﻿116 
  E. 
  J. 
  ALLEN. 
  

  

  mouth 
  to 
  the 
  posterior 
  septum 
  of 
  the 
  eighth 
  segment 
  may 
  be 
  

   conveniently 
  divided 
  into 
  two 
  parts 
  corresponding 
  to 
  what 
  

   are 
  known 
  in 
  other 
  Po]ycha3tes 
  as 
  oesophagus 
  and 
  pharynx 
  

   (or 
  gizzard). 
  There 
  is, 
  however, 
  no 
  definite 
  line 
  of 
  demarca- 
  

   tion 
  between 
  these 
  two 
  parts. 
  The 
  oesophagus 
  is 
  lined 
  by 
  

   elongated, 
  ciliated 
  epithelial 
  cells, 
  outside 
  which 
  is 
  a 
  thin 
  

   layer 
  of 
  annular 
  muscles 
  followed 
  by 
  a 
  thin 
  layer 
  of 
  longi- 
  

   tudinal 
  ones. 
  Proceeding 
  further 
  backwards 
  the 
  epithelial 
  

   layer 
  becomes 
  narroAver, 
  the 
  cells 
  being 
  considerably 
  less 
  

   elongated, 
  whilst, 
  on 
  the 
  other 
  hand, 
  the 
  muscular 
  layers, 
  

   especially 
  the 
  layer 
  of 
  annular 
  muscles, 
  become 
  much 
  more 
  

   strongly 
  developed 
  (cf. 
  figs. 
  42 
  and 
  48 
  with 
  fig. 
  43). 
  It 
  

   is 
  this 
  muscular 
  portion 
  which 
  may 
  be 
  termed 
  the 
  pharyux 
  

   (ph.) 
  In 
  its 
  hinder 
  part 
  the 
  epithelium 
  is 
  thrown 
  into 
  folds 
  

   or 
  villi, 
  and 
  at 
  the 
  point 
  where 
  the 
  septum 
  of 
  segment 
  8 
  is 
  

   attached 
  to 
  the 
  alimentary 
  canal 
  one 
  large 
  fold, 
  forming 
  a 
  

   kind 
  of 
  valve 
  (fig. 
  43, 
  v.) 
  seems 
  to 
  constitute 
  a 
  definite 
  line 
  of 
  

   demarcation 
  between 
  the 
  pharynx 
  and 
  the 
  intestine. 
  As 
  is 
  

   explained 
  on 
  p. 
  123, 
  the 
  septa 
  in 
  this 
  region 
  of 
  the 
  body 
  are 
  

   pushed 
  very 
  much 
  backwards 
  ; 
  the 
  junction 
  of 
  the 
  pharynx 
  

   and 
  intestine, 
  although 
  really 
  at 
  the 
  posterior 
  end 
  of 
  the 
  eighth 
  

   segment, 
  may 
  lie 
  as 
  far 
  back 
  as 
  the 
  level 
  of 
  the 
  twelfth 
  

   parapodia. 
  From 
  the 
  posterior 
  septum 
  of 
  segment 
  8 
  to 
  the 
  

   posterior 
  septum 
  of 
  segment 
  13, 
  the 
  intestine 
  continues 
  as 
  a 
  

   comparatively 
  straight 
  tube, 
  not 
  diifering 
  much 
  in 
  structure 
  

   from 
  the 
  pharjnx, 
  excepting 
  that 
  the 
  muscular 
  layers 
  are 
  

   rapidly 
  reduced 
  until 
  they 
  almost 
  entirely 
  disappear 
  (figs. 
  43 
  

   and 
  50). 
  In 
  segments 
  14 
  and 
  15 
  the 
  intestine 
  is 
  con- 
  

   siderably 
  dilated, 
  but 
  narrows 
  again 
  as 
  it 
  passes 
  through 
  

   each 
  septum. 
  In 
  the 
  segments 
  from 
  16 
  backwards, 
  this 
  

   enlargement 
  of 
  the 
  intestine 
  in 
  each 
  segment 
  becomes 
  very 
  

   great, 
  so 
  that 
  the 
  dilated 
  intestine 
  occupies 
  a 
  large 
  part 
  of 
  

   the 
  body-cavity 
  (figs. 
  1 
  and 
  58). 
  In 
  the 
  living 
  worm 
  these 
  

   intestinal 
  pouches 
  are 
  constantly 
  expanding 
  and 
  contracting, 
  

   the 
  movements 
  of 
  the 
  intestine 
  constituting, 
  in 
  the 
  posterior 
  

   region 
  of 
  the 
  body, 
  the 
  principal 
  mode 
  of 
  circulation 
  of 
  the 
  

   blood. 
  

  

  