﻿JOURNAL 
  

  

  OF 
  THE 
  

  

  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  

  

  Vol. 
  12 
  January 
  19, 
  1922 
  No. 
  2 
  

  

  ZOOLOGY. 
  — 
  The 
  evolution 
  of 
  the 
  animal 
  body} 
  Austin 
  H. 
  Clark, 
  

   U. 
  S. 
  National 
  Museum. 
  

  

  In 
  a 
  recent 
  number 
  of 
  this 
  Journal^ 
  I 
  gave 
  a 
  brief 
  synopsis 
  of 
  

   the 
  steps 
  in 
  the 
  evolution 
  of 
  animals 
  based 
  upon 
  the 
  progressively 
  

   increasing 
  complexity 
  of 
  structure 
  correlated 
  with 
  increased 
  economic 
  

   efficiency. 
  The 
  subject 
  was 
  treated 
  in 
  much 
  greater 
  detail 
  in 
  a 
  later 
  

   paper. 
  ^ 
  

  

  Superposed 
  upon 
  this 
  evolutionary 
  line 
  there 
  is 
  another 
  having 
  

   to 
  do 
  with 
  the 
  development 
  of 
  the 
  body 
  as 
  a 
  whole 
  instead 
  of 
  with 
  the 
  

   refinement 
  of 
  its 
  internal 
  organization, 
  and 
  to 
  a 
  large 
  extent 
  the 
  two 
  

   are 
  quite 
  independent. 
  

  

  All 
  the 
  higher 
  animals 
  are 
  ultimately 
  derived 
  from 
  an 
  attached 
  

   animal 
  colony 
  within 
  which 
  the 
  component 
  zooids 
  are 
  more 
  or 
  less 
  

   differentiated 
  for 
  the 
  better 
  performance 
  of 
  certain 
  more 
  or 
  less 
  definite 
  

   functions, 
  this 
  animal 
  colony 
  being 
  in 
  general 
  comparable 
  to 
  the 
  colony 
  

   of 
  phytons 
  known 
  as 
  a 
  flowering 
  plant. 
  

  

  In 
  the 
  sponges 
  the 
  colonial 
  nature 
  of 
  the 
  animal 
  is 
  evident, 
  but 
  

   there 
  are 
  no 
  definite 
  organs 
  or 
  tissues, 
  and 
  the 
  mass 
  is 
  imperfectly 
  

   or 
  not 
  at 
  all 
  divided. 
  The 
  sponges 
  are 
  thus 
  comparable 
  to 
  certain 
  

   of 
  the 
  so-called 
  thallophytes. 
  

  

  The 
  coelenterates 
  have 
  a 
  definite 
  body 
  structure 
  and 
  are 
  funda- 
  

   mentally 
  colonial, 
  the 
  colony 
  being 
  produced 
  asexually 
  by 
  budding 
  and 
  

   the 
  component 
  individuals 
  usually 
  showing 
  more 
  or 
  less 
  differentiation 
  

   into 
  (a) 
  nutritive, 
  {h) 
  reproductive 
  and 
  (c) 
  excretionary 
  ("defensive") 
  

   types, 
  the 
  latter 
  bearing 
  numerous 
  cells 
  containing 
  a 
  secretion 
  and 
  also 
  

   a 
  coiled 
  tubule. 
  Free 
  living 
  coelenterates 
  occur, 
  and 
  these 
  arise 
  (1) 
  

   through 
  the 
  assumption 
  of 
  a 
  free 
  floating 
  existence 
  by 
  the 
  colony 
  as 
  a 
  

   whole 
  (siphonophores), 
  or 
  (2) 
  through 
  the 
  partial 
  (medusae 
  of 
  hydroids) 
  

   or 
  complete 
  {Aurelia, 
  Trachomedusae, 
  most 
  actinians, 
  etc.) 
  dissociation 
  

   of 
  the 
  units 
  of 
  the 
  colony. 
  

  

  ^ 
  Received 
  December 
  16, 
  1921. 
  

  

  2 
  This 
  Journal 
  11: 
  207-208. 
  May 
  4, 
  1921. 
  

  

  3 
  Bull, 
  de 
  ITnstit. 
  Oceanographique 
  (Monaco), 
  400: 
  1-24. 
  20 
  septembre, 
  1921. 
  

  

  25 
  

  

  