602  ME.  C.  SPENCE  BATE  ON  THE  DEYELOP3^IENT  OP  DECAPOD  CEDSTACEA. 
has  completed  its  number  by  the  addition  of  one  immediately  preceding  the  telson.  In 
the  larva  of  the  Brachyura  this  new  segment  fonns  no  very  important  part  in  the  general 
economy,  since  the  appendages,  which  exhibit  a prominent  feature  in  the  later  stages  of 
the  young  condition,  again  assume  an  atrophied  condition.  But  in  the  Macroura  the 
appendages  of  the  penultimate  segment  increase  to  a very  considerable  degree  at  a very 
early  period,  and  together  with  the  last  segment  form  the  fan-like  caudal  apparatus  of 
the  adult.  According  to  the  statement  of  Eathke,  in  the  lar\'a  of  the  Anomov.ra  the 
pleopoda  are  early  developed  to  a considerable  extent.  Thus  at  this  immature 
age  this  intermediate  form  exhibits  conditions  of  a mixed  character.  It  has  the  J/ff- 
croura  type  posteriorly,  while  anteriorly  it  exhibits  that  of  the  Brachyura^  in  ha’^ing  only 
two  pairs  of  natatory  feet  in  the  earliest  state. 
But  perhaps  of  all  the  parts  in  the  Brachyura,  the  posterior  segment  or  telson  is  that 
which  undergoes  the  greatest  visible  alteration  of  form,  passing,  as  it  does  in  the  laiwa 
of  Carcinus,  from  a large  fork-like  appendage  to  that  which  is  exactly  the  reverse.  The 
telson  of  the  adult  terminates  posteriorly  in  a blunt  point  fringed  with  hairs.  But  even 
this  extreme  alteration  is  but  the  result  of  a succession  of  gradual  changes.  In  the  d/«- 
croura  the  change  from  first  to  last  is  not  so  great.  It  is  little  more  than  an  elongation, 
without  a corresponding  increase  in  the  width  of  the  segment. 
In  contemplating  the  development  of  the  decapod  Crustacea,  from  the  youngest  and 
most  anomalous  form  to  that  of  the  adult,  we  perceive  that  the  greatest  amount  of 
change,  both  in  appearance  and  the  development  of  parts,  takes  place  in  ih.e  Brachyura; 
but  that  even  here  the  change  is  no  sudden  transformation  of  one  form  into  another, 
but  a gradual  and  persistent  growth,  following  each  successive  moult.  Every  part  that 
is  present  in  the  larva,  though  not  permanent  in  itself,  is  to  be  found  in  a permanent 
condition  in  one  or  other  form  of  adult  Crustacea.  And  moreover,  those  appendages 
which  play  the  most  important  parts  in  the  larva,  fulfil  only  secondary  conditions  in 
relation  to  the  adult.  Thus  the  large  natatory  limbs  in  the  larva  become  the  palps  of 
the  adult  gnathopoda,  and  the  appendages  of  the  second  antenna  either  represent  imim- 
portant  parts  of  the  same  organ,  or  are  altogether  wanting.  Again,  we  perceive  that 
certain  parts  which  continue  present  with  but  a small  amount  of  alteration,  such  as  the 
mandibles  and  the  maxillse,  bear  a close  resemblance  of  form  to  the  same  organs  in  adult 
Crustacea  of  an  order  lower  than  the  parent  of  these.  Again,  where  development  of 
new  parts  takes  place,  it  follows  a law  that  is  in  accordance  with  the  absence  or  presence 
of  the  same  parts  in  Crustacea  in  the  descending  grade.  Thus  we  find  that  the  poste- 
rior segments  are  those  which  are  not  present  in  the  anterior  and  second  dmsion  of  the 
animal ; but  in  i\\epleon,  instead  of  being  the  last,  it  is  the  penultimate  that  is  wanting. 
Among  the  Amphipoda  there  are  certain  forms  more  or  less  perfect  in  then  complete 
character.  In  the  descending  scale  we  perceive  that  the  penultimate  segment  of  the 
pleon,  with  its  appendages,  first  becomes  rudimentary,  as  in  Cyrtophium,  and  ultimately 
disappears,  as  in  Bulichia,  before  any  other  segments  of  the  pleon  are  affected  by  the 
depressing  change. 
