596  ME.  C.  SPENCE  BATE  ON  THE  DE^T:L0P]MENT  OE  DECAPOD  CEUSTACEA. 
The  ])ereio])oda  (fig.  E,  10  to  14)  have  become  organs  sufficiently  poweifiil  to  enable 
the  animal  to  use  them  in  walking.  The  anterior  two  or  three  pairs  have  a tooth-like 
spine  attached  to  the  anterior  edge  of  the  coxa  and  lasos;  and  the  branchite  have 
assumed  the  compound  character  approaching  that  of  the  perfect  organ. 
The  ])leopoda  (figs.  18,  19,  20)  have  all  the  external  or  secondary  branches  largely 
developed  and  furnished  with  plumose  cilia,  thus  forming  efficient  natatory  organs. 
The  telson  (fig.  21)  has  no  appendage.  It  is  not  forked  as  in  the  early  period,  but  is 
imperfectly  square  and  squamiform.  On  each  side  of  the  posterior  margin  the  ividiment 
of  the  styliform  process  may  still  be  observed  in  the  form  of  a small  denticle.  The  three 
central  hairs  still  remain. 
Soon  other  moults  deprive  the  animal  of  the  dorsal  spine  (Plate  XLIII.  F).  But  the 
loss  of  this  peculiar  feature  in  the  external  aspect  of  the  animal,  although  the  most 
apparent,  is  not,  I think,  the  most  important  alteration  at  this  period.  The  anterior 
margin  of  the  carapax,  including  the  rostrum,  continues  much  depressed,  but  the  lateral 
margin  of  the  same  anterior  portion  is  much  raised ; there  is  also  a considerable  lobe 
(fig.  F a,  F"«)  developed  on  each  side  posterior  to  the  eyes.  The  upper  antenna  (fig.  2) 
is  more  complete,  and  the  internal  structure  of  the  acoustic  organ  (fig.  2')  may  be 
detected  in  the  fii’st  articulation.  The  second  antenna  (fig.  3)  has  increased  but  little  in 
length ; and  a few  hairs,  a pair  of  which  are  exceedingly  long,  are  developed  towards 
the  tip.  The  maxillm  (%s.  5 and  6)  are  nearly  of  the  same  form  and  character  as  the 
preceding ; so  also  are  most  of  the  succeeding  members.  Three  small  tubercles  (r,  s,  t) 
are  seen  to  be  developed  upon  the  oral  margin  of  the  posterior  gnathopoda  (fig.  9). 
The  small  protuberances,  which  Avere  previously  scarcely  risible  upon  the  coxa  (a) 
and  basos  (b)  of  the  duateriov  pereiopoda  (figs.  10,  11,  12),  haA'e  become  developed  into 
spines  of  considerable  importance.  The  appendages  (figs,  15  to  20)  to  the  pleon  ha^e 
undergone  no  appreciable  change.  The  telson  (hg.  21)  has  lost  the  last  rudiments  of  the 
styliform  caudal  processes ; but  the  central  hairs,  still  limited  to  three,  remain  as  in  the 
preceding. 
Plate  XLIV. — Successive  moults  rob  the  young  animal  soon  of  the  frontal  spine. 
Contemporary  vrith  its  decreasing  importance,  the  pleon  becomes  gradually  folded  nearer 
and  nearer,  until  it  is  closely  compressed  agamst  the  inferior  smTace  of  the  pereion. 
Upon  each  side  of  the  carapax  posterior  to  the  eyes  four  small  denticles  (I,  ni,  o)  are 
developed.  These  at  first  are  scarcely  visible  in  the  contour  of  the  animal,  Avhich  has 
considerably  increased  in  diameter  at  this  point,  where  it  is  as  wide  as  at  the  posterior 
extremity  of  the  carapax. 
The  eyes  (fig.  G,  1)  are  still  large  and  prominent.  The  organs  (figs.  5,  G,  7,  8,  9) 
attendant  upon  the  mouth  in  all  essential  points  approximate  nearer  to  those  of  the  adult 
form ; and  where  there  is  a distinction,  it  is  in  their  resemblance  to  the  younger  stage. 
The  pereiopoda  are  all  strong  and  capable  of  supporting  the  animal  in  its  perambulations. 
The  spines  upon  the  basos  in  the  pereiopoda  are  gone,  and  the  pleopoda  haA  e lost  their 
natatory  character  and  assumed  an  atrophied  appearance  (figs.  17,  18,  19,  20). 
