ME.  C.  SPENCE  BATE  ON  THE  DEVELOPMENT  OF  DECAPOD  CEHSTACEA.  601 
anterior  pair  become  the  cheliform  organs,  and  therefore  homologize  with  the  first  pair 
oi pereiopoda^  the  large-clawed  legs  in  the  Brachyura. 
This  observation  is  not  supported  by  the  researches  of  Captain  Du  Cane  on  PalcBmon; 
but  I have,  in  the  dissection  of  the  larva  of  an  unknown  Macroma,  removed  the  whole 
of  the  five  pairs  of  pereiopoda  from  behind  the  larval  appendages.  Captain  Du  Cane’s 
observations  appear  to  have  been  conducted  by  watching  the  animal  in  its  habits  and 
parts  without  dissection.  I therefore  feel  more  confidence  in  my  own  observations, 
since,  by  removing  every  member,  I could  examine  each  as  to  its  individual  form. 
These  remarks  are  also  supported  by  the  opinion  of  Eathke,  who  states  that  the  larval 
natatory  appendages  homologize  with  the  “ three  pairs  of  maxillipeds  ” of  the  adult ; 
that  is,  the  posterior  pair  of  the  cephalic  region,  and  the  two  anterior  pairs  of  the 
pereion — “ the  gnathopoda''  But,  as  I have  previously  stated,  in  the  Brachyura  the 
larva  has  but  two  pairs,  and  these  homologize  with  the  gnathopoda  only. 
Having  estabhshed  this  position,  we  shall  find  that  an  examination  of  their  develop- 
ment will  confirm,  by  the  forms  that  they  assume,  the  correctness  of  these  conclusions. 
In  the  earliest  form  each  of  these  organs  consists  of  a basal  articulation,  that  supports 
two  separate  appendages  (Plate  XL VI.  figs.  8,  9,  A).  One,  and  this  the  more  important 
(S),  in  this  young  condition  exhibits  alike  in  each  pair  but  two  articulations,  the  joint 
being  only  visible  after  being  treated  with  hquor  potassse.  The  other  appendage  (T) 
differs  in  both  pairs.  It  consists  of  five  distinct  articulations  in  the  anterior  (8),  and  but 
of  three  in  the  posterior  (9).  It  is  these  two  branches  (T)  that  represent  the  true  limbs 
of  the  adult.  The  others  (S),  without  passing  through  any  important  change,  become 
the  secondary  appendages  in  each. 
There  appears  to  be  no  very  important  alteration  in  the  separate  parts  of  the  animal 
until  ^0  pereiopoda  exhibit  appearance  of  strength ; but,  in  accordance  with  the  changing 
condition  in  its  habits  from  a swimming  to  a walking  animal,  the  gnathopoda  cease  to  be 
required  as  natatory  organs : they  consequently  lose  that  power,  proceed  in  the  progress 
of  their  development,  and  assume  that  which  enables  them  to  fulfil  the  functions  of 
external  permanent  organs  attendant  upon  the  mouth. 
But  the  great  distinction  between  the  early  and  the  late  forms  of  the  Brachyura  is 
dependent  upon  the  absence  in  the  early  stage  of  the  'pereiopoda, — the  five  posterior 
pahs  of  limbs  belonging  to  the  pereion. 
In  the  early  larva,  when  it  has  just  escaped  from  the  egg,  the  rudiments,  undistin- 
guishable  without  care,  alone  are  present,  in  the  form  of  sac-like  appendages.  After  a 
few  moults  they  are  visible  to  casual  observation,  and  increase  gradually  in  size,  joint 
by  joint,  until  they  are  of  sufficient  importance  to  assist  as  organs  of  locomotion. 
Gradually,  as  this  state  becomes  more  perfect,  the  gnathopoda  lose  their  natatory  cha- 
racter, and  assimilate  to  those  of  the  mature  animal. 
Posterior  to  these  there  are  no  appendages  in  the  larva,  but  the  germs  of  the  pleopoda 
are  capable  of  being  detected  shortly  after  bhth.  These  soon  increase  to  their  greatest 
comparative  extent.  At  the  same  time,  the  pleon,  which  at  first  had  but  six  segments, 
4 K 2 
