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XXVIII.  On  the  Development  of  Decapod  Crustacea.  By  C.  Spence  Bate,  F.L.S.,  <&c. 
Communicated  hy  Sir  W.  Snow  Haeeis,  F.B.S. 
Eeceived  May  1, — Eead  June  18,  1857. 
Many  years  have  elapsed  since  Mr.  Vaughan  Thomson  made  the  important  discovery 
that  the  Zo  'ea  of  naturahsts  was  but  the  immature  form  of  an  adult  Decapod. 
His  fortunate  observation  was  received  with  the  greatest  caution  by  zoologists  in 
general,  and  several  undertook  researches  upon  the  subject  to  demonstrate  the  error 
into  which  they  assumed  he  had  fallen.  The  researches  of  Kathke  upon  the  Astacus 
Jluviatilis,  of  MM.  Milne-Edwaeds  and  Audouin  upon  marine  Decapoda,  together  with 
those  of  Mr.  Westwood  upon  the  genus  Gegarcinus,  for  a considerable  period  delayed 
the  general  acceptation  of  the  fact  that  the  Zoea  is  the  larva  of  a Decapod. 
Rathee,  in  1840*,  published  the  results  of  a second  investigation  on  the  subject,  being 
his  researches  upon  Pagurus^  Astacus  marinus,  and  Hyas.  In  these  he  corroborated 
the  statements  of  Mr.  Thomson,  and  admitted  that,  “ relying  on  the  history  of  the  deve- 
lopment of  the  Cra}'tish,  and  trusting  too  much  to  analogies  in  the  structure  of  full- 
grown  Decapods,”  he  had  done  Mr.  Thomson  injustice  in  not  putting  faith  in  his  dis- 
covery. 
It  was  not  long  after  Mr.  Thomson’s  fii-st  valuable  discovery  that  good  fortune  placed 
within  his  reach  the  opportunity  to  show  some  other  changes  that  the  young  animal 
passed  through  in  its  transition  to  a perfect  creature ; and  by  a singular  coincidence,  the 
same  volume  of  the  Philosophical  Transactions  which  contains  Mr.  Westwood’s  refuta- 
tion of  what  he  calls  Mr.  Thomson’s  theory,  also  contains  that  latter  gentleman’s  further 
discoveries  of  what  has  been  termed  the  metamorphoses  through  which  the  animal  passed 
in  its  growth  to  the  mature  Crab ; and  where  he  demonstrates  that  the  Zo  'ea  and  Mega- 
lopa  of  naturalists  are  but  two  forms  in  the  development  of  one  and  the  same  animal. 
Mr.  Couch,  Mr.  Goodsie,  M.  Jolt,  Captain  Du  Cane,  Rathke,  Bell,  and  most  natural- 
ists have  since  confirmed  the  assertion  of  Mr.  Thomson,  and  affirm  that  two  remarkable 
metamorphoses  take  place  during  the  progressive  development  to  the  adult  stage.  This 
is  accepted  as  the  present  state  of  our  knowledge  in  Dr.  Caepentee’s  recent  work  on  the 
microscope,  and  in  Professor  Bell’s  ‘ British  Crustacea.’ 
To  this  general  rule  a few  exceptions  exist.  The  Land-crabs  and  freshwater  Crayfish 
form  the  most  important.  Beyond  these  we  know  of  none  in  the  same  orders  of  Crus- 
tacea, the  young  of  which  quits  the  ovum  with  all  its  members  developed  and  closely 
approximating  in  form  to  that  of  the  adult. 
* Annals  of  Natural  History,  vol.  vi. 
