﻿438 
  Trof. 
  M'lntosli 
  on 
  the 
  

  

  ward 
  and 
  inward. 
  The 
  foregoing 
  arrangement 
  tlras 
  holds 
  

   the 
  whole 
  body-cavity 
  in 
  control, 
  and, 
  like 
  the 
  complex 
  

   muscles 
  of 
  the 
  vertebrate 
  heart, 
  must 
  materially 
  aid 
  the 
  

   blood-vessels 
  in 
  propelling 
  the 
  blood 
  forward 
  into 
  the 
  

   anterior 
  region 
  and 
  in 
  keeping 
  it 
  there 
  — 
  during 
  the 
  con- 
  

   traction 
  of 
  the 
  other 
  muscles 
  of 
  the 
  part 
  — 
  by 
  firm 
  closure 
  of 
  

   the 
  channel 
  of 
  communication. 
  Its 
  importance, 
  therefore, 
  

   in 
  regard 
  to 
  the 
  functions 
  of 
  the 
  proboscis 
  is 
  apparent. 
  

  

  With 
  the 
  diminution 
  and 
  atrophy 
  of 
  the 
  transverse 
  

   ventral 
  muscles 
  over 
  the 
  nerve-cords 
  in 
  the 
  posterior 
  region, 
  

   the 
  vertical 
  and 
  oblique 
  also 
  coincide, 
  so 
  that 
  by-and-by 
  

   only 
  a 
  few 
  slender 
  fasciculi 
  of 
  each 
  remain 
  — 
  stretched 
  from 
  

   the 
  dorsal 
  raphe 
  in 
  the 
  case 
  of 
  the 
  vertical 
  and 
  the 
  lateral 
  

   wall 
  in 
  the 
  oblique, 
  to 
  the 
  atrophied 
  transverse 
  on 
  each 
  side 
  

   of 
  the 
  ventral 
  blood-vessel. 
  Toward 
  the 
  extremity, 
  again, 
  

   both 
  become 
  well 
  developed, 
  the 
  oblique 
  spreading 
  from 
  

   the 
  outer 
  part 
  of 
  the 
  arch 
  over 
  the 
  nerve-cord 
  upward 
  and 
  

   outward 
  in 
  a 
  fan-like 
  manner 
  to 
  the 
  body-wall 
  ; 
  while 
  the 
  

   vertical 
  extend 
  from 
  the 
  median 
  part 
  of 
  the 
  arch 
  outward 
  

   and 
  then 
  upward 
  to 
  the 
  exterior 
  of 
  the 
  dorsal 
  longitudinal 
  

   muscles. 
  

  

  As 
  soon 
  as 
  the 
  under 
  lip 
  of 
  the 
  animal 
  becomes 
  continuous 
  

   beneath 
  the 
  extruded 
  proboscis, 
  a 
  series 
  of 
  transverse 
  fibres 
  

   pass 
  — 
  from 
  the 
  raphe 
  at 
  the 
  inner 
  side 
  of 
  each 
  longitudinal 
  

   ventral 
  muscle 
  — 
  quite 
  across 
  the 
  body, 
  and 
  thus 
  form 
  a 
  very 
  

   efficient 
  expulsive 
  system, 
  as 
  well 
  as 
  a 
  barrier 
  to 
  return 
  after 
  

   extrusion. 
  It 
  is 
  probably 
  this 
  muscle 
  which 
  is 
  observed 
  in 
  

   the 
  living 
  animal, 
  after 
  expulsion 
  of 
  the 
  proboscis, 
  stretched 
  

   as 
  a 
  very 
  strong 
  transverse 
  band 
  or 
  arch 
  between 
  the 
  bases 
  

   of 
  the 
  tentacles, 
  apparently 
  blocking 
  (with 
  the 
  pre-oral, 
  

   transverse 
  and 
  vertical) 
  the 
  passage 
  of 
  blood 
  forward 
  into 
  

   the 
  cephalic 
  contractile 
  space, 
  and 
  forming 
  an 
  abutment 
  

   against 
  the 
  posterior 
  part 
  of 
  the 
  vascular 
  sinus 
  behind 
  the 
  

   triangular 
  region. 
  When 
  the 
  animal 
  is 
  viewed 
  from 
  the 
  

   ventral 
  surface 
  (the 
  proboscis 
  being 
  withdrawn) 
  the 
  fibres 
  

   pass 
  right 
  across 
  the 
  body-wall 
  in 
  front 
  of 
  the 
  first 
  bristle- 
  

   bundles. 
  Behind 
  the 
  latter 
  they 
  become 
  somewhat 
  indistinct, 
  

   but 
  at 
  the 
  third 
  pair 
  they 
  are 
  very 
  evident 
  — 
  extending 
  in- 
  

   ward 
  and 
  slightly 
  backward 
  to 
  the 
  central 
  raphe. 
  They 
  are 
  

   broad 
  in 
  front 
  and 
  gradually 
  diminish 
  toward 
  the 
  posterior 
  

   end 
  of 
  the 
  region. 
  This 
  ventral 
  transverse 
  muscle 
  continues 
  

   for 
  some 
  distance 
  backward 
  as 
  a 
  powerful 
  layer, 
  and 
  then 
  

   presents 
  a 
  chitinous 
  raphe 
  in 
  the 
  median 
  line, 
  so 
  as 
  to 
  form 
  

   two 
  muscles. 
  Throughout 
  the 
  anterior 
  region 
  the 
  latter 
  

   stretch 
  from 
  the 
  common 
  raphe 
  externally 
  to 
  the 
  ventral 
  

   median 
  line, 
  under 
  the 
  hypoderm, 
  and 
  they 
  are 
  sometimes 
  

  

  