﻿FRESH-WATER 
  TRICLADS 
  OF 
  ALASKA 
  — 
  ^KENK 
  169 
  

  

  from 
  some 
  localities, 
  an 
  indistinct 
  lighter 
  median 
  line 
  occurred 
  dor- 
  

   sally 
  in 
  the 
  prepharyngeal 
  region, 
  with 
  lighter 
  areas 
  above 
  the 
  pharynx 
  

   and 
  the 
  copulatory 
  organs. 
  

  

  The 
  species 
  has 
  many 
  small 
  eyes 
  (a 
  generic 
  character) 
  arranged 
  

   in 
  a 
  band 
  along 
  the 
  frontal 
  margin 
  of 
  the 
  head, 
  the 
  base 
  of 
  the 
  auricles, 
  

   and 
  the 
  lateral 
  margins 
  of 
  the 
  body 
  a 
  short 
  distance 
  behind 
  the 
  head. 
  

   Anteriorly 
  the 
  eyes 
  are 
  placed 
  in 
  more 
  than 
  one 
  row, 
  somewhat 
  

   irregularly 
  scattered 
  ; 
  behind 
  the 
  head 
  they 
  are 
  in 
  a 
  single 
  row 
  reaching 
  

   backward 
  about 
  one-third 
  to 
  one-half 
  the 
  length 
  of 
  the 
  prepharyngeal 
  

   region. 
  There 
  is 
  no 
  narrowing 
  of 
  the 
  band 
  of 
  eyes 
  at 
  the 
  base 
  of 
  the 
  

   auricles. 
  

  

  The 
  pharynx 
  is 
  inserted 
  at 
  about 
  the 
  middle 
  of 
  the 
  body 
  and 
  

   measures 
  in 
  length 
  almost 
  one-fourth 
  the 
  length 
  of 
  the 
  body. 
  The 
  

   copulatory 
  organs 
  occupy 
  more 
  than 
  half 
  the 
  post-pharyngeal 
  region. 
  

  

  The 
  animal 
  moves 
  by 
  gliding 
  only. 
  

  

  The 
  pharynx 
  of 
  Polycelis 
  borealis 
  is 
  structurally 
  typical 
  of 
  the 
  genus 
  

   Polycelis 
  and 
  of 
  the 
  family 
  Planariidae, 
  the 
  muscle 
  zone 
  being 
  formed 
  

   by 
  two 
  distinct 
  layers, 
  a 
  thick 
  circular 
  layer 
  adjoining 
  the 
  epithelium 
  

   of 
  the 
  pharyngeal 
  lumen 
  and 
  a 
  thinner 
  layer 
  of 
  longitudinal 
  fibers. 
  

   The 
  anterior 
  trunk 
  of 
  the 
  intestine 
  bears 
  5 
  to 
  6 
  lateral 
  branches. 
  

  

  The 
  numerous, 
  fairly 
  large 
  testes 
  are 
  arranged 
  in 
  two 
  zones, 
  to 
  

   the 
  right 
  and 
  left 
  of 
  the 
  anterior 
  intestinal 
  trunk, 
  extending 
  from 
  the 
  

   level 
  of 
  the 
  ovaries 
  posteriorly 
  to 
  the 
  base 
  of 
  the 
  pharynx. 
  The 
  

   testes 
  are 
  essentially 
  ventral, 
  though 
  individual 
  vesticles 
  may 
  extend 
  

   dorsally 
  in 
  the 
  mesenchymatic 
  spaces 
  between 
  the 
  intestinal 
  branches 
  

   and 
  occupy 
  almost 
  the 
  entire 
  dorsoventral 
  diameter 
  of 
  the 
  body. 
  

   Where 
  intestinal 
  branches 
  are 
  present, 
  however, 
  the 
  testes 
  develop 
  

   only 
  below 
  them. 
  Each 
  zone 
  of 
  testes 
  reaches 
  laterally 
  only 
  little 
  

   beyond 
  the 
  ventral 
  nerve 
  cord. 
  

  

  The 
  ovaries 
  are 
  situated 
  behind 
  the 
  first 
  pair 
  of 
  lateral 
  branches 
  of 
  

   the 
  anterior 
  trunk 
  of 
  the 
  intestine. 
  

  

  The 
  genital 
  pore 
  (j)g), 
  situated 
  in 
  the 
  midline 
  behind 
  the 
  middle 
  of 
  

   the 
  postpharyngeal 
  region, 
  connects 
  with 
  a 
  narrow 
  posterior 
  extension 
  

   of 
  the 
  genital 
  atrium 
  (a) 
  . 
  This 
  extension 
  leads 
  dorsally 
  and 
  somewhat 
  

   to 
  the 
  left 
  into 
  the 
  duct 
  of 
  the 
  copulatory 
  bursa 
  and 
  anteriorly 
  into 
  

   the 
  widened 
  portion 
  of 
  the 
  atrium 
  containing 
  the 
  penis. 
  There 
  is 
  no 
  

   marked 
  division 
  of 
  the 
  atrium 
  into 
  a 
  posterior 
  common 
  atrium 
  and 
  

   an 
  anterior 
  male 
  atrium. 
  The 
  wall 
  of 
  the 
  atrium 
  is 
  lined 
  with 
  a 
  

   cubical 
  epithelium 
  and 
  equipped 
  with 
  two 
  muscle 
  layers, 
  one 
  circular 
  

   and 
  the 
  other 
  longitudinal. 
  

  

  The 
  penis 
  consists 
  of 
  a 
  bulb 
  embedded 
  in 
  the 
  mesenchyme 
  and 
  a 
  

   papUla 
  projecting 
  into 
  the 
  atrium. 
  Neither 
  of 
  the 
  two 
  parts 
  is 
  very 
  

   muscular. 
  The 
  shape 
  of 
  the 
  penis, 
  particularly 
  of 
  the 
  papilla, 
  is 
  very 
  

  

  