﻿ANATOMY 
  AND 
  AFFINITIES 
  OF 
  THE 
  TROCHID.E. 
  55 
  

  

  plasmatic 
  cells 
  of 
  Brock 
  (9); 
  and 
  appear 
  to 
  be 
  of 
  common 
  

   occurrence 
  in 
  the 
  connective 
  tissue 
  of 
  Gasteropods. 
  

  

  The 
  intestine 
  either 
  leads 
  directly 
  out 
  of 
  the 
  anterior 
  or 
  

   intestinal 
  chamber 
  of 
  the 
  stomach 
  without 
  becoming 
  folded 
  

   upon 
  itself 
  as 
  in 
  T. 
  zizyphinus, 
  or 
  it 
  recurves 
  and 
  crosses 
  

   over 
  the 
  stomach 
  as 
  in 
  T. 
  lineatus 
  (figs. 
  40, 
  45, 
  int.); 
  

   becoming 
  folded 
  upon 
  itself 
  several 
  times, 
  it 
  then 
  runs 
  

   forward 
  to 
  about 
  the 
  level 
  of 
  the 
  terminal 
  portion 
  of 
  the 
  

   radula 
  sac, 
  where, 
  bending 
  on 
  itself 
  to 
  form 
  a 
  y-shaped 
  loop, 
  

   it 
  retraces 
  its 
  course 
  towards 
  the 
  posterior 
  end 
  of 
  the 
  body, 
  

   and 
  on 
  reaching 
  the 
  level 
  of 
  the 
  pericardium 
  curves 
  dorsally 
  

   and 
  horizontally, 
  entering 
  the 
  pericardium 
  and 
  penetrating 
  

   the 
  ventricle. 
  After 
  emerging 
  from 
  the 
  pericardium 
  it 
  again 
  

   curves, 
  and 
  entering 
  the 
  mantle-cavity 
  runs 
  along 
  the 
  roof 
  of 
  

   that 
  structure 
  towards 
  the 
  anterior 
  end 
  of 
  the 
  body, 
  debouch- 
  

   ing 
  into 
  the 
  mantle-cavity 
  by 
  the 
  anus, 
  which 
  is 
  situated 
  near 
  

   the 
  middle 
  line. 
  The 
  terminal 
  portion 
  of 
  the 
  rectum 
  (/'.) 
  is 
  

   enveloped 
  by 
  the 
  hypobranchial 
  gland 
  {m.g.). 
  

  

  The 
  Vascular 
  System. 
  — 
  The 
  heart 
  (figs. 
  39, 
  47) 
  is 
  

   enclosed 
  within 
  a 
  large 
  pericardium, 
  which 
  is 
  situated 
  at 
  the 
  

   distal 
  end 
  of 
  the 
  mantle-cavity, 
  abuts 
  on 
  the 
  left 
  kidney, 
  

   and 
  is 
  bounded 
  on 
  its 
  posterior 
  border 
  by 
  the 
  right 
  kidney 
  and 
  

   stomach. 
  The 
  ventricle 
  {v.) 
  is 
  traversed 
  by 
  the 
  rectum 
  and 
  

   is 
  very 
  muscular. 
  It 
  is 
  situated 
  nearly 
  transversely, 
  passing 
  

   from 
  right 
  to 
  left 
  of 
  the 
  body 
  ; 
  on 
  the 
  left 
  side 
  the 
  ventricle 
  

   is 
  enlarged 
  into 
  a 
  bulbous 
  structure, 
  the 
  aortic 
  bnlb, 
  from 
  

   which 
  arise 
  two 
  large 
  arteries, 
  the 
  posterior 
  and 
  anterior 
  

   aortae. 
  Communicating 
  with 
  the 
  ventricle 
  are 
  two 
  thin-walled 
  

   auricles; 
  of 
  these 
  the 
  left 
  [1. 
  au.) 
  is 
  the 
  larger, 
  and 
  is 
  

   situated 
  in 
  the 
  anterior 
  portion 
  of 
  the 
  pericardium 
  ; 
  the 
  

   right 
  auricle 
  {r. 
  an.) 
  is 
  situated 
  in 
  the 
  posterior 
  region 
  of 
  the 
  

   pericardium, 
  and, 
  though 
  of 
  smaller 
  calibre 
  than 
  the 
  left, 
  is 
  

   much 
  longer. 
  The 
  walls 
  of 
  both 
  right 
  and 
  left 
  auricles 
  are 
  

   very 
  thin, 
  and 
  are 
  produced 
  into 
  numerous 
  fringe-like 
  

   processes 
  which, 
  when 
  examined 
  microscopically, 
  are 
  seen 
  to 
  

   be 
  clothed 
  with 
  numerous 
  large 
  epithelial 
  cells 
  (fig. 
  38), 
  each 
  

   containing 
  a 
  large 
  round 
  nucleus 
  and 
  protoplasm 
  having 
  

  

  