﻿SOME 
  NEW 
  SPECIES 
  OF 
  THE 
  GENUS 
  PHEEODRILUS. 
  293 
  

  

  to 
  form 
  a 
  muscular 
  " 
  atrial 
  sac.'' 
  In 
  P. 
  beddardi 
  the 
  atrium 
  

   is 
  much 
  larger 
  than 
  in 
  the 
  previous 
  species, 
  and 
  is 
  coiled, 
  

   and 
  terminates 
  in 
  a 
  small 
  penis. 
  And 
  finally, 
  in 
  P. 
  subter- 
  

   raneus 
  the 
  atrium 
  becomes 
  drawn 
  out 
  to 
  an 
  extraordinary 
  

   length, 
  and 
  is 
  differentiated 
  into 
  a 
  long 
  narrow, 
  and 
  a 
  short 
  

   glandular 
  region, 
  while 
  the 
  penis 
  is 
  quite 
  small. 
  

  

  Mr. 
  Beddard, 
  in 
  his 
  memoir 
  on 
  P. 
  subterraneus, 
  has 
  

   called 
  attention 
  to 
  the 
  peculiarity 
  and 
  unique 
  character 
  of 
  

   this 
  atrial 
  sac, 
  and 
  has 
  compared 
  it 
  Avith 
  certain 
  other 
  

   structures, 
  and 
  the 
  inclusion 
  of 
  Hesperodrilus 
  in 
  the 
  

   genus 
  renders 
  it 
  easier 
  to 
  make 
  comparisons 
  with 
  the 
  closely 
  

   allied 
  family, 
  the 
  Tubificid^e. 
  

  

  In 
  Tubifex 
  itself 
  there 
  is 
  a 
  comparatively 
  simple 
  pro- 
  

   trusible 
  penis, 
  surrounded 
  by 
  a 
  muscular 
  wall, 
  forming 
  a 
  sac 
  

   in 
  which 
  it 
  lies. 
  This 
  is 
  quite 
  comparable 
  to 
  the 
  arrange- 
  

   ment 
  in 
  P. 
  lacustris. 
  But 
  in 
  other 
  genera, 
  such 
  as 
  Lim- 
  

   nodrilus, 
  the 
  muscular 
  investment 
  extends 
  much 
  further 
  up 
  

   the 
  wall 
  of 
  the 
  apparatus 
  ; 
  the 
  penis 
  is 
  much 
  more 
  powerfully 
  

   developed 
  it 
  is 
  true, 
  but 
  we 
  do 
  not 
  know 
  to 
  what 
  extent 
  it 
  can 
  

   be 
  protruded. 
  From 
  its 
  general 
  structure 
  one 
  is 
  inclined 
  to 
  

   think 
  that 
  the 
  extent 
  is 
  limited. 
  This 
  muscular 
  sheath, 
  which 
  

   in 
  some 
  species 
  (constituting 
  the 
  genus 
  '^Camptodrilus" 
  of 
  

   Eisen) 
  is 
  formed 
  of 
  spirally 
  ai"ranged 
  fibres, 
  surrounds 
  a 
  con- 
  

   siderable 
  length 
  of 
  the 
  " 
  atrium," 
  narrowed 
  of 
  course 
  as 
  com- 
  

   pared 
  with 
  the 
  saccular 
  region 
  higher 
  up 
  the 
  tube. 
  It 
  seems 
  

   to 
  me 
  that 
  the 
  atrial 
  sac 
  of 
  Phreodrilus 
  beddardi 
  is 
  a 
  step 
  

   further 
  than 
  this, 
  in 
  which 
  the 
  whole 
  atrium 
  has 
  become 
  

   surrounded 
  by 
  muscle. 
  

  

  It 
  appears 
  that 
  protrusion 
  in 
  this 
  case 
  is 
  effected 
  by 
  the 
  

   compression 
  of 
  the 
  fluid 
  within 
  the 
  sac 
  by 
  the 
  contraction 
  of 
  

   the 
  muscle 
  in 
  the 
  wall, 
  which 
  is 
  aided 
  by 
  the 
  contraction 
  of 
  

   the 
  fan-shaped 
  muscle 
  (m. 
  ic.) 
  above 
  described. 
  This 
  muscle 
  

   is 
  probably 
  homologous 
  with 
  those 
  surrounding 
  the 
  penial 
  

   sac 
  of 
  P. 
  lacustris, 
  themselves 
  in 
  continuity 
  with 
  the 
  

   longitudinal 
  muscles 
  of 
  the 
  body-wall, 
  and 
  acting 
  as 
  " 
  pro- 
  

   trusors." 
  In 
  P. 
  beddardi 
  the 
  penial 
  sac 
  itself 
  is 
  provided 
  

   with 
  longitudinal 
  muscles 
  alone, 
  which 
  appear 
  to 
  act 
  as 
  

  

  