﻿1874.] 
  and 
  Skull 
  of 
  Amphioxus 
  lanceolatus. 
  131 
  

  

  segment, 
  it 
  would 
  appear 
  that 
  this 
  segment 
  is, 
  in 
  the 
  main, 
  the 
  result 
  

   of 
  the 
  chondrification, 
  with 
  or 
  without 
  subsequent 
  ossification, 
  of 
  the 
  four- 
  

   teenth 
  protovertebra. 
  

  

  There 
  is 
  no 
  evidence, 
  at 
  present, 
  that 
  the 
  ear-capsule 
  represents 
  a 
  

   modification 
  of 
  any 
  part 
  of 
  the 
  vertebral 
  skeleton, 
  nor 
  that 
  the 
  trabecule 
  

   are 
  any 
  thing 
  but 
  an 
  anterior 
  pair 
  of 
  visceral 
  arches. 
  And 
  if 
  these 
  parts 
  

   have 
  nothing 
  to 
  do 
  with 
  centra, 
  or 
  arches, 
  of 
  vertebrae, 
  it 
  follows 
  that 
  

   the 
  numerous 
  protovertebrse, 
  which 
  lie 
  in 
  front 
  of 
  the 
  fourteenth 
  in 
  

   Amphioxus, 
  are 
  represented 
  only 
  by 
  muscles 
  and 
  nerves 
  in 
  the 
  higher 
  

   Vertebrata. 
  

  

  The 
  anterior 
  end 
  of 
  the 
  cerebro-spinal 
  axis 
  of 
  Amphioxus 
  answers 
  

   to 
  the 
  lamina 
  terminalis 
  of 
  the 
  thalamencephalon 
  of 
  the 
  higher 
  Verte- 
  

   brata, 
  the 
  cerebral 
  hemispheres 
  and 
  olfactory 
  lobes 
  remaining 
  unde- 
  

   veloped. 
  

  

  If 
  the 
  auditory 
  nerve 
  is, 
  as 
  Gegenbaur 
  has 
  suggested, 
  the 
  dorsal 
  branch 
  

   of 
  a 
  single 
  nerve 
  which 
  represents 
  both 
  the 
  portio 
  dura 
  and 
  the 
  portio 
  

   mollis, 
  the 
  auditory 
  organ 
  of 
  Amphioxus 
  is 
  to 
  be 
  sought 
  in 
  connexion 
  with 
  

   the 
  dorsal 
  branch 
  of 
  its 
  eighth 
  nerve. 
  I 
  have 
  found 
  nothing 
  representing 
  

   an 
  auditory 
  organ 
  in 
  this 
  position 
  ; 
  and 
  I 
  can 
  only 
  conclude 
  that 
  Amphioxus 
  

   really 
  has 
  no 
  auditory 
  apparatus. 
  In 
  all 
  other 
  respects, 
  however, 
  it 
  con- 
  

   forms 
  to 
  the 
  Vertebrate 
  type 
  ; 
  and, 
  considering 
  its 
  resemblance 
  to 
  the 
  

   early 
  stages 
  of 
  Petromyzon 
  described 
  by 
  Schulze, 
  I 
  can 
  see 
  no 
  reason 
  

   for 
  removing 
  it 
  from 
  the 
  class 
  Pisces. 
  But 
  its 
  permanently 
  segmented 
  

   skull 
  and 
  its 
  many 
  other 
  peculiarities 
  suggest 
  that 
  it 
  should 
  be 
  regarded 
  

   as 
  the 
  type 
  of 
  a 
  primary 
  division 
  or 
  subclass 
  of 
  the 
  class 
  Pisces, 
  to 
  which 
  

   the 
  name 
  of 
  Entomocrania 
  may 
  be 
  applied, 
  in 
  contrast 
  to 
  the 
  rest, 
  in 
  

   which 
  the 
  primary 
  segmentation 
  of 
  the 
  skull 
  is 
  lost, 
  and 
  which 
  may 
  be 
  

   termed 
  Holocrania. 
  On 
  a 
  future 
  occasion 
  I 
  propose 
  to 
  show 
  in 
  what 
  

   manner 
  the 
  skull 
  of 
  the 
  Marsipobranch 
  is 
  related 
  to 
  that 
  of 
  the 
  higher 
  

   Vertebrata, 
  and 
  more 
  especially 
  to 
  the 
  skull 
  of 
  the 
  Prog 
  in 
  its 
  youug 
  

   tadpole 
  state. 
  

  

  EXPLANATION 
  OF 
  THE 
  FIGURES. 
  

  

  A, 
  C, 
  D 
  are 
  diagrammatic, 
  but 
  accurate, 
  representations 
  of 
  the 
  anterior 
  part 
  of 
  the 
  

   body 
  in 
  Amphioxus 
  (A), 
  in 
  an 
  Ammocozte 
  1*6 
  inch 
  long 
  (C), 
  and 
  in 
  a 
  fully 
  

   grown 
  Ammocoefe 
  5*7 
  inches 
  long 
  (D). 
  B 
  is 
  a 
  copy 
  of 
  the 
  furthest 
  advanced 
  stage 
  

   of 
  the 
  young 
  Petrcmyzon 
  planeri 
  six 
  weeks 
  after 
  hatching, 
  as 
  figured 
  by 
  Schultze 
  

   in 
  his 
  memoir 
  on 
  the 
  development 
  of 
  that 
  fish. 
  The 
  figures 
  are 
  magnified 
  to 
  the 
  

   same 
  vertical 
  dimension, 
  so 
  as 
  to 
  afford 
  a 
  means 
  of 
  estimating, 
  roughly, 
  the 
  changes 
  

   in 
  the 
  proportional 
  growth 
  of 
  the 
  various 
  parts 
  of 
  the 
  head 
  of 
  the 
  Lamprey 
  in 
  its 
  

   progress 
  from 
  the 
  embryonic 
  towards 
  the 
  adult 
  condition. 
  In 
  C, 
  the 
  brain 
  is 
  already 
  

   differentiated 
  into 
  the 
  three 
  primary 
  vesicles 
  and 
  the 
  vesicles 
  of 
  the 
  cerebral 
  hemi- 
  

   spheres, 
  though 
  they 
  are 
  not 
  shown, 
  the 
  whole 
  brain 
  being 
  merely 
  indicated 
  by 
  the 
  

   dark 
  shading. 
  The 
  trabecule 
  (Tr), 
  which 
  have 
  already 
  united 
  in 
  front, 
  are 
  indi- 
  

   cated, 
  but 
  not 
  the 
  semilunar 
  ethmoidal 
  cartilage, 
  which 
  lies 
  above 
  and 
  behind 
  the 
  

  

  