﻿POMATIOPSIS 
  LAPIDARIA 
  — 
  ABBOTT 
  65 
  

  

  An 
  amphibious 
  nature 
  appears 
  to 
  be 
  a 
  prime 
  requisite 
  of 
  a 
  snail 
  in 
  

   order 
  to 
  serve 
  successfully 
  as 
  an 
  intermediate 
  host 
  to 
  Schistosoma 
  

   japonicum. 
  The 
  reason 
  for 
  this 
  is 
  still 
  little 
  understood. 
  Whether 
  

   the 
  nature 
  of 
  the 
  environment, 
  such 
  as 
  periodic 
  fluctuation 
  in 
  the 
  tem- 
  

   perature 
  of 
  the 
  snail 
  due 
  to 
  resubmergence 
  in 
  rising 
  and 
  ebbing 
  waters, 
  

   or 
  a 
  host 
  of 
  other 
  conditions 
  brought 
  about 
  by 
  the 
  external 
  environ- 
  

   ment, 
  or 
  whether 
  the 
  nature 
  of 
  the 
  physiological 
  environment 
  of 
  the 
  

   snail 
  itself 
  (conditioned 
  in 
  turn 
  by 
  the 
  external 
  habitat) 
  is 
  what 
  makes 
  

   this 
  type 
  of 
  snail 
  a 
  suitable 
  host 
  is 
  as 
  yet 
  unknown. 
  

  

  A 
  few 
  scanty 
  observations 
  have 
  been 
  made 
  that 
  aid 
  in 
  defining 
  the 
  

   habitudinal 
  limits 
  of 
  a 
  schistosome-carrying 
  snail. 
  In 
  the 
  family 
  

   Amnicolidae 
  there 
  are 
  various 
  genera 
  whose 
  species 
  possess 
  characters 
  

   that 
  appear 
  to 
  be 
  correlated 
  with 
  a 
  general 
  evolutionary 
  trend 
  toward 
  

   more 
  and 
  more 
  terrestrial 
  habits. 
  These 
  characters 
  are 
  the 
  number 
  of 
  

   gills, 
  degree 
  of 
  development 
  of 
  the 
  pedal 
  mucus 
  gland, 
  and 
  the 
  pro- 
  

   duction 
  of 
  mucus. 
  Other 
  characters 
  take 
  a 
  special 
  course 
  in 
  modifica- 
  

   tions, 
  but 
  their 
  correlation 
  with 
  the 
  trend 
  toward 
  amphibious 
  life 
  is 
  

   probably 
  accidental. 
  

  

  Number 
  of 
  

   Genus 
  Habitat 
  gills 
  Pedal 
  gland 
  

  

  Bulimus 
  Completely 
  aquatic 
  150-100 
  Slightly 
  developed. 
  

  

  Oncomdania 
  Moderately 
  amphibious- 
  50 
  Moderately 
  developed. 
  

  

  Pomatiopsis 
  Distinctly 
  amphibious 
  27-28 
  Well 
  developed. 
  

  

  (Synceridae 
  Amphibious 
  to 
  terrestrial 
  Enormously 
  developed.) 
  

  

  Zoogeography. 
  — 
  The 
  distribution 
  of 
  Poinatiopsis 
  lapidaria 
  (Say), 
  

   as 
  indicated 
  on 
  the 
  map 
  (fig. 
  10) 
  , 
  extends 
  throughout 
  the 
  Mississippi 
  

   River 
  drainage 
  system, 
  into 
  southern 
  Canada, 
  and 
  across 
  into 
  the 
  

   central 
  portion 
  of 
  the 
  Atlantic 
  Seaboard. 
  The 
  distribution 
  of 
  the 
  

   lung-fluke 
  disease, 
  Paragoimnus 
  kelUcotti 
  Ward, 
  can 
  be 
  superimposed 
  

   directly 
  on 
  that 
  of 
  the 
  snail 
  except 
  for 
  the 
  Atlantic 
  seaboard 
  area. 
  

   The 
  second 
  intermediate 
  crayfish 
  hosts, 
  some 
  ten 
  species 
  of 
  the 
  closely 
  

   related 
  genera 
  Camharus., 
  Procamhanis, 
  and 
  OroGonectes., 
  have 
  a 
  much 
  

   wider 
  range 
  than 
  either 
  the 
  first 
  intermediate 
  snail 
  host 
  or 
  the 
  parasitic 
  

   trematode, 
  and 
  although 
  they 
  are 
  necessary 
  members 
  of 
  the 
  host 
  cycle 
  

   they 
  do 
  not 
  delimit 
  the 
  distribution 
  of 
  the 
  disease 
  as 
  does 
  the 
  snail. 
  

  

  The 
  origins 
  of 
  the 
  various 
  species 
  of 
  Pomutiopsis 
  in 
  North 
  America 
  

   have 
  not 
  yet 
  been 
  speculated 
  upon, 
  although 
  the 
  close 
  morphological 
  

   similarity 
  of 
  P. 
  lapidaria 
  (Say) 
  and 
  Oncomelania 
  nosophora 
  (Rob- 
  

   son) 
  suggests 
  that 
  their 
  common 
  ancestor's 
  range 
  extended 
  across 
  the 
  

   old 
  Bering 
  Sea 
  connection. 
  Some 
  hope 
  may 
  be 
  held 
  for 
  a 
  fairly 
  com- 
  

   plete 
  picture 
  of 
  the 
  systematic 
  relationships 
  of 
  the 
  various 
  species 
  of 
  

   American 
  Pomatiopsis^ 
  but 
  this 
  will 
  have 
  to 
  come 
  through 
  cytological 
  

   observation 
  of 
  chromosomal 
  differences, 
  similar 
  to 
  that 
  of 
  Dobzhansky 
  

   and 
  others 
  with 
  insects. 
  The 
  divergence 
  in 
  external 
  morphology 
  

  

  