ME.  C.  SPENCE  BATE  ON  THE  DEVELOPMENT  OF  DECAPOD  CEUSTACEA.  593 
supports  two  distinct  appendages,  the  one  consisting  of  five  short  articulations,  the  other 
of  two  long  ones,  terminating  with  four  long  finely-ciliated  hairs. 
The  next  pair  of  members  difier  little  from  the  last,  and  represent  the  second  pair  of 
gnatJiopoda  (Plate  XL.  figs.  A and  B,  9)  in  the  adult.  Like  the  preceding,  each  consists 
of  a basal  joint  supporting  two  appendages,  one  of  which  is  formed  of  three  short  arti- 
culations, the  other  of  two  long  ones,  furnished  at  the  extremity  with  four  long  hairs 
delicately  ciliated.  At  the  base  of  these  two  last  pairs  of  appendages,  the  germ  of  the 
future  branchiae  may  be  distinguished.  These  two  last  pairs  fulfil  the  office  of  natatory 
organs  in  this  early  stage  of  the  animal. 
Posterior  to  the  last  pair  of  members  that  I have  described  several  sacs  are  visible 
Plate  XL.  figs.  A and  B,  10).  These  e’sfidently  contain  the  germs  of  the  five  pairs  of 
jpereiojooda,  or  true  perambulatory  legs,  the  most  anterior  of  which  I think  I have  been 
enabled  to  perceive  lying  folded  within  the  sac,  as  shown  in  Plate  XL.  fig.  10.  Some 
of  these  small  sac-buds  probably  are  the  germs  of  the  future  branchiae  ; and  it  is  not  im- 
probable that  in  this  embryonic  condition  they  fulfil  the  object  of  their  design  suffi- 
ciently well  for  so  immature  a creature. 
We  now  come  to  the  third  division  of  the^ mimal,  the  pleon,  or  that  portion  which 
in  adult  Crastacea  supports  the  swimming  legs.  It  consists  at  this  stage  of  six  segments ; 
five  upon  one  type,  the  last  difierent.  The  first  five  are  simple  articulations,  and  carry 
no  developed  appendage,  except  two  small  tooth-like  processes  upon  the  second.  But 
close  observation  will  detect  the  germs  of  the  iwtme  pleopoda  (Plate  XL.  fig.  17)  upon 
one  or  two,  situated  laterally  and  inferiorly.  Each  of  them  consists  of  but  a simple  bud, 
on  the  margin  of  which  can  be  detected  three  or  four  clear  bead-like  spots,  the  rudi- 
ments of  the  undeveloped  cilia. 
There  are  no  data  by  which  to  determine  the  homologue  of  the  two  denticular  pro- 
cesses (Plates  XL.  and  XLl.  figs.  B,  C"',  D",  16)  upon  the  second  segment.  They  are 
directed  forwards,  and  agree  somewhat  in  position  to  the  external  organs  of  the  adult 
male.  They  are  constant  upon  all  the  specimens  in  this  species,  and  in  that  of  Cancer 
Pagurus  also,  the  lan  a of  which  agrees  so  nearly  with  that  of  Carcinus^  that  it  is  doubt- 
ful if  the  closest  observation  could  identify  them  apart.  The  jpleopoda  in  an  early  stage 
are  attached  to  all  except  the  first  and  last  of  the  segments  of  the  pleon,  but  are  present 
only  in  the  adult  female  of  the  Brachyura. 
If  there  be  any  truth  in  the  hypothesis  relative  to  the  denticular  processes  upon  the 
second  segment,  each  animal  must  in  its  larval  condition  contain  within  itself  the  germs 
of  both  sexes.  The  one  or  the  other  becomes  developed  according  to  more  or  less 
favourable  circumstances.  If  this  be  the  case,  it  is  not  difficult  to  account  for  the  occa- 
sional development  of  distinct  sexes  upon  opposite  sides  of  the  same  animal ; or,  as  I 
have  in  my  possession,  a Corystes  cassivelaunus  bearing  the  aspect  of  the  female,  and  yet 
possessing  male  organs. 
The  sixth  or  last  appendage  (Plate  XL.  figs.  A and  B,  21)  to  the  larva  homologizes  with 
the  last,  or  telson  of  the  adult.  It  forms  a prominently  forked  appendage,  consisting  of  two 
4 I 2 
