:ME.  C.  SPENCE  BATE  ON  THE  DEVELOPMENT  OF  DECAPOD  CRUSTACEA.  591 
of  the  ewtixe  pleon,  and  partly  by  the  gnathopoda,  the  (so-called)  natatory  feet.  They 
together  induce  a jerking  motion  in  a progressively  circuitous  course. 
In  this  stage  I have  carefully  and  extensively  examined  every  part  of  the  animal  both 
in  position  and  in  detail. 
The  carapax  covers  almost  the  w'^hole  of  the  pereion,  and  extends  in  front  to  a long 
projecting  rostrum,  which  is  nearly  half  the  length  of  the  entire  animal.  A similar 
projection  arises  from  the  centre  of  the  dorsal  region,  and  gives  an  increased  eccentricity 
to  its  general  appearance — a circumstance  which  has  lent  considerable  force  to  the  belief 
of  an  exaggerated  metamorphosis  in  the  history  of  the  development  of  the  Crab. 
In  this  early  stage  the  dorsal  spine  is  no  mere  ornament,  but  possesses  a specific  use 
in  supporting  the  heart  in  its  position.  We  thus  are  enabled  to  identify  its  exact  posi- 
tion in  the  homologies  of  the  adult,  with  the  cardiac  region  on  the  carapax,  which  in 
some  genera  are  actually  represented  by  a stout  spinous  protuberance.  It  appears 
moreover  to  be  analogous  in  pm’pose  to  the  two  strong  processes  found  upon  the 
internal  surface  of  the  same  region  of  the  carapax  in  Carcinus  and  other  genera. 
Posteriorly,  as  in  the  adult  animal,  the  carapax  is  centrally  attached  to  the  pleon  by 
a membrane,  while  on  each  side  it  is  free,  so  as  to  admit  even  at  this  early  period  an 
inpom’ing  current  of  water  to  aerate  the  branchiee.  Laterally  the  carapax  overhangs  the 
pereion  and  protects  the  coxse  of  the  developed  members,  but  anteriorly  it  is  compressed 
so  as  to  leave  the  eyes  prominent. 
In  the  early  form  the  eye  is  generally  considered  by  naturalists  to  be  sessile ; and  if 
om*  observations  were  to  be  confined  to  certain  Macroura,  it  would  be  difficult  to  dis- 
prove the  assertion,  since  the  distinction  between  eyes  that  are  supported  upon  almost 
obsolete  peduncles  and  those  which  have  none  is  very  limited. 
the  wafking  legs  : hence  pereiopoda.  Fleon  is  taken  from  TrXew,  to  navigate,  being  that  part  of  the  animal 
which  supports  the  limbs  by  which  it  swims  about : hence  pleopoda,  the  smmming  feet. 
B 
Cephalon. 
1.  Eye. 
2.  Superior  antenna. 
3.  Inferior  antenna. 
4.  Mandible. 
5.  First  maxilla. 
6.  Second  maxilla. 
7.  Maxilliped. 
Pereion. 
8.  First  gnathopod. 
9.  Second  gnathopod. 
10  to  14.  Pereiopoda. 
Pleon. 
15  to  20.  Pleopoda. 
21.  Telson. 
MDCCCLVIII. 
