﻿JAN. 
  19, 
  1922 
  CLARK: 
  EVOLUTION 
  OF 
  THE 
  ANIMAL 
  BODY 
  27 
  

  

  a 
  sort 
  of 
  invaginated 
  strobilization, 
  the 
  larvae 
  (cercariae, 
  correspond- 
  

   ing 
  to 
  ephyrae) 
  finally 
  escaping 
  by 
  the 
  disintegration 
  of 
  the 
  nurse. 
  

  

  The 
  unsegmented 
  cestodes 
  bear 
  approximately 
  the 
  same 
  relation 
  to 
  

   the 
  tapeworms 
  that 
  Lucernaria 
  does 
  to 
  the 
  scyphistoma 
  of 
  Aurelia, 
  

   and 
  the 
  turbellarians 
  in 
  their 
  relations 
  to 
  the 
  liver 
  flukes 
  and 
  their 
  

   allies 
  are 
  comparable 
  to 
  the 
  Trachomedusae 
  as 
  compared 
  with 
  the 
  

   colonial 
  coelenterates 
  ; 
  that 
  is, 
  they 
  are 
  solitary 
  animals 
  ultimately 
  

   derived 
  through 
  the 
  dissociation 
  of 
  the 
  units 
  of 
  a 
  primarily 
  colonial 
  type. 
  

  

  Of 
  the 
  remaining 
  acoelomate 
  Kumorphozoa 
  the 
  Polyzoa 
  and 
  Calysso- 
  

   zoa 
  are 
  clearly 
  comparable 
  to 
  colonial 
  coelenterates 
  ; 
  the 
  rotifers 
  in 
  their 
  

   asexual 
  and 
  direct 
  development 
  suggest 
  a 
  fragmented 
  colony 
  while 
  

   the 
  round 
  worms 
  and 
  the 
  Acanthocephala 
  are 
  solitary, 
  like 
  the 
  Tracho- 
  

   medusae, 
  some 
  cestodes, 
  and 
  the 
  turbellarians. 
  

  

  All 
  other 
  animals 
  agree 
  in 
  the 
  possession 
  of 
  that 
  structure 
  known 
  as 
  

   a 
  coelome. 
  The 
  coelome, 
  which 
  arises 
  by 
  budding 
  from 
  the 
  enteron, 
  

   consists 
  of 
  three 
  sections, 
  (a) 
  the 
  perivisceral, 
  forming 
  a 
  body 
  cavity, 
  

   (b) 
  the 
  gonadial, 
  and 
  (c) 
  the 
  nephridial. 
  There 
  is 
  thus 
  a 
  curious 
  

   correspondence 
  between 
  the 
  three 
  divisions 
  of 
  the 
  coelome 
  and 
  the 
  

   three 
  classes 
  into 
  which 
  the 
  polyps 
  of 
  the 
  coelenterates 
  naturally 
  fall, 
  

   and 
  this 
  suggests 
  the 
  possibility 
  of 
  coelomate 
  animals 
  having 
  arisen 
  

   through 
  a 
  gastruloid 
  structure 
  resembling 
  a 
  redia 
  by 
  the 
  budding 
  off 
  

   from 
  the 
  enteron 
  of 
  three 
  units 
  which 
  remained 
  within 
  the 
  gastruloid 
  

   and 
  there 
  became 
  differentiated 
  into 
  the 
  three 
  types 
  characteristic 
  of 
  

   the 
  externally 
  budded 
  coelenterate 
  polyps, 
  subsequently 
  undergoing 
  

   further 
  development. 
  

  

  The 
  priapulids, 
  sipunculids, 
  molluscs, 
  nemerteans, 
  phoronids, 
  

   brachiopods, 
  chaetognaths, 
  enteropneusts, 
  tunicates, 
  cephalochordates 
  

   and 
  vertebrates 
  would 
  thus 
  be 
  explained 
  as 
  colonial 
  animals 
  derived 
  

   from 
  a 
  coelenterate-like 
  colonial 
  type 
  through 
  a 
  process 
  of 
  invagination 
  

   by 
  which 
  the 
  additional 
  units 
  were 
  produced 
  within 
  the 
  original 
  

   gastruloid 
  ancestor 
  by 
  budding 
  from 
  the 
  enteron 
  instead 
  of 
  externally 
  

   as 
  in 
  the 
  coelenterates 
  and 
  polyzoans. 
  

  

  Such 
  an 
  interpretation 
  would 
  account 
  for 
  (1) 
  the 
  entire 
  absence 
  

   in 
  these 
  groups 
  of 
  that 
  external 
  segmentation 
  so 
  characteristic 
  of 
  

   the 
  cestodes, 
  the 
  annelids, 
  the 
  arthropods 
  and 
  the 
  echinoderms; 
  (2) 
  

   the 
  entire 
  absence, 
  except 
  in 
  the 
  enteropneusts 
  and 
  tunicates, 
  which 
  

   stand 
  quite 
  apart 
  from 
  the 
  other 
  phyla, 
  of 
  all 
  forms 
  of 
  asexual 
  repro- 
  

   duction, 
  this 
  being 
  here 
  represented 
  by 
  internal 
  budding; 
  (3) 
  the 
  al- 
  

   most 
  complete 
  absence 
  of 
  parasitism 
  (occurring 
  only 
  in 
  a 
  very 
  few 
  

   molluscs 
  and 
  nemerteans), 
  since 
  the 
  transference 
  of 
  the 
  asexual 
  bud- 
  

  

  