﻿FRESH-WATER 
  TRICLADS 
  OF 
  ALASKA 
  — 
  ^KENK 
  165 
  

  

  There 
  are 
  two 
  rather 
  small 
  eyes, 
  situated 
  close 
  together 
  (about 
  one- 
  

   fourth 
  the 
  body 
  width 
  apart 
  at 
  the 
  level 
  of 
  the 
  eyes) 
  and 
  far 
  removed 
  

   from 
  the 
  frontal 
  end. 
  This 
  character, 
  easily 
  recognized 
  in 
  life, 
  dis- 
  

   tinguishes 
  the 
  species 
  from 
  another 
  white 
  triclad 
  with 
  which 
  it 
  shares 
  

   its 
  habitat, 
  Dendrocoelopsis 
  alaskensis, 
  described 
  as 
  new 
  on 
  p. 
  178. 
  

  

  The 
  pharynx 
  is 
  inserted, 
  in 
  sexually 
  mature 
  specimens, 
  somewhat 
  

   behind 
  the 
  middle 
  of 
  the 
  body 
  and 
  measures 
  about 
  one-sixth 
  of 
  the 
  

   body 
  length. 
  The 
  copulatory 
  organs 
  occupy 
  the 
  anterior 
  half 
  of 
  the 
  

   postpharyngeal 
  region. 
  

  

  The 
  animal 
  moves 
  by 
  gliding 
  only; 
  crawling 
  movements, 
  such 
  as 
  

   are 
  seen 
  in 
  other 
  triclads, 
  particularly 
  in 
  those 
  equipped 
  with 
  anterior 
  

   adhesive 
  organs, 
  have 
  not 
  been 
  observed 
  in 
  this 
  species. 
  

  

  From 
  the 
  description 
  it 
  may 
  be 
  seen 
  that 
  the 
  species 
  in 
  life 
  shows 
  

   a 
  close 
  resemblance 
  to 
  other 
  species 
  of 
  the 
  same 
  genus, 
  particularly 
  

   to 
  the 
  American 
  Phagocata 
  morgani, 
  the 
  European 
  P. 
  albissima, 
  P. 
  

   vitta, 
  and 
  related 
  forms. 
  A 
  separation 
  of 
  these 
  species 
  can 
  be 
  made 
  

   only 
  on 
  the 
  basis 
  of 
  anatomical 
  characters. 
  

  

  Only 
  those 
  characters 
  of 
  the 
  digestive 
  system 
  that 
  have 
  a 
  taxonomic 
  

   significance 
  are 
  discussed 
  here. 
  The 
  pharynx 
  has 
  a 
  structure 
  typical 
  

   of 
  the 
  family 
  Planariidae; 
  i. 
  e., 
  the 
  fibers 
  of 
  the 
  internal 
  muscle 
  zone 
  

   are 
  arranged 
  in 
  two 
  distinct 
  layers, 
  a 
  thick 
  inner 
  circular 
  layer 
  and 
  a 
  

   narrower 
  outer 
  longitudinal 
  one. 
  The 
  anterior 
  intestinal 
  trunk 
  bears 
  

   10 
  or 
  11 
  branches 
  on 
  each 
  side. 
  Each 
  posterior 
  trunk 
  has 
  21 
  to 
  27 
  

   lateral 
  branches 
  and 
  numerous 
  short 
  medial 
  branches 
  in 
  both 
  the 
  

   pharyngeal 
  and 
  postpharyngeal 
  regions. 
  

  

  The 
  testes 
  are 
  numerous 
  and 
  are 
  arranged, 
  on 
  each 
  side 
  of 
  the 
  

   midline, 
  in 
  a 
  longitudinal 
  zone 
  extending 
  from 
  a 
  short 
  distance 
  behind 
  

   the 
  ovary 
  almost 
  to 
  the 
  posterior 
  end 
  of 
  the 
  body. 
  Their 
  position 
  is 
  

   predominantly 
  ventral, 
  below 
  the 
  intestinal 
  branches. 
  Only 
  a 
  few 
  

   testes 
  extend 
  into 
  the 
  mesenchymatic 
  "septa" 
  between 
  the 
  branches 
  

   toward 
  the 
  dorsal 
  side. 
  

  

  The 
  two 
  ovaries 
  are 
  typical, 
  each 
  situated 
  approximately 
  below 
  the 
  

   second 
  intestinal 
  branch. 
  An 
  undifferentiated 
  mass 
  of 
  cells, 
  the 
  par- 
  

   ovarium, 
  is 
  attached 
  to 
  the 
  dorsolateral 
  side 
  of 
  each 
  ovary. 
  

  

  The 
  genital 
  pore 
  (pg), 
  situated 
  about 
  halfway 
  between 
  the 
  mouth 
  

   and 
  the 
  posterior 
  end 
  of 
  the 
  body, 
  leads 
  into 
  a 
  small 
  cavity 
  (ac) 
  

   which 
  continues 
  to 
  the 
  left 
  and 
  dorsally 
  into 
  the 
  duct 
  of 
  the 
  copulatory 
  

   bursa 
  (bd) 
  and 
  to 
  the 
  right 
  and 
  anteriorly 
  into 
  the 
  male 
  atrium 
  (am) 
  . 
  

   This 
  cavity 
  may 
  be 
  considered 
  to 
  represent 
  a 
  common 
  genital 
  atrium. 
  

   In 
  some 
  specimens, 
  however, 
  there 
  appears 
  to 
  be 
  no 
  differentiation 
  

   of 
  the 
  atrium 
  into 
  male 
  and 
  common 
  parts, 
  and 
  the 
  bursa 
  duct 
  and 
  

   an 
  undivided 
  atrium 
  meet 
  at 
  or 
  near 
  the 
  genital 
  pore. 
  These 
  varia- 
  

   tions 
  are 
  obviously 
  due 
  to 
  the 
  different 
  states 
  of 
  muscular 
  contraction 
  

   in 
  which 
  the 
  animals 
  were 
  killed. 
  The 
  atrium, 
  narrow 
  at 
  the 
  genital 
  

   aperture, 
  widens 
  as 
  it 
  extends 
  forward, 
  to 
  the 
  right 
  side 
  of 
  the 
  midline. 
  

  

  