﻿FRESH-WATER 
  TRICLADS 
  OF 
  ALASKA 
  — 
  KENT 
  175 
  

  

  laterad 
  to 
  the 
  eyes. 
  A 
  pair 
  of 
  indistinct, 
  converging 
  streaks 
  of 
  lighter 
  

   coloration 
  may 
  run 
  posteriorly 
  from 
  the 
  lateral 
  angles 
  of 
  the 
  head. 
  

  

  The 
  ventral 
  side 
  is 
  light 
  gray. 
  

  

  The 
  pharynx 
  is 
  situated, 
  in 
  mature 
  animals, 
  behind 
  the 
  middle 
  of 
  

   the 
  body; 
  the 
  mouth, 
  at 
  the 
  beginning 
  of 
  the 
  last 
  third 
  of 
  the 
  body; 
  

   and 
  the 
  genital 
  aperture, 
  midway 
  between 
  the 
  mouth 
  and 
  the 
  hind 
  end. 
  

  

  The 
  normal 
  locomotion 
  of 
  the 
  animal 
  is 
  quiet 
  gliding. 
  When 
  dis- 
  

   turbed, 
  it 
  may 
  attach 
  itself 
  to 
  the 
  substratum 
  (apparently 
  with 
  the 
  

   marginal 
  adhesive 
  zones) 
  or 
  move 
  by 
  "crawling." 
  Young 
  specimens 
  

   do 
  not 
  crawl 
  as 
  readily 
  as 
  do 
  adult 
  ones. 
  

  

  The 
  grasping 
  organ, 
  or 
  sucker, 
  in 
  living 
  animals, 
  appears 
  as 
  a 
  well- 
  

   marked 
  bulge 
  on 
  the 
  frontal 
  margin 
  of 
  the 
  head, 
  showing 
  a 
  concave 
  

   ventral 
  depression. 
  In 
  preserved 
  specimens, 
  the 
  frontal 
  margin 
  is 
  

   generally 
  curved 
  ventrally 
  and 
  the 
  site 
  of 
  the 
  organ 
  forms 
  a 
  thick, 
  

   grooved 
  rim. 
  Anatomically, 
  the 
  organ 
  consists 
  of 
  glandular 
  and 
  

   muscular 
  elements. 
  The 
  subterminal 
  adhesive 
  surface 
  is 
  covered 
  with 
  

   an 
  epithelium 
  devoid 
  of 
  rhabdites 
  and 
  pierced 
  by 
  numerous 
  gland 
  

   ducts 
  filled 
  with 
  a 
  granular, 
  eosinophilic 
  secretion. 
  The 
  cell 
  bodies 
  

   of 
  the 
  glands 
  are 
  scattered 
  through 
  the 
  mesenchyme 
  of 
  the 
  anterior 
  

   half 
  of 
  the 
  prepharyngeal 
  region, 
  particularly 
  above 
  the 
  intestine. 
  

   The 
  gland 
  ducts 
  run 
  anteriorly 
  in 
  dense 
  bundles 
  and, 
  in 
  their 
  terminal 
  

   sections, 
  are 
  thickly 
  swollen 
  with 
  secretion. 
  The 
  muscular 
  system 
  of 
  

   the 
  organ, 
  which 
  could 
  not 
  be 
  analyzed 
  in 
  detail 
  on 
  account 
  of 
  the 
  

   density 
  of 
  the 
  glandular 
  structures, 
  has 
  fibers 
  attached 
  to 
  the 
  adhesive 
  

   surface, 
  serving 
  presumably 
  as 
  retractors. 
  

  

  The 
  rather 
  short 
  and 
  thick 
  pharynx 
  is 
  structurally 
  typical 
  of 
  the 
  

   family 
  Dendrocoelidae; 
  its 
  internal 
  muscular 
  zone 
  consists 
  of 
  inter- 
  

   mingled 
  circular 
  and 
  longitudinal 
  fibers. 
  

  

  Auricular 
  sense 
  organs 
  are 
  represented 
  by 
  two 
  bands 
  of 
  sensory 
  

   epithelium 
  extending 
  posteriorly 
  from 
  the 
  sides 
  of 
  the 
  frontal 
  margin. 
  

   The 
  cells 
  of 
  this 
  epithelium 
  are 
  less 
  tall 
  than 
  are 
  those 
  of 
  the 
  sur- 
  

   rounding 
  body 
  epithelum 
  ; 
  they 
  lack 
  rhabdites 
  and 
  are 
  approached 
  by 
  

   nerve 
  fibers 
  from 
  the 
  underlying 
  mesenchyme. 
  The 
  location 
  of 
  these 
  

   organs 
  corresponds 
  to 
  the 
  two 
  light, 
  longitudinal 
  streaks 
  on 
  the 
  sides 
  

   of 
  the 
  head 
  seen 
  in 
  the 
  living 
  animal. 
  

  

  The 
  testes 
  are 
  of 
  moderate 
  size, 
  numerous, 
  densely 
  packed, 
  and 
  

   occupy 
  the 
  dorsal 
  half 
  of 
  the 
  mesenchyme, 
  generally 
  above 
  the 
  intes- 
  

   tinal 
  branches. 
  They 
  are 
  arranged 
  in 
  two 
  wide 
  areas 
  on 
  both 
  sides 
  

   of 
  the 
  midline, 
  extending 
  from 
  the 
  level 
  of 
  the 
  ovaries 
  close 
  to 
  the 
  

   posterior 
  end. 
  

  

  The 
  ovaries, 
  situated 
  at 
  the 
  level 
  of 
  the 
  second 
  pair 
  of 
  branches 
  of 
  

   the 
  anterior 
  intestinal 
  trunk, 
  show 
  no 
  structural 
  peculiarities. 
  

  

  The 
  genital 
  pore 
  (pg) 
  leads 
  immediately 
  into 
  two 
  cavities, 
  the 
  male 
  

   atrium 
  (am) 
  and, 
  to 
  the 
  right 
  and 
  somewhat 
  posteriorly, 
  the 
  duct 
  of 
  

  

  