﻿NIAGARA 
  FALLS 
  AND 
  VICINITY 
  1 
  33 
  

  

  ANIMALS 
  

  

  Class 
  HYDROZOA 
  Owen 
  

  

  This 
  class 
  includes 
  the 
  simplest 
  polyps, 
  oi 
  which 
  the 
  fresh-water 
  

   Hydra 
  is 
  an 
  example. 
  The 
  body 
  consists 
  of 
  a 
  hollow 
  tube, 
  the 
  walls 
  

   of 
  which 
  are 
  composed 
  of 
  two 
  cellular 
  layers, 
  ectoderm 
  and 
  endodemiy 
  

   with 
  a 
  non-cellular 
  layer, 
  the 
  mesogloea, 
  between 
  them. 
  These 
  lay- 
  

   ers 
  meet 
  at 
  the 
  mouth, 
  w^hich 
  is 
  the 
  only 
  opening 
  into 
  the 
  gastric 
  

   space 
  inclosed 
  by 
  the 
  body 
  wall. 
  Tentacles, 
  furnished 
  with 
  nettle 
  

   cells, 
  surround 
  the 
  mouth. 
  

  

  A 
  few 
  hydroids 
  are 
  simple 
  forms, 
  but 
  the 
  majority 
  are 
  united 
  into 
  

   colonies, 
  which 
  frequently 
  assume 
  a 
  branching 
  or 
  tree-like 
  char- 
  

   acter, 
  a 
  polyp 
  occupying 
  the 
  end 
  of 
  each 
  branch. 
  Reproduction 
  is 
  

   usually 
  carried 
  on 
  by 
  specially 
  modified 
  polyps, 
  the 
  goiwpolyps, 
  

   which 
  produce 
  jellyfish 
  or 
  medusae. 
  These 
  may 
  remain 
  attached 
  

   to 
  the 
  colony 
  or 
  become 
  free-swimming. 
  

  

  Some 
  hydroids 
  are 
  entirely 
  unprotected, 
  no 
  hard 
  structures 
  being 
  

   developed, 
  and 
  these 
  consequently 
  leave 
  no 
  remains. 
  The 
  majority 
  

   of 
  species, 
  however, 
  secrete 
  a 
  horny 
  or 
  chitinous 
  covering, 
  the 
  peri- 
  

   derm, 
  which 
  invests 
  the 
  whole 
  stock, 
  and 
  in 
  one 
  group 
  is 
  expanded 
  

   at 
  the 
  ends 
  of 
  the 
  branches 
  into 
  cups 
  or 
  hydrothecae, 
  into 
  which 
  the 
  

   polyps 
  can 
  withdraw. 
  This 
  chitinous 
  periderm 
  may 
  be 
  preserved 
  

   in 
  the 
  form 
  of 
  a 
  carbonaceous 
  film 
  (e. 
  g. 
  D 
  i 
  c 
  t 
  y 
  o 
  n 
  e 
  m 
  a 
  and 
  

   G 
  r 
  a 
  p 
  t 
  o 
  1 
  i 
  t 
  e 
  s). 
  

  

  Some 
  hydroid 
  colonies, 
  i. 
  e. 
  the 
  hydrocorallines, 
  secrete 
  at 
  the 
  

   base 
  a 
  dense 
  calcareous 
  covering, 
  which 
  has 
  much 
  the 
  aspect 
  of 
  

   coral, 
  and 
  is 
  frequently 
  mistaken 
  for 
  that 
  (e, 
  g. 
  M 
  i 
  1 
  1 
  e 
  p 
  o 
  r 
  a, 
  

   Stromatopora). 
  Most 
  hydroid 
  colonies 
  are 
  permanently 
  at- 
  

   tached 
  to 
  rocks, 
  seaweeds, 
  or 
  other 
  objects 
  of 
  support. 
  

  

  Genus 
  dictyonema 
  Hall 
  

  

  [Ety. 
  : 
  d{-/.Tu<p^, 
  net; 
  v^/za, 
  thread] 
  

  

  (1852. 
  Pal. 
  N. 
  Y. 
  2:174) 
  

  

  Colony 
  forming 
  a 
  network 
  of 
  anastomosing 
  branches, 
  the 
  whole 
  

   commonly 
  flattened 
  on 
  the 
  rock 
  surface, 
  but 
  originally 
  forming 
  a 
  

   funnel 
  or 
  fan-shaped 
  expansion. 
  The 
  branches 
  proceed 
  from 
  a 
  

   common 
  acute 
  base, 
  divide 
  frequently, 
  and 
  are 
  at 
  intervals 
  united 
  

   again 
  by 
  transverse 
  dissepiments. 
  The 
  outer 
  surfaces 
  of 
  the 
  

   branches 
  are 
  striated; 
  the 
  inner 
  bear 
  hydrothecae, 
  though 
  these 
  are 
  

   seldom 
  seen 
  in 
  the 
  flattened 
  specimens. 
  

  

  Dictyonema 
  retiforme 
  Hall. 
  (Fig. 
  27) 
  (1852. 
  Pal. 
  N. 
  Y. 
  2:174, 
  

   pi. 
  40F) 
  

  

  