﻿ANATOMY 
  AND 
  AFFINITIES 
  OF 
  THE 
  TEOCHIDJl. 
  43 
  

  

  Very 
  small 
  and 
  inconspicuous 
  papillas 
  occur 
  on 
  the 
  margin. 
  

   The 
  mantle 
  completely 
  encircles 
  the 
  body, 
  but 
  the 
  posterior 
  

   portion 
  (fig. 
  40, 
  m. 
  a.) 
  is 
  very 
  small, 
  its 
  margin 
  being 
  thin. 
  

   This 
  part 
  of 
  the 
  mantle 
  is 
  closely 
  attached 
  to 
  the 
  columella 
  

   muscle. 
  

  

  The 
  man 
  tie 
  -cavity 
  is 
  large, 
  and 
  is 
  divided 
  by 
  the 
  gill- 
  

   septum 
  into 
  two 
  chambers, 
  a 
  large 
  right 
  chamber, 
  into 
  which 
  

   the 
  excretory 
  and 
  anal 
  orifices 
  open, 
  and 
  a 
  much 
  smaller 
  left 
  

   (dorsal) 
  one, 
  which 
  encloses 
  the 
  lamellfe 
  of 
  the 
  left 
  side 
  of 
  the 
  

   gill. 
  

  

  The 
  gill 
  (figs. 
  89 
  — 
  43, 
  p.), 
  is 
  characteristically 
  bipecti- 
  

   nate, 
  the 
  gill-axis 
  or 
  septum 
  bearing 
  on 
  either 
  side 
  a 
  series 
  of 
  

   triangular 
  gill-plates 
  or 
  lamella). 
  This 
  septum 
  is 
  attached 
  to 
  

   the 
  mantle-wall 
  along 
  two 
  lines 
  of 
  insertion, 
  on 
  the 
  left 
  side 
  

   the 
  attachment 
  is 
  near 
  the 
  junction 
  between 
  the 
  mantle 
  and 
  

   left 
  body-wall, 
  whilst 
  the 
  other 
  line 
  of 
  insertion 
  of 
  the 
  gill- 
  

   septum 
  is 
  near 
  the 
  mid-line 
  of 
  thereof 
  of 
  the 
  pallia! 
  chamber. 
  

   The 
  gill, 
  and 
  consequently 
  the 
  septum, 
  extends 
  to 
  the 
  posterior 
  

   extremity 
  of 
  the 
  mantle-cavity, 
  thus 
  dividing 
  it 
  into 
  the 
  two 
  

   chambers 
  previously 
  mentioned. 
  The 
  afferent 
  and 
  efferent 
  

   blood-vessels 
  of 
  the 
  gill 
  are 
  situated 
  on 
  the 
  dorsal 
  and 
  ventral 
  

   sides 
  respectively 
  of 
  the 
  gill-septum. 
  

  

  The 
  anterior 
  extremity 
  of 
  the 
  gill 
  is 
  free, 
  and 
  is 
  supported 
  

   by 
  a 
  rod-like 
  structure 
  of 
  cartilaginous 
  consistency. 
  

  

  The 
  gill-lamella 
  are 
  not 
  equally 
  well 
  developed 
  on 
  both 
  

   sides 
  of 
  the 
  septum, 
  those 
  on 
  the 
  inner 
  (left) 
  side 
  are 
  much 
  

   smaller 
  than 
  those 
  on 
  the 
  outer 
  (right). 
  

  

  The 
  microscopic 
  structure 
  of 
  the 
  gill 
  and 
  gill-lamell£e 
  of 
  

   Trochusis 
  so 
  essentially 
  similar 
  to 
  that 
  of 
  Pleurotomaria 
  

   that 
  it 
  will 
  suffice 
  to 
  refer 
  to 
  Woodward's 
  paper 
  on 
  that 
  genus 
  

   (45, 
  pp. 
  223 
  — 
  226) 
  for 
  a 
  detailed 
  account. 
  

  

  The 
  hypobranchial 
  gland 
  occupies 
  the 
  customary 
  

   position 
  between 
  the 
  rectum 
  and 
  afferent 
  border 
  of 
  the 
  gill. 
  

   Various 
  degrees 
  of 
  differentation 
  are 
  presented 
  in 
  the 
  different 
  

   species. 
  In 
  T. 
  cinerarius 
  and 
  T. 
  umbilicatus 
  the 
  gland 
  

   is 
  comparatively 
  small, 
  in 
  T. 
  magus 
  (fig. 
  41, 
  m. 
  g.) 
  it 
  is 
  much 
  

   better 
  developed, 
  and 
  the 
  glandular 
  tissue 
  covers 
  the 
  trans- 
  

  

  