ME.  C.  SPENCE  BATE  ON  THE  DEVELOPMENT  OF  DECAPOD  CEUSTACEA.  597 
When  the  animal  is  a little  larger,  though  still  of  the  same  form,  the  segments  of  the 
pleon  in  the  male  commence  a fusion  together.  The  intromittent  organs  are  visible. 
The  animal  (exclusive  of  i\ie  pleon)  scarcely  differs  in  appearance  from  the  time  w^hen 
it  was  half  a line  long  until  it  has  reached  to  two  and  a half  lines ; even  then  the  altera- 
tion in  appearance  consists  chiefly  in  the  increase  of  the  animal  on  each  side,  and  the 
comparative  shortening  of  the  interocular  portion  of  the  carapax.  The  eyes  are  still 
prominent,  but  the  organs  generally  have  assumed  a closer  resemblance  to  those  of  the 
adult  form,  from  which  they  appear  to  differ  but  little.  That  which  is  most  conspicuous 
in  the  animal,  exists  in  the  gradual  extension  of  the  post-orbital  or  hepatic  regions. 
Having  pursued  the  course  of  development  from  the  larva  to  the  mature  form  of  the 
Brachyura  decapod,  a moderate-sized  specimen  of  which  produces  about  two  millions  of 
ova,  we  perceive  that  the  progress  made  is  not  by  any  sudden  metamorphosis,  but  by  a 
series  of  moultings  similar  to  those  which  take  place  in  the  adult ; and  that  with  each 
successive  moult  there  is  a corresponding  degree  of  progress  in  its  development.  But 
the  amount  of  change  at  each  moult  is  so  little,  that  it  gives  to  the  animal  but  a very 
small  degree  of  difference  in  its  general  appearance ; and  it  is  only  by  a comparison  of 
the  earliest  form  with  the  last,  and  that  -svithout  any  consideration  of  the  intermediate 
stages  in  its  growth,  that  the  idea  of  a true  metamorphosis  in  decapod  Crustacea  has 
existed. 
There  are  six  or  seven  well-marked  stages  or  forms  that  the  growing  animal  passes 
through  in  its  progress  to  maturity,  and  each  of  these  is  linked  to  the  preceding,  as  well 
as  to  that  which  follows,  by  a succession  of  changes  that  are  but  just  appreciable. 
The  form  of  the  Zoea  when  it  first  quits  the  ovum  is  distinctly  embryonic.  The 
whole  of  the  parts  are  confined  within  a case,  which  gives  it  an  immature  character. 
The  great  dorsal  and  frontal  spines  are  confined  and  folded  close  to  the  animal,  and  all 
the  cilia  enclosed  within  the  tunic  or  outer  case  extend  but  a very  limited  way  beyond 
that  of  the  true  limbs,  whereas  the  greater  portion  of  each  is  drawn  back  within  it. 
This  is  correct  of  every  member,  and  the  large  caudal  spines  upon  ihe  pleon.  Besides 
this,  the  whole  animal  exhibits  the  appearance  of  contraction  within  the  tunic,  the  skin 
being  seen  to  be  corrugated,  so  that  when  the  animal  throws  off  the  case  it  immediately 
increases  considerably  in  size. 
It  undergoes  the  first  moult  within  a very  few  hours  after  its  exclusion  from  the  egg, 
when  it  loses  much  of  its  embryonic  appearance,  and  swims  about  with  more  apparent 
control  over  its  movements,  and  evidently  seeks  its  own  food.  It  keeps  near  the  surface 
of  the  water,  and  always  directs  its  head  towards  the  light. 
The  large  dorsal  and  frontal  spines  are  extended,  and  exhibit  a marked  peculiarity  in 
this  stage  of  the  young  creature.  The  latter  represents  the  rostrum,  and  is  scarcely  more 
important  than  is  found  among  certain  Macroura\  whereas  the  former  is  occupied  by 
the  heart,  and  evidently  homologizes  with  the  pointed  tubercle,  which  is  situated  upon 
the  cardiac  region  in  Stenorhynclms,  &c.  These  two  appendages  gradually  disappear,  or 
to  speak  more  correctly,  their  importance  is  outgrown  by  the  relative  increase  of  the 
