﻿LOBSTER 
  AND 
  CLAM 
  INVESTIGATIONS. 
  185 
  

  

  STRUCTURE 
  AND 
  LIFE 
  HISTORY 
  OF 
  DIATOMS. 
  

  

  The 
  diatoms 
  are 
  a 
  well-defined 
  group 
  of 
  aquatic 
  plants 
  not 
  closely 
  

   related 
  to 
  an}^ 
  other. 
  Perhaps 
  they 
  should 
  be 
  placed 
  nearer 
  the 
  brown 
  

   alj^», 
  Phteoph3^cepe, 
  than 
  any 
  other 
  group, 
  and 
  might 
  be 
  defined 
  as 
  

   unicellular 
  alga, 
  characterized 
  by 
  a 
  silicification 
  of 
  the 
  cell 
  wall 
  and 
  

   l),v 
  the 
  presence 
  of 
  chlorophyl 
  and 
  a 
  brown 
  pigment, 
  diatomin. 
  

   Though 
  unicellular, 
  they 
  ma}" 
  be 
  united 
  in 
  chains 
  or 
  filaments, 
  or, 
  by 
  

   the 
  secretion 
  of 
  a 
  gelatinous 
  material 
  in 
  the 
  form 
  of 
  an 
  inclosing 
  

   sheath 
  or 
  a 
  supporting 
  stipe, 
  they 
  may 
  form 
  colonies 
  of 
  character- 
  

   istic 
  shape 
  adhering 
  to 
  plants 
  or 
  other 
  submerged 
  objects. 
  

  

  Cell 
  structure. 
  — 
  Though 
  the 
  diatoms 
  appear 
  in 
  a 
  great 
  variet}^ 
  of 
  

   forms 
  and 
  sizes, 
  their 
  structure 
  is 
  essentially 
  the 
  same 
  in 
  all. 
  The 
  cell 
  

   is 
  inclosed 
  in 
  a 
  shell 
  composed 
  of 
  silica, 
  consisting 
  of 
  two 
  sj^nmietrical 
  

   parts 
  or 
  valves, 
  which 
  are 
  in 
  contact 
  at 
  their 
  margins 
  with 
  an 
  inter- 
  

   mediate 
  hoop 
  or 
  girdle. 
  In 
  some 
  forms 
  one 
  valv^e 
  fits 
  over 
  the 
  other 
  

   like 
  the 
  cover 
  of 
  a 
  pill 
  box. 
  The 
  girdle 
  may 
  be 
  single 
  or 
  double 
  or 
  

   complex 
  in 
  structure, 
  with 
  one 
  or 
  more 
  plates 
  inserted 
  between 
  the 
  top 
  

   of 
  the 
  valve 
  and 
  the 
  girdle. 
  The 
  siliceous 
  shell 
  is 
  usually 
  elaborately 
  

   and 
  exquisitely 
  sculptured, 
  the 
  extreme 
  delicacy 
  of 
  the 
  details 
  with 
  

   which 
  the 
  valves 
  are 
  ornamented 
  making 
  the 
  diatoms 
  most 
  beautiful 
  

   o))jects 
  under 
  the 
  microscope, 
  and 
  testing 
  its 
  highest 
  powers. 
  

  

  The 
  form 
  of 
  the 
  diatom 
  varies 
  with 
  the 
  habits 
  of 
  the 
  species. 
  Most 
  

   of 
  the 
  free-swimming 
  forms 
  are 
  o))long, 
  oval, 
  or 
  spindle-shaped; 
  the 
  

   fixed 
  species 
  are 
  usually 
  of 
  different 
  shape 
  at 
  their 
  free 
  and 
  attached 
  

   ends; 
  the 
  floating 
  forms 
  have 
  special 
  contrivances 
  for 
  increasing 
  their 
  

   1)Uoyancy. 
  

  

  The 
  cytoplasm 
  is 
  disposed, 
  peripherally, 
  as 
  a 
  lining 
  to 
  the 
  cell 
  wall; 
  

   centrally, 
  it 
  ma}" 
  form 
  a 
  ])ridge 
  across 
  the 
  center 
  of 
  the 
  cell 
  or 
  may 
  

   take 
  the 
  form 
  of 
  a 
  stellate 
  mass 
  with 
  a 
  series 
  of 
  radiating 
  threads 
  

   extending 
  out 
  to 
  the 
  peripheral 
  cytoplasm. 
  The 
  nucleus 
  is 
  in 
  the 
  

   peripheral 
  cytoplasm 
  close 
  to 
  the 
  cell 
  wall, 
  is 
  suspended 
  by 
  the 
  

   protoplasmic 
  bridge, 
  or 
  is 
  in 
  the 
  center 
  of 
  the 
  stellate 
  mass 
  of 
  

   cytoplasm. 
  

  

  Chromatophores 
  are 
  always 
  present 
  in 
  the 
  cells. 
  They 
  are 
  

   yellowish-brown 
  in 
  color 
  and 
  contain 
  besides 
  chlorophyl 
  the 
  peculiar 
  

   pigment 
  diatomin. 
  The 
  chromatophores 
  vary 
  in 
  number 
  in 
  the 
  difi'er- 
  

   ent 
  species 
  and 
  take 
  the 
  form 
  of 
  bands, 
  granules, 
  or 
  rounded 
  masses 
  

   arranged 
  irregularly 
  or 
  in 
  radiating 
  lines. 
  The 
  arrangement 
  of 
  the 
  

   chromatophores 
  is 
  not 
  constant 
  even 
  in 
  the 
  same 
  individual. 
  An 
  

   amorphous 
  mass 
  may 
  become 
  divided 
  into 
  numerous 
  granules 
  of 
  equal 
  

   size 
  and 
  definite 
  outline. 
  There 
  seems 
  to 
  be 
  some 
  definite 
  relation 
  

   between 
  the 
  arrangement 
  of 
  the 
  chromatophores 
  and 
  the 
  growth 
  and 
  

   division 
  of 
  the 
  whole 
  frustule. 
  In 
  some 
  species 
  a 
  few 
  round 
  oil 
  

   globules 
  are 
  also 
  present 
  in 
  the 
  cyto})lasm. 
  

  

  