﻿252 
  

  

  Prof. 
  Allman 
  on 
  the 
  Structure 
  

  

  [Feb. 
  11, 
  

  

  their 
  narrow 
  stalk-like 
  portion, 
  the 
  condition 
  of 
  the 
  endoderm 
  departs 
  

   widely 
  from 
  that 
  of 
  this 
  tissue 
  in 
  the 
  tentacles 
  of 
  other 
  marine 
  hydroids 
  ; 
  

   for 
  it 
  presents 
  no 
  trace 
  of 
  the 
  septate 
  disposition 
  so 
  well 
  marked 
  in 
  these. 
  

   It 
  is, 
  on 
  the 
  contrary, 
  composed 
  of 
  a 
  layer 
  of 
  small 
  cells 
  loaded 
  with 
  

   opaque 
  granules 
  and 
  surrounding 
  a 
  continuous 
  wide 
  axile 
  cavity. 
  

  

  It 
  is, 
  however, 
  in 
  the 
  terminal 
  capitulum 
  of 
  the 
  tentacle 
  that 
  the 
  

   structure 
  of 
  these 
  organs 
  departs 
  most 
  widely 
  from 
  any 
  thing 
  that 
  has 
  

   as 
  yet 
  been 
  recognized 
  in 
  the 
  tentacles 
  of 
  other 
  hydroids. 
  Here 
  a 
  very 
  

   peculiar 
  tissue 
  is 
  developed 
  between 
  the 
  muscular 
  layer 
  and 
  the 
  proper 
  

   ectoderm, 
  where 
  it 
  takes 
  the 
  place 
  of 
  the 
  zone 
  of 
  claviform 
  tissue. 
  It 
  

   forms 
  a 
  thick 
  hemispherical 
  cap 
  over 
  the 
  muscular 
  lamella 
  and 
  endoderm 
  

   of 
  the 
  tentacle, 
  and 
  is 
  composed 
  of 
  closely 
  applied 
  exceedingly 
  slender 
  

   prisms, 
  with 
  their 
  inner 
  ends 
  resting 
  on 
  the 
  muscular 
  lamella, 
  to 
  which 
  

   the 
  prisms 
  are 
  perpendicular, 
  the 
  whole 
  structure 
  forcibly 
  suggesting 
  the 
  

   rod-like 
  tissue 
  associated 
  with 
  special 
  sense-apparatus 
  in 
  higher 
  animals. 
  

   It 
  appears 
  to 
  be 
  but 
  a 
  modification 
  of 
  the 
  tissue 
  which 
  elsewhere 
  forms 
  

   the 
  zoue 
  of 
  claviform 
  tissue. 
  

  

  Extending 
  in 
  a 
  radiating 
  direction 
  from 
  the 
  convex 
  surface 
  of 
  this 
  rod- 
  

   like 
  tissue, 
  towards 
  the 
  external 
  surface 
  ef 
  the 
  tentacle, 
  may 
  be 
  seen 
  

   numerous 
  firm 
  filaments, 
  each 
  of 
  which, 
  making 
  its 
  way 
  among 
  the 
  

   cells 
  of 
  the 
  ectoderm, 
  terminates 
  distally 
  in 
  a 
  very 
  delicate 
  transparent 
  

   oviform 
  sac, 
  which 
  carries, 
  near 
  its 
  distal 
  end, 
  a 
  minute 
  styliform 
  

   process. 
  "Within 
  this 
  sac, 
  and 
  completely 
  filling 
  it, 
  is 
  an 
  oviform 
  capsule 
  

   with 
  firm 
  transparent 
  walls, 
  and 
  having 
  immersed 
  in 
  its 
  clear 
  refringent 
  

   contents 
  a 
  cylindrical 
  cord 
  wound 
  upon 
  itself 
  in 
  two 
  or 
  three 
  coils. 
  

   Under 
  pressure, 
  the 
  contained 
  cord 
  may 
  be 
  sometimes 
  forced 
  out 
  through 
  

   the 
  smaller 
  or 
  distal 
  end 
  of 
  the 
  capsule. 
  Notwithstanding 
  the 
  obvious 
  

   resemblance 
  of 
  these 
  bodies 
  to 
  thread-cells, 
  their 
  significance 
  is, 
  without 
  

   doubt, 
  something 
  entirely 
  different. 
  Indeed 
  their 
  resemblance 
  to 
  the 
  

   Pacinian 
  bodies 
  of 
  Yertebrata 
  is 
  too 
  strong 
  to 
  be 
  overlooked. 
  Their 
  as- 
  

   semblage 
  constitutes 
  a 
  zone 
  parallel 
  to 
  the 
  spherical 
  surface 
  of 
  the 
  capi- 
  

   tulum, 
  and 
  lying 
  at 
  a 
  slight 
  distance 
  within 
  it. 
  Though 
  it 
  is 
  impossible 
  

   to 
  assign 
  to 
  them, 
  with 
  certainty, 
  their 
  exact 
  function, 
  we 
  feel 
  compelled 
  

   to 
  regard 
  the 
  whole 
  system, 
  including 
  the 
  bacillar 
  tissue 
  to 
  which 
  their 
  

   stalks 
  can 
  be 
  traced 
  (and 
  which 
  is 
  only 
  a 
  locally 
  modified 
  portion 
  of 
  the 
  

   nervous 
  zone, 
  or 
  zone 
  of 
  claviform 
  tissue), 
  as 
  an 
  apparatus 
  of 
  sense. 
  It 
  

   would 
  almost 
  seem 
  to 
  represent 
  a 
  form 
  of 
  sense-organ, 
  in 
  which 
  sight 
  and 
  

   t 
  ouch 
  show 
  themselves 
  in 
  one 
  of 
  their 
  earliest 
  phylogenetic 
  stages, 
  in 
  which 
  

   they 
  have 
  not 
  yet 
  become 
  fully 
  differentiated 
  from 
  one 
  another. 
  This 
  is 
  

   the 
  only 
  known 
  instance 
  of 
  the 
  existence 
  in 
  a 
  hydriod 
  trophosome 
  of 
  any 
  

   thing 
  which 
  may 
  with 
  fair 
  reason 
  be 
  regarded 
  as 
  a 
  special 
  apparatus 
  of 
  sense. 
  

  

  The 
  male 
  and 
  female 
  sporosacs 
  are 
  borne 
  by 
  the 
  same 
  trophosome. 
  

  

  The 
  generative 
  elements, 
  whether 
  male 
  or 
  female, 
  originate 
  in 
  a 
  special 
  

   cavity 
  (gonogenetic 
  chamber), 
  which 
  is 
  formed 
  in 
  the 
  substance 
  of 
  the 
  

   endoderm 
  of 
  the 
  sporosac. 
  

  

  