600  ME.  C.  SPENCE  BATE  ON  THE  DEVELOP:HENT  OE  DECAPOD  CEESTACEA. 
embryonic  appendages  are  most  commonly  present  in  the  adults  of  the  lower  forms  of 
Crustacea. 
Perceiving  the  squamiform  articulation  to  be  common  to  the  larva  of  Macroura  as  well 
as  the  Brachyura^  we  are  enabled  to  identify  it  as  the  homologue  of  the  same  appendage 
in  the  adult  Macroura,  and  to  state  that  the  articulation  from  which  it  springs  is  the 
homologue  of  the  hasos  articulation  of  the  adult.  Unlike  as  is  the  earhest  condition  of 
the  second  antenna  to  the  mature  organ,  yet  before  the  larva  has  lost  the  dorsal  spine  it 
has  assumed  much  the  appearance  of  the  fnlly  developed  state,  and  exhibits  upon  the 
coxa  the  yet  imperfect  stage  of  the  olfactory  organ,  which  shortly  afterwards  is  pro- 
tected by  a strong  operculum,  that  permits  the  internal  organ  to  be  exposed,  or  shut  out 
from  external  sensations.  Early  in  its  existence  this  antenna  is  attached  at  the  base 
only,  but  in  the  adult  form  the  lateral  portion  of  the  carapax  has  so  increased  upon 
its  structure,  that  the  filamentary  termination  only  extends  free  (Plate  XLV.  fig.  3. 
XL VI.  fig.  3,  J) ; the  peduncle,  except  the  last  articulation,  without  being  actually  fused, 
is  closely  incorporated  with  the  surrounding  structure,  and  forms  the  internal  wall  of 
the  orbit,  and  becomes  the  partition  between  it  and  the  foramen  in  which  the  anterior 
antenna  is  planted. 
The  mandible  (Plate  XLVI.  fig.  B,  4)  likewise  undergoes  considerable  changes  between 
the  early  and  late  conditions.  At  first  there  appears  no  internal  lever  or  calcareous 
tendon,  but  it  consists  of  a strong  denticulated  curved  scale  articulating  with  the 
integument.  It  bears  a near  resemblance  to  its  homologue  in  the  Ediiophthalmous 
Crustacea  (fig.  4 Q).  It  is  without  the  (mis-named)  palp*,  which  appears  soon  after 
and  is  produced  contemporary  with  the  internal  continuation  or  lever  of  the  organ. 
The  maxillae  (Plate  XLVI.  figs.  5,  6)  differ'  but  in  the  relative  dimensions  of  their 
respective  parts  from  those  of  the  adult ; and  the  first,  hke  the  mandible,  bears  an  ex- 
tremely close  resemblance  to  that  of  the  Amphipoda  (fig.  5 Q). 
The  next  pair  of  organs  present  in  the  adult  are  wanting  in  the  larva  of  the  Bra- 
chyura.  These  are  the  posterior  cephalic  appendages,  and  are  developed  only  contem- 
porary with  the  pereiopoda.  The  absence  of  these  last  appendages  appears  peculiar'  to 
the  Brachyura.  My  own  observations  on  the  Macroura  support  those  made  by  Captain 
Du  Cane,  M.  Pathke,  and  M.  Jolt,  who  describe  three  pairs  of  natatory  feet  as  belong- 
ing to  the  larva  of  the  Macroura. 
The  two  pairs  of  appendages  present  in  the  young  Brachyura  homologize  uith  the 
two  pairs  of  gnathopoda  in  the  adult  (Plate  XLVI.  figs.  8,  9,  A to  J). 
If  the  animal  be  examined  after  a few  moultings,  it  rvill  be  found,  as  previously 
described,  that  the  pereiopoda,  which  can  be  detected  immediately  after  birth,  are 
developed  directly  posterior  to  those  existing  in  the  larva.  These  have  also  been  per- 
ceived by  Captain  Du  Cane  in  Palcemon  in  the  same  relative  position.  Examination  of 
these  members  individually  enables  the  relative  character  of  each  to  be  detected.  The 
* The  so-called  palp  is  the  homologue  of  the  continuous  joints  of  the  member,  and  not  a secondiu’v 
appendage,  and  probably  represents  the  fourth,  fifth  and  sixth  segments  of  the  true  leg. 
