598  ME.  C.  SPENCE  BATE  ON  THE  DETELOP:^IENT  OE  DECAPOD  CEI'STACEA. 
other  parts,  since  they  can  be  perceived  in  their  position  less  and  less  prominent  until 
the  young  creature  has  obtained  the  more  important  featru’es  of  the  adult.  Looking  at 
the  carapax  dorsally,  it  gradually  increases  in  breadth  from  the  extreme  point  of  the 
frontal  spine  to  the  eyes,  when,  lea\dng  orbits  for  the  eyes,  it  widens  a little  imme- 
diately behind  them,  and  then  again  it  increases  in  diameter  unto  the  posterior  margin. 
The  larva,  like  the  adult,  has  the  power  of  elevating  the  carapax.  The  cuiTent  of 
water  that  laves  the  branchise  flows  in  beneath  the  posterior  dorsal  and  lateral  edges, 
while  a membrane  unites  the  centre  of  the  carapax  with  the  pleon,  and  precludes  the 
admission  of  water  to  the  internal  viscera. 
The  larval*  form  continues  with  a slight  tendency  to  an  increase  in  the  relative  length 
and  width  of  the  whole  until  the  hepatic  regions  (Plate  XLIII.  flg.  F a)  become  visible  ; 
and  when  these  appear,  the  dorsal  spine  is  lost,  and  the  animal  assumes  the  general 
aspect  of  a Decapod,  approximating  nearest  in  appearance  to  the  form  of  the  Spider- 
crabs  among  adults. 
Soon  after  the  hepatic  lobes  are  visible,  they  become  cronmed  first  with  one  (Plate 
XLIV.  G.  1)  anterior  tooth,  then  two  (m),  three  (?i),  and  four  (o).  These  ultimately 
increase  in  diameter  and  number,  and  become  the  latero-anterior  denticulated  mai'gin 
of  the  adult  Carcinus. 
This  margin  on  each  side  generally  extends  outwards  and  forwards,  and  the  anterior 
tooth  (/)  becomes  the  outer  limit  of  the  orbit ; while  its  under  margin  is  formed  by  the 
carapax  (fig.  G")  being  reflected  inferiorly ; its  external  hmit,  -nith  the  appendage  which 
it  supports — the  second  antenna  (3), — being  planted  upon  the  inner  side  of  the  eye. 
In  the  earliest  form  the  eyes  appear  sessile.  This,  however,  continues  but  a very  Kttle 
while.  Before  the  animal  has  undergone  a change  in  its  general  form,  the  eyes  are 
elevated  upon  prominent  footstalks.  But  even  in  its  earliest  stage  this  condition  of  the 
eyes  is  to  our  conception  sessile,  rather  because  the  peduncle  is  limited,  than  because  it 
is  altogether  absent.  Thus  in  the  genus  Cyrphiops,  Dana,  of  the  southern  seas,  the  eyes 
are  hid  entirely  beneath  the  carapax,  and  in  some  of  our  (subterranean  1)  Macroura  the 
peduncle  is  so  short  that  the  eye  cannot  readily  be  distinguished  from  the  sessile  form. 
This,  which  we  find  in  the  adult  of  different  genera,  is  but  a condition  similar  to  that  in 
the  Zo'ea.  Thus  in  the  Macroura^  where  the  ophthalmic  segment  is  absent,  as  I have 
observed  in  the  young  of  Crangon,  and  has  been  represented  in  Palcemon  by  Captain 
Du  Cajne,  the  eyes  are  united  at  the  base,  as  Lvo  organs  springing  from  one  origin ; but 
in  the  Brachyura^  where  the  ophthalmic  segment  is  present, — though  hid  and  very  unim- 
portant,— the  eyes  of  the  larva  do  not  so  closely  approach  each  other ; and,  when  seen 
dorsally  (as  in  fig.  B",  Plate  XL.),  exhibit  evidences  that  they  are  actually  pedmiculated 
in  this  early  stage*.  With  the  development  of  these  organs,  the  angle  formed  by  the 
* Since  this  paper  has  been  laid  before  the  Society,  Mr.  E.  Q.  CotJCH  has  sliowu,  in  a paper  read  before 
the  British  Association  at  Dublin,  and  published  in  the  ‘ Natmal  History  Eeview  ’ for  October,  1857,  that  in 
Palinurus,  in  which  the  adult  has  the  ophthalmic  segment  largely  developed,  the  eyes  in  the  lai’val  condition 
are  distinctly  pedunculated. 
