ME.  C.  SPENCE  BATE  ON  THE  DEVELOPMENT  OF  DECAPOD  CEHSTACEA.  599 
reflexion  of  the  carapax  upon  the  inferior  surface  of  the  animal  continues  to  increase, 
and,  as  previously  shown,  forms  the  orbit  that  protects  the  eye. 
At  fli’st  the  upper  antenna  is  embryonic  (Plate  XL.  fig.  2)  in  its  character,  and  bears 
a more  imperfect  appearance  than  is  seen  in  any  of  the  adult  Crustacea,  but  approxi- 
mates nearer  to  that  of  Biastylis  Bathkii  than  to  any  of  which  we  are  aware.  But,  as 
we  have  before  observed,  there  are  conditions  in  the  structure,  even  at  this  early  period, 
which  enable  us  not  only  to  identify  the  organ  with  its  homologue  in  the  adult,  but 
also  to  demonstrate  the  parts  of  it  that  are  actually  present. 
In  all  adult  Crustacea,  as  we  have  elsewhere  shown*,  the  anterior  antenna  has  upon 
each  articulation  of  the  filament  two  or  more  membranous  hair-like  appendages,  which 
I have  thought,  from  their  being  constant  to  the  auditory  antenna,  and  never  found  on 
any  other  part,  to  have  an  intimate  connexion  with  the  sense  of  hearing,  and  therefore 
call  them  auditory  cilia. 
A\Tien  the  animal  throws  off  the  earliest  integumentary  surface,  there  are  but  two  of 
these  auditory  cilia  visible ; but  them  presence  indicates  not  only  that  the  organ  is,  as 
its  position  implies,  the  homologue  of  the  anterior  antenna,  but  that  the  germ  of  the 
future  fllament  is  present  in  a single  articulation  already  developed.  After  several 
moults,  two,  three  and  more  articulations  appear  successively,  and  a corresponding 
increase  to  the  number  of  auditory  cilia ; and  soon  the  germ  of  the  secondary  appen- 
dage is  visible.  We  next  perceive  that  the  base  of  the  organ  has  increased  in  diameter, 
and  assumes  a general  form  differing  but  httle  from  that  in  the  adult  organ,  and  this 
while  the  animal  is  yet  so  yoimg  that  it  has  lost  nothing  of  its  larval  appearance. 
In  the  mean  time  a corresponding  change  has  taken  place  in  the  other  organs.  The 
second  antenna  is  considerably  produced  in  length,  but  as  yet  it  has  not  assumed  the 
form  of  the  adult  organ. 
When  flrst  developed,  as  we  have  before  seen,  the  second  antenna  consists  of  two 
branches  springing  from  the  same  base.  These  branches  we  believe  homologize  with 
similar  appendages  to  the  adult  organ,  found  mostly  in  the  Macroura  and  Anamoura 
decapods.  The  true  antenna  springs  from  a small  bud-like  germ  at  their  base  (Plates 
XL.  and  XLVI.  flg.  B,  3). 
The  squamiform  appendage  in  Macroura  (the  Scaphocerite  of  M.  Milne-Ed wards’s 
later  writings)  universally  springs  from  the  basos  articulation ; and  therefore  it  is  the 
homotype  of  the  exognathe  or  palp,  which  springs  from  the  same  articulation  of  each  of 
the  posterior  organs  of  the  mouth,  and  therefore  holds  the  same  relation  (as  we  hope 
shortly  to  demonstrate)  to  the  more  important  parts  in  the  young  stage  of  the  so-called 
natatory  feet  of  the  larva. 
The  antenna  originating  from  the  almost  invisible  germ  (a)  progressively  developes, 
but  its  secondary  appendages  remain  with  each  successive  moult  unaltered.  They 
decrease  in  relative  importance,  and  are  ultimately  lost,  and  this  considerably  before 
the  animal  has  assumed  the  adult  form.  But  it  is  interesting  to  observe  that  these 
* British  Association  Eeports,  1855. 
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MDCCCLVIII. 
