﻿Structure 
  of 
  Astr^eospongia 
  meniscus. 
  113 
  

  

  We 
  have 
  also 
  treated 
  some 
  of 
  the 
  microscopical 
  sections 
  

   with 
  acids, 
  producing 
  most 
  satisfactory 
  results. 
  

  

  The 
  microscopic 
  examination 
  of 
  the 
  sediment 
  left 
  after 
  the 
  

   etching 
  process, 
  was 
  likewise 
  important, 
  as 
  in 
  many 
  instances 
  

   we 
  found 
  it, 
  in 
  a 
  great 
  measure, 
  composed, 
  of 
  tubes 
  (perhaps 
  

   spiculse) 
  the 
  presence 
  of 
  which 
  we 
  had 
  not 
  before 
  detected 
  in 
  

   the 
  sponge. 
  

  

  The 
  external 
  form 
  of 
  the 
  sponge 
  has 
  already 
  been 
  given 
  in 
  

   our 
  description 
  taken 
  from 
  Professor 
  Romer, 
  from 
  a 
  simple 
  

   disc 
  gradually 
  passing 
  into 
  the 
  form 
  of 
  a 
  cup. 
  

  

  The 
  diameters 
  of 
  our 
  specimens 
  vary 
  from 
  two 
  to 
  eight 
  

   centimetres, 
  and 
  the 
  perpendicular 
  depth 
  is 
  from 
  one-half 
  to 
  

   four 
  centimetres. 
  

  

  Judging 
  from 
  the 
  examination 
  of 
  a 
  great 
  number 
  of 
  speci- 
  

   mens, 
  that 
  the 
  original 
  form 
  of 
  all 
  these 
  sponges 
  was 
  a 
  simple 
  

   thin 
  flat 
  disc, 
  containing 
  only 
  one 
  or 
  two 
  layers 
  of 
  colons, 
  it 
  is 
  

   by 
  the 
  gradual 
  growth 
  and 
  addition 
  of 
  cells 
  and 
  colons 
  that 
  it 
  

   takes 
  its 
  final 
  cup-like 
  form 
  ; 
  the 
  central 
  depth 
  of 
  which 
  is 
  

   about 
  half 
  the 
  height 
  of 
  the 
  cup. 
  

  

  The 
  convex 
  side 
  of 
  many 
  of 
  the 
  specimens 
  before 
  us 
  show 
  a 
  

   slight 
  depression, 
  nearly 
  central, 
  and 
  this 
  peculiarity 
  is 
  much 
  

   more 
  marked 
  upon 
  the 
  flatter 
  specimens. 
  In 
  many 
  cases 
  a 
  well 
  

   defined 
  star 
  marks 
  the 
  middle 
  of 
  this 
  indentation. 
  

  

  The 
  concave 
  surface, 
  as 
  a 
  rule, 
  exhibits 
  the 
  stars 
  more 
  finely 
  

   developed 
  and 
  in 
  a 
  much 
  better 
  state 
  of 
  preservation. 
  The 
  

   central 
  depression 
  is 
  also 
  noticed, 
  in 
  some 
  instances, 
  upon 
  this 
  

   side, 
  conforming 
  in 
  position 
  to 
  that 
  which 
  appears 
  upon 
  the 
  

   convex 
  surface;- 
  but 
  neither 
  of 
  these 
  depressions 
  seem 
  to 
  mark 
  

   a 
  point 
  of 
  attachment 
  to 
  any 
  extraneous 
  object. 
  As 
  Professor 
  

   Romer 
  has 
  already 
  stated, 
  there 
  appears 
  to 
  be 
  no 
  doubt 
  that 
  

   these 
  sponges 
  were 
  free, 
  either 
  floating 
  in 
  the 
  water 
  or 
  bedded 
  

   in 
  the 
  mud. 
  

  

  According 
  to 
  Dr. 
  Bronn 
  in 
  his 
  u 
  Thier 
  Reichs," 
  we 
  can 
  con- 
  

   clude, 
  from 
  the 
  minuteness 
  of 
  the 
  pores 
  in 
  the 
  species 
  under 
  

   consideration, 
  that 
  they 
  inhabited 
  shallow 
  waters; 
  as 
  from 
  

   observation 
  upon 
  recent 
  sponges 
  it 
  is 
  found 
  that 
  the 
  size 
  of 
  the 
  

   pores, 
  bears 
  certain 
  relation 
  to 
  the 
  depth 
  of 
  the 
  water 
  in 
  which 
  

   they 
  live, 
  increasing 
  in 
  size 
  somewhat 
  in 
  proportion 
  to 
  the 
  

   depth 
  of 
  the 
  water. 
  

  

  Dr. 
  Romer 
  states, 
  evidently 
  from 
  analogy 
  with 
  the 
  recent 
  

   8 
  

  

  