﻿428 
  Prof. 
  M'Intosh 
  on 
  the 
  

  

  second 
  chamber 
  is 
  formed 
  by 
  a 
  loop 
  which 
  adjoins 
  the 
  lateral 
  

   and 
  inferior 
  part 
  of 
  the 
  former 
  and 
  surrounds 
  the 
  tip 
  of 
  the 
  

   blood-vessels, 
  as 
  well 
  as 
  inferiorly 
  the 
  origin 
  of 
  the 
  lateral 
  

   muscles. 
  It 
  is 
  from 
  this 
  anterior 
  loop 
  that 
  the 
  chitinous 
  

   lamella 
  of 
  the 
  lateral 
  expansion 
  originates. 
  By 
  the 
  gradual 
  

   coalescence 
  of 
  the 
  inner 
  margins 
  of 
  the 
  chitinous 
  rings 
  of 
  

   the 
  median 
  muscles 
  their 
  outline 
  in 
  transverse 
  section 
  

   resembles 
  a 
  figure 
  of 
  eight; 
  and 
  the 
  lateral 
  loop 
  forms 
  quite 
  

   as 
  large 
  a 
  chamber 
  appended 
  to 
  each 
  side. 
  Coalescence 
  of 
  

   the 
  chitinous 
  basement-tissue 
  goes 
  on 
  rapidly, 
  so 
  that 
  a 
  

   figure 
  resembling 
  a 
  crown 
  is 
  next 
  formed 
  by 
  the 
  investment 
  

   of 
  the 
  central 
  muscles. 
  Dorsally 
  is 
  a 
  gently 
  convex 
  arch, 
  

   which 
  dips 
  downward 
  at 
  the 
  median 
  line 
  to 
  join 
  the 
  strong 
  

   vertical 
  partition, 
  and 
  curves 
  downward 
  externally 
  at 
  the 
  

   point 
  of 
  junction 
  with 
  the 
  outer 
  wall 
  of 
  the 
  external 
  chamber 
  

   and 
  the 
  intermediate 
  septum. 
  The 
  latter 
  passes 
  downward 
  

   and 
  inward 
  to 
  meet 
  the 
  short 
  ventral 
  (chitinous) 
  floor, 
  which 
  

   in 
  the 
  preparations 
  is 
  slightly 
  drawn 
  upward, 
  where 
  it 
  unites 
  

   with 
  the 
  vertical 
  septum. 
  The 
  external 
  loop 
  is 
  large 
  and 
  

   rounded, 
  but 
  is 
  thinner 
  than 
  the 
  investment 
  of 
  the 
  central 
  

   muscle. 
  It 
  joins 
  the 
  transverse 
  floor 
  at 
  the 
  junction 
  of 
  the 
  

   external 
  septum, 
  and 
  on 
  each 
  side 
  encloses 
  the 
  external 
  

   muscle 
  and 
  the 
  special 
  space 
  for 
  the 
  blood-vessels, 
  which 
  lie 
  

   on 
  the 
  septum 
  between 
  it 
  and 
  the 
  inner 
  chamber. 
  The 
  

   crown-like 
  arrangement 
  of 
  the 
  chitinous 
  basement-tissue 
  is 
  

   retained 
  throughout 
  the 
  greater 
  part 
  of 
  the 
  snout, 
  though, 
  by 
  

   the 
  increase 
  of 
  the 
  lateral 
  chamber 
  and 
  the 
  change 
  in 
  position 
  

   of 
  the 
  vascular 
  area, 
  the 
  appearances 
  around 
  it 
  are 
  somewhat 
  

   altered. 
  The 
  median 
  and 
  the 
  lateral 
  septa 
  and. 
  the 
  transverse 
  

   ventral 
  plate 
  are 
  the 
  strongest 
  portions 
  of 
  the 
  chitinous 
  

   apparatus. 
  The 
  investment 
  of 
  the 
  outer 
  compartment 
  and 
  

   the 
  loop 
  enclosing 
  the 
  vascular 
  area 
  are 
  thin. 
  

  

  When 
  the 
  ventral 
  hypodermic 
  tissue 
  in 
  front 
  of 
  the 
  mouth 
  

   attains 
  considerable 
  depth, 
  the 
  median 
  sulcus 
  of 
  the 
  chitinous 
  

   crown 
  superiorly 
  is 
  much 
  increased 
  by 
  the 
  shortening 
  of 
  the 
  

   septum 
  ; 
  and 
  by-aud-by 
  the 
  figure-of-eight 
  shape 
  is 
  again 
  

   assumed, 
  the 
  lateral 
  septum 
  being 
  almost 
  horizontal 
  

   throughout 
  the 
  middle 
  third, 
  over 
  the 
  vascular 
  area, 
  which 
  

   now 
  projects 
  inferiorly 
  in 
  its 
  special 
  chitinous 
  investment. 
  

   Both 
  chambers 
  are 
  sensibly 
  diminished, 
  and 
  finally 
  the 
  

   median 
  separate 
  to 
  form 
  with 
  the 
  lateral 
  a 
  figure 
  of 
  eight 
  in 
  

   transverse 
  section 
  on 
  each 
  side 
  of 
  the 
  central 
  hypoderm. 
  

   The 
  chitinous 
  ring 
  for 
  the 
  central 
  muscle 
  is 
  irregularly 
  round 
  

   and 
  has 
  a 
  firm 
  loop 
  for 
  the 
  vessels 
  attached 
  inferiorly, 
  while 
  

   the 
  outer 
  still 
  gives 
  off 
  the 
  lateral 
  lamella. 
  Then 
  the 
  

   chitinous 
  investment 
  of 
  the 
  vessels 
  moves 
  to 
  the 
  inner 
  and 
  

  

  