﻿186 
  KEPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  

  

  Motility. 
  — 
  Many 
  of 
  the 
  free-living 
  diatoms 
  have 
  the 
  power 
  of 
  move- 
  

   ment. 
  The 
  mechanism 
  of 
  this 
  motion 
  has 
  been 
  variously 
  explained: 
  

   (1) 
  As 
  produced 
  by 
  pseudopodia 
  of 
  protoplasm 
  extending- 
  through 
  open- 
  

   ings 
  in 
  the 
  cell 
  wall; 
  (2) 
  by 
  the 
  presence 
  of 
  cilia 
  extending 
  through 
  

   the 
  cell 
  wall; 
  (3) 
  by 
  endosmatic 
  currents 
  of 
  water 
  passing 
  in 
  and 
  out 
  

   of 
  the 
  cell. 
  As 
  the 
  diatoms 
  on 
  the 
  lobster 
  were 
  all 
  fixed 
  species, 
  the 
  

   matter 
  of 
  motion 
  was 
  not 
  specially 
  investigated 
  in 
  connection 
  with 
  the 
  

   present 
  question, 
  although 
  frequently 
  in 
  stained 
  specimens 
  apparent 
  

   cilia 
  were 
  observed 
  extending 
  from 
  all 
  sides 
  of 
  the 
  f 
  rustule. 
  This 
  same 
  

   appearance 
  has 
  been 
  noted 
  in 
  some 
  of 
  the 
  motile 
  forms, 
  and 
  it 
  has 
  been 
  

   suggested, 
  on 
  what 
  grounds 
  I 
  can 
  not 
  say, 
  that 
  these 
  cilia 
  are 
  probably 
  

   fungoid 
  growths. 
  

  

  Reproduction. 
  — 
  The 
  ordinary 
  method 
  of 
  multiplication 
  of 
  the 
  dia- 
  

   toms 
  is 
  simple 
  cell 
  division. 
  The 
  nucleus 
  divides 
  first, 
  the 
  chromato- 
  

   phores 
  divide 
  either 
  before 
  or 
  after 
  the 
  division 
  of 
  the 
  protoplasm, 
  

   and 
  two 
  new 
  cells 
  are 
  formed 
  within 
  the 
  old 
  pair 
  of 
  valves. 
  Each 
  

   of 
  the 
  new 
  cells 
  forms 
  a 
  new 
  valve 
  on 
  its 
  inner 
  side, 
  so 
  that 
  the 
  new 
  

   valves 
  lie 
  back 
  to 
  back 
  along 
  the 
  line 
  of 
  division. 
  In 
  cases 
  where 
  the 
  

   valves 
  are 
  of 
  unequal 
  size, 
  as 
  each 
  old 
  valve 
  becomes 
  the 
  larger 
  valve 
  

   of 
  the 
  new 
  diatom, 
  it 
  follows 
  that 
  after 
  division 
  the 
  daughter 
  diatoms 
  

   are 
  smaller 
  than 
  the 
  original. 
  In 
  those 
  species 
  in 
  which 
  the 
  valves 
  do 
  

   not 
  increase 
  in 
  size 
  this 
  results 
  in 
  a 
  great 
  diminution 
  in 
  the 
  size 
  of 
  the 
  

   new 
  diatoms, 
  and 
  the 
  original 
  size 
  is 
  again 
  restored 
  by 
  the 
  formation 
  

   of 
  "auxospores." 
  

  

  The 
  formation 
  of 
  auxospores 
  was 
  at 
  one 
  time 
  supposed 
  to 
  take 
  place 
  

   merel}^ 
  to 
  compensate 
  for 
  the 
  reduction 
  in 
  the 
  size 
  of 
  the 
  diatoms 
  by 
  

   repeated 
  cell 
  divisions. 
  They 
  are 
  more 
  properly 
  considered 
  as 
  forms 
  

   of 
  reproduction. 
  Two 
  kinds 
  must 
  be 
  considered 
  — 
  the 
  asexual 
  and 
  the 
  

   sexual 
  auxospores. 
  In 
  the 
  former 
  the 
  cell 
  contents 
  separate 
  from 
  the 
  

   cell 
  wall, 
  increase 
  greatly 
  in 
  size, 
  with 
  or 
  without 
  division 
  and 
  subse- 
  

   quent 
  coalition, 
  surround 
  themselves 
  with 
  a 
  membrane, 
  and 
  finally 
  form 
  

   a 
  new 
  diatom 
  within, 
  of 
  the 
  maximum 
  size 
  of 
  the 
  species. 
  In 
  the 
  sexual 
  

   method 
  the 
  cell 
  contents 
  escape 
  from 
  two 
  cells, 
  fuse, 
  a 
  true 
  fertiliza- 
  

   tion 
  takes 
  place, 
  and 
  a 
  new 
  diatom 
  is 
  formed 
  from 
  the 
  resulting 
  cell 
  

   either 
  at 
  once 
  or 
  after 
  a 
  preliminary 
  division. 
  

  

  The 
  formation 
  of 
  swarm 
  spores 
  in 
  the 
  diatoms 
  has 
  not 
  been 
  observed, 
  

   although 
  in 
  many 
  species 
  there 
  are 
  indications 
  that 
  some 
  such 
  phe- 
  

   nomenon 
  occurs. 
  From 
  the 
  relation 
  of 
  the 
  diatoms 
  to 
  other 
  alga? 
  in 
  

   which 
  this 
  is 
  a 
  common 
  method 
  of 
  reproduction 
  it 
  seems 
  most 
  prob- 
  

   able 
  that 
  it 
  does 
  occur. 
  A 
  few 
  species 
  form 
  resting 
  spores. 
  The 
  

   protoplasm 
  of 
  the 
  cell 
  becomes 
  condensed 
  into 
  about 
  one-third 
  of 
  its 
  

   normal 
  volume, 
  and 
  a 
  thick 
  cell 
  wall 
  of 
  definite 
  and 
  peculiar 
  shape 
  is 
  

   formed 
  about 
  it. 
  In 
  this 
  state 
  the 
  diatoms 
  are 
  extremely 
  resistant 
  and 
  

   are 
  able 
  to 
  await 
  the 
  return 
  of 
  more 
  favorable 
  conditions, 
  when 
  they 
  

   reassume 
  the 
  original 
  form 
  of 
  the 
  species. 
  

  

  