Hence, 
  as 
  each 
  myotome 
  of 
  Amphioxus 
  represents 
  the 
  corresponding 
  

   portion 
  of 
  a 
  protovertebra, 
  it 
  follows 
  that 
  the 
  same 
  region 
  of 
  the 
  skull 
  

   in 
  the 
  Lamprey 
  and 
  other 
  Vertebrata 
  represents, 
  at 
  fewest, 
  six 
  protover- 
  

   tebrse, 
  almost 
  all 
  traces 
  of 
  which 
  are 
  lost, 
  even 
  in 
  the 
  embryo 
  condition 
  

   of 
  the 
  higher 
  Vertebrata. 
  

  

  It 
  may 
  further 
  be 
  concluded 
  that 
  the 
  several 
  pairs 
  of 
  nerves 
  which 
  

   leave 
  the 
  cerebro-spinal 
  axis, 
  between 
  those 
  which 
  answer 
  to 
  the 
  portio 
  

  

  