590  ME.  C.  SPENCE  BATE  ON  THE  DEYELOP3IENT  OF  DECAPOD  CECSTACEA. 
This  is  the  case  with  all  the  Edriophthalma ; and  in  the  Eeport  upon  the  British 
Species  which  I was  requested  to  lay  before  the  Members  of  the  British  Association, 
the  following  passage  occurs : — “ In  the  highest  types  of  Crustacea,  the  immense  variety 
of  change  from  the  Zo  'ea  to  the  adult  animal  is  but  the  result  of  subordinate  becoming 
more  imjportmit  parts,  together  with  the  development  of  others  not  yet  present,  and 
therefore  hardly  acceptable  under  the  signification  of  metamorphosis*.” 
It  is  this  opinion  that  I am  now,  to  the  best  of  my  ability,  about  to  demonstrate,  that 
from  the  Zoea  or  larval  form  to  that  of  the  well-developed  Decapod,  there  is  no  sudden 
change  such  as  is  understood  by  the  term  metamorphosis,  and  generally  believed  by 
naturalists ; but  that  the  development  is  gradually  progressive  throughout ; that  some 
parts  are  absent,  and  that  some  which  are  present  may  become  obsolete,  but  that  each 
part  undergoes  gradual  transformation. 
In  order  to  arrive  at  a degree  of  certainty,  the  researches  upon  this  subject  have  been 
confined  to  one  species,  others  being  used  as  collateral  evidence  only.  The  larvae  in  the 
earliest  stages  were  procured  direct  from  Carcinus  Mcenas,  being  hatched  in  my  own 
possession.  Those  of  a later  period  have  been  the  result  of  two  or  thi’ee  years’  accumu- 
lations, most  of  which  were  taken  in  Plymouth  Sound.  Thus  procuring  a number,  I 
have  been  enabled  to  trace  through  a scarcely  broken  chain  the  progressive  development 
of  the  respective  parts  up  to  that  which  they  assume  in  the  adult  animal. 
The  embryo,  when  it  first  quits  the  egg  (Plate  XL.  fig.  A),  is  enclosed  within  a general 
tunic,  which  embraces  each  individual  member  and  is  conformable  to  the  whole  animal. 
This  tunic  is  of  extreme  tenuity  and  exceedingly  transparent;  it  is  very  liable  to  be 
overlooked  without  close  observation.  Within  this  tunic  may  be  distinctly  seen  the  hairs 
belonging  to  each  part  respectively.  These  are  not  extended,  but  retracted  ufith  tele- 
scopic joints,  within  the  members  of  which  they  form  appendages.  In  this  way  also  the 
larger  processes  of  the  caudal  extremity  (fig.  21')  are  contracted  within  shorter  limits,  as 
are  also  the  horn-like  processes  upon  the  dorsal  surface  and  frontal  region.  These  rivo 
are  moreover  folded  down,  the  dorsal  laid  forward  upon  the  top  of  the  head,  the  frontal 
brought  down  between  the  eyes  and  compressed  beneath  the  frontal  region  (fig.  A).  In 
this  embryonic  condition  the  animal  swims  about  for  a few  hours.  It  then  liberates  itself 
from  the  general  tunic  by  throwing  ofi*  that  portion  which  covered  the  head  and  pereion, 
anteriorly,  while  that  which  protects  the  ipleon  is  draA\m  off  posteriorly  f. 
The  animal  is  now  freed  from  its  embryonic  condition,  and  more  actively  sports  in 
the  water,  and  procures  its  own  food.  Its  gambols  are  produced,  partly  by  the  action 
* Tlie  author  is  perfectly  aware  that  in  Insecta  the  change  of  the  animal  within  the  chrysalis  is  gradual 
in  development ; but  he  wishes  to  show  that  there  is  no  stage  in  Crustacea  answermg  to  the  chrysalis ; 
that  the  moults  in  process  of  development  of  the  Crustacea  are  of  the  same  kind  as  those  which  take  place 
in  the  adult  condition. 
t The  terms  used  in  this  paper  are  those  suggested  by  the  author  in  his  Eeport  on  the  British  Edrioph- 
thalma  (British  Association  Eeport  for  1855,  p.  27),  instead  of  the  old  and  incorrect  synonyms  of  Thorax, 
Abdomen,  &c.  The  accompanying  diagram  of  an  imaginary  Zoea  exhibits  the  details  in  position. 
The  term  pereion  is  derived  from  Trepatow,  to  lualk  about,  being  that  portion  of  the  animal  which  supports 
