﻿218 
  Prof. 
  Carl 
  Chun 
  on 
  the 
  

  

  into 
  the 
  chambers, 
  and 
  out 
  through 
  the 
  stigmata 
  of 
  the 
  latter. 
  

   On 
  the 
  cessation 
  of 
  the 
  contraction 
  fresh 
  air, 
  richer 
  in 
  oxygen, 
  

   flows 
  in 
  again. 
  The 
  structure 
  of 
  the 
  chambers, 
  their 
  endow- 
  

   ment 
  with 
  stigmata 
  and 
  trachea?, 
  and 
  the 
  regular 
  respiratory 
  

   movements 
  serving 
  for 
  the 
  renewal 
  of 
  the 
  air 
  lead 
  me 
  (little 
  

   as 
  this 
  opinion 
  may 
  harmonize 
  with 
  previous 
  conceptions) 
  to 
  

   see 
  in 
  the 
  Velellse 
  and 
  Porpita? 
  Ccelenterata 
  breathing 
  air, 
  and, 
  

   indeed, 
  breathing 
  by 
  tracheai. 
  This 
  notion 
  by 
  no 
  means 
  

   excludes 
  the 
  occurrence 
  at 
  the 
  same 
  time 
  at 
  all 
  parts 
  turned 
  

   towards 
  the 
  water 
  of 
  a 
  diffusional 
  exchange 
  with 
  the 
  air 
  

   absorbed 
  in 
  the 
  sea-water, 
  an 
  exchange 
  which, 
  indeed, 
  plays 
  

   a 
  part 
  in 
  all 
  air-breathing 
  aquatic 
  animals 
  *. 
  

  

  The 
  Velellidse 
  certainly 
  represent 
  not 
  only 
  the 
  most 
  com- 
  

   plicated 
  in 
  structure, 
  but 
  also 
  the 
  most 
  divergent 
  of 
  the 
  Physo- 
  

   phoridae. 
  But, 
  as 
  I 
  have 
  already 
  expressly 
  asserted, 
  there 
  is 
  

   no 
  feature 
  in 
  their 
  organization 
  which 
  cannot 
  be 
  explained 
  by 
  

   gradual 
  adaptation 
  to 
  an 
  existence 
  at 
  the 
  surface 
  of 
  the 
  sea. 
  

   I 
  think 
  that 
  I 
  fix 
  the 
  position 
  of 
  the 
  Velelhe 
  and 
  Porpitce 
  in 
  

   the 
  system 
  more 
  correctly 
  by 
  proposing 
  to 
  divide 
  the 
  order 
  

   Physophoridae 
  into 
  two 
  suborders, 
  of 
  which 
  one 
  includes 
  

   all 
  Physophoridse 
  with 
  an 
  unchambered 
  pneumatophore 
  

   functioning 
  as 
  a 
  gas- 
  gland 
  (Ilajrfophysce) 
  , 
  while 
  the 
  other 
  

   embraces 
  the 
  (partially) 
  air-breathing 
  Velellida? 
  (Tracho- 
  

   physce) 
  furnished 
  with 
  a 
  chambered 
  pneumatophore, 
  stigmata, 
  

   and 
  trachea?. 
  

  

  As 
  regards 
  Hiickel's 
  systematic 
  arrangement 
  in 
  other 
  

   respects 
  I 
  shall 
  take 
  occasion 
  in 
  the 
  sequel 
  to 
  express 
  diver- 
  

   gent 
  opinions 
  of 
  subordinate 
  importance. 
  I 
  see 
  with 
  satis- 
  

   faction 
  that 
  my 
  opinion 
  formerly 
  expressed 
  in 
  these 
  ' 
  Sitz- 
  

   ungsberichte 
  ' 
  (1882, 
  lii. 
  p. 
  1170) 
  as 
  to 
  the 
  near 
  relationship 
  

   between 
  the 
  Rhizophysce 
  and 
  the 
  Physaliw, 
  to 
  which 
  I 
  endea- 
  

   voured 
  to 
  give 
  expression 
  by 
  the 
  creation 
  of 
  the 
  family 
  Pneu- 
  

   inatophoridaj, 
  is 
  not 
  only 
  adopted 
  by 
  Hackel, 
  but 
  also 
  well 
  

   illustrated 
  by 
  a 
  number 
  of 
  most 
  interesting 
  intermediate 
  

   forms 
  which 
  were 
  obtained 
  by 
  the 
  ' 
  Challenger 
  ' 
  from 
  the 
  

   deep 
  sea. 
  

  

  As 
  regards 
  Hackel's 
  peculiar 
  " 
  Medusa-theory," 
  which 
  is 
  

   intended 
  to 
  occupy 
  a 
  conciliatory 
  position 
  between 
  the 
  poly- 
  

  

  * 
  These 
  statements 
  of 
  course 
  do 
  not 
  apply 
  to 
  the 
  interesting 
  " 
  Disca- 
  

   lidas 
  " 
  discovered 
  by 
  the 
  ' 
  Challenger 
  ' 
  in 
  the 
  deep 
  sea. 
  These 
  in 
  a 
  certain 
  

   degree 
  represent 
  Disconulse 
  which 
  have 
  become 
  sexually 
  mature, 
  and 
  it 
  

   is 
  to 
  be 
  supposed 
  that 
  in 
  them, 
  as 
  in 
  the 
  young 
  stages 
  of 
  Porpitce 
  and 
  

   Velellm, 
  the 
  gas-secreting 
  ectoderm 
  persists. 
  At 
  any 
  rate 
  they 
  partly 
  

   want 
  the 
  tracheae, 
  which 
  occur 
  only 
  from 
  the 
  third 
  or 
  fourth 
  concentric 
  

   air-chamber 
  onwards. 
  It 
  is 
  possible 
  that 
  they 
  represent 
  young 
  stages 
  of 
  

   Porpitce 
  which, 
  as 
  I 
  have 
  found 
  to 
  be 
  the 
  case 
  with 
  many 
  Siphonophora, 
  

   ascend 
  to 
  the 
  surface 
  at 
  the 
  commencement 
  of 
  sexual 
  maturity. 
  

  

  