﻿FRESH-WATER 
  TRICLADS 
  OF 
  ALASKA 
  — 
  KENK 
  

  

  181 
  

  

  I 
  I 
  I 
  

  

  a 
  ode 
  pg 
  bd 
  

  

  Figure 
  24. 
  — 
  Dendrocoelopsis 
  alaskensis, 
  diagram 
  of 
  the 
  copulatory 
  organs 
  in 
  longitudinal 
  

   section, 
  X 
  70. 
  a, 
  genital 
  atrium; 
  b, 
  bursa; 
  bd, 
  bursa 
  stalk; 
  ode, 
  common 
  oviduct; 
  

   pg, 
  genital 
  pore; 
  ph, 
  pharynx 
  (extended 
  through 
  the 
  mouth); 
  vd, 
  vas 
  deferens; 
  

   vs, 
  seminal 
  vesicle. 
  

  

  Zoogeographic 
  conclusions 
  

  

  The 
  occurrence 
  in 
  Alaska 
  of 
  four 
  endemic 
  species 
  of 
  triclads 
  poses 
  

   several 
  interesting 
  questions. 
  It 
  is 
  known 
  that 
  fresh-water 
  triclads 
  

   are 
  most 
  abundantly 
  represented 
  in 
  the 
  northern 
  Temperate 
  Zone 
  

   and 
  that 
  the 
  number 
  of 
  species 
  declines 
  toward 
  both 
  the 
  Arctic 
  and 
  

   the 
  Tropical 
  Zones. 
  The 
  paucity 
  of 
  the 
  triclad 
  fauna 
  at 
  high 
  lati- 
  

   tudes 
  has 
  frequently 
  been 
  attributed 
  to 
  the 
  effects 
  of 
  glaciation 
  in 
  

   rather 
  recent 
  geologic 
  time. 
  During 
  the 
  glacial 
  period, 
  when 
  huge 
  

   ice 
  masses 
  covered 
  great 
  parts 
  of 
  the 
  northern 
  hemisphere, 
  all 
  fresh- 
  

   water 
  life 
  over 
  wide 
  areas 
  must 
  have 
  vanished. 
  After 
  the 
  glaciers 
  

   had 
  receded, 
  the 
  areas 
  were 
  gradually 
  repopulated 
  by 
  species 
  entering 
  

   them 
  from 
  adjacent 
  territories. 
  The 
  pattern 
  of 
  distribution 
  of 
  

   freshwater 
  triclads 
  m 
  Europe 
  shows 
  evidence 
  of 
  a 
  definite 
  succession 
  

   of 
  species 
  which 
  migrated 
  into 
  the 
  previously 
  glaciated 
  areas 
  in 
  the 
  

   postglacial 
  period 
  (cf. 
  Thienemann's 
  1950 
  summary 
  of 
  the 
  pertinent 
  

   literature) 
  . 
  

  

  It 
  is 
  well 
  known, 
  however, 
  that 
  the 
  greater 
  part 
  of 
  Alaska 
  was 
  

   not 
  covered 
  with 
  ice 
  in 
  the 
  glacial 
  period. 
  Geologic 
  evidence 
  of 
  

   glaciation 
  has 
  been 
  found 
  only 
  in 
  the 
  Brooks 
  Range, 
  the 
  Alaska 
  

   Range 
  (and 
  areas 
  south 
  of 
  it), 
  and 
  in 
  small 
  isolated 
  spots 
  in 
  the 
  

   Yukon 
  and 
  Kuskokwim 
  River 
  Valleys 
  while 
  the 
  remaining 
  surface 
  

   of 
  Alaska 
  remained 
  free 
  of 
  ice 
  (cf. 
  Geological 
  Society 
  of 
  America, 
  

  

  