﻿43i 
  Kindle 
  — 
  Factor 
  in 
  Rounding 
  of 
  Sand 
  Grains. 
  

  

  The 
  sea-urchin 
  is 
  one 
  of 
  the 
  few 
  invertebrates 
  which 
  

   appear 
  to 
  thrive 
  best 
  on 
  a 
  sandy 
  bottom. 
  The 
  effect 
  on 
  

   the 
  sand 
  of 
  being 
  passed 
  through 
  the 
  echinoderm 
  stomach 
  

   and 
  subjected 
  to 
  the 
  rubbing 
  and 
  grinding 
  which 
  it 
  must 
  

   there 
  undergo 
  must 
  be 
  of 
  considerable 
  importance 
  in 
  

   reducing 
  both 
  the 
  size 
  of 
  the 
  grains 
  and 
  their 
  angularity. 
  

   Under 
  the 
  influence 
  of 
  muscular 
  compression 
  there 
  would 
  

   appear 
  to 
  be 
  scarcely 
  any 
  limit 
  to 
  the 
  small 
  size 
  to 
  which 
  

   the 
  muscular 
  attrition 
  could 
  reduce 
  the 
  sand 
  particles, 
  as 
  

   there 
  is 
  in 
  the 
  case 
  of 
  their 
  reduction 
  in 
  water. 
  

  

  Darwin 
  estimated 
  that 
  earthworms 
  on 
  each 
  acre 
  of 
  land 
  

   suitable 
  for 
  their 
  habitat 
  passed 
  through 
  their 
  bodies 
  

   annually 
  a 
  weight 
  of 
  more 
  than 
  ten 
  tons 
  of 
  earth. 
  13 
  It 
  

   appears 
  not 
  improbable 
  that 
  echinoderms 
  and 
  holothu- 
  

   rians 
  on 
  sea 
  bottom 
  areas 
  which 
  afford 
  a 
  suitable 
  habitat 
  

   may 
  subject 
  an 
  equally 
  large 
  volume 
  of 
  sand 
  to 
  trituration 
  

   in 
  their 
  stomachs. 
  In 
  any 
  event 
  it 
  must 
  be 
  granted 
  that 
  

   a 
  very 
  large 
  volume 
  of 
  sand 
  is 
  annually 
  passed 
  through 
  

   the 
  bodies 
  of 
  these 
  voracious 
  creatures 
  and 
  that 
  they 
  play 
  

   an 
  important 
  role 
  in 
  the 
  rounding 
  of 
  sand 
  grains. 
  

  

  Eecognition 
  of 
  the 
  agency 
  of 
  echinoderms 
  in 
  rounding 
  

   sand 
  grains 
  seems 
  to 
  lead 
  to 
  the 
  conclusion 
  that 
  more 
  

   rounding 
  of 
  sand 
  grains 
  occurs 
  on 
  the 
  sea 
  bottom 
  than 
  

   has 
  hitherto 
  been 
  suspected. 
  It 
  can 
  hardly 
  be 
  claimed 
  

   however 
  that 
  the 
  work 
  of 
  echinoderms, 
  important 
  as 
  it 
  

   may 
  be, 
  rivals 
  that 
  of 
  the 
  wind 
  in 
  rounding 
  grains 
  of 
  sand. 
  

   It 
  should 
  also 
  be 
  noted 
  that 
  the 
  wind 
  probably 
  exercises 
  

   a 
  selective 
  process 
  in 
  removing 
  sand 
  from 
  the 
  sea 
  shore 
  

   to 
  the 
  dunes, 
  taking 
  more 
  of 
  the 
  rounded 
  than 
  of 
  the 
  

   angular 
  grains 
  because 
  "a 
  round 
  sand 
  grain 
  will 
  prob- 
  

   ably 
  run 
  before 
  the 
  wind 
  better 
  than 
  a 
  flat 
  one." 
  14 
  

  

  13 
  Darwin, 
  Chas. 
  : 
  The 
  formation 
  of 
  vegetable 
  mould 
  through 
  the 
  action 
  

   of 
  worms, 
  p. 
  236, 
  1904, 
  ed. 
  (1st 
  ed.) 
  1881. 
  

  

  14 
  Richardson, 
  Hugh: 
  Sea 
  Sand. 
  — 
  Ann. 
  Eept. 
  Yorkshire 
  Philosophical 
  

   Soc. 
  for 
  1902 
  (1903) 
  pp. 
  57, 
  58. 
  

  

  