﻿266 
  

  

  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  

  

  while 
  the 
  posterior 
  border 
  is 
  spaced 
  behind 
  the 
  third 
  pair 
  a 
  distance 
  

   about 
  equal 
  to 
  interval 
  between 
  second 
  and 
  third 
  pairs. 
  This 
  zone 
  

   contains 
  the 
  slime-net 
  glands. 
  ''Three 
  to 
  10 
  or 
  even 
  more 
  glands 
  

   are 
  located 
  on 
  the 
  outer 
  surface 
  and 
  in 
  the 
  outer 
  part 
  of 
  the 
  sides 
  of 
  

   the 
  papillae. 
  These 
  glands 
  develop 
  from 
  the 
  surface 
  epithelium 
  

   and 
  are 
  formed 
  by 
  an 
  invagination 
  of 
  this 
  layer 
  of 
  cells 
  and 
  the 
  out- 
  

   lying 
  cuticle. 
  The 
  gland 
  cells 
  develop 
  numerous 
  terminal 
  cones 
  

   which 
  penetrate 
  into 
  the 
  cuticle 
  of 
  the 
  duct 
  (formed 
  by 
  invagination) 
  . 
  

   During 
  the 
  secretion 
  of 
  any 
  one 
  slime-net 
  only 
  about 
  one-eighth 
  of 
  

   the 
  gland 
  cells 
  are 
  active. 
  In 
  these 
  cells 
  the 
  terminal 
  cones 
  perforate 
  

   the 
  cuticle 
  and 
  open 
  into 
  the 
  duct 
  as 
  minute 
  tubules 
  from 
  which 
  the 
  

   fibrous 
  secretion 
  which 
  forms 
  the 
  slime-net 
  is 
  discharged. 
  The 
  net 
  

  

  Figure 
  17. 
  — 
  Urechis 
  caupo 
  Fisher 
  and 
  MacGintie: 
  A 
  vertical 
  section 
  of 
  a 
  mature 
  slime- 
  

   net 
  gland 
  taken 
  while 
  actively 
  secreting; 
  drawing 
  by 
  W. 
  W. 
  Newby 
  (1941). 
  

  

  