392 
MR.  R.  OWEN'S  REMARKS  ON  THE  ENTOZOA. 
lation  in  the  disc  of  Rhizostoma,  &c.  ;  a  structure  which,  when  compared  to  the  di- 
stinctly developed  vessels  manifested  in  the  Echinodermata,  presents  a  strong  argument 
for  retaining  the  Acalepha  in  the  more  simple  division  of  Cuvier's  Radiata. 
If  it  be  true,  as  has  been  stated,  that  the  Medusa  produce  not  ova,  but  locomotive 
ciliated  gemmules,  we  have  an  additional  reason  for  placing  the  Acalepha  among  the 
Acrita,  in  which  division  of  the  animal  kingdom  only  is  the  plant-like  generation  by 
gemmation  external  or  internal,  or  by  spontaneous  fission,  observed.  This  character 
is  not,  however,  generally  applicable  to  the  Acrita  ;  for  the  Sterelmintha  propagate  by 
ova,  and  have  appropriate  organs,  distinct  from  those  of  the  digestive  system.  These 
organs  are  either  cryptandrous,  or  productive  only,  as  in  Cystica  and  Cestoidea ;  or  a 
fecundating  gland  is  superadded  to  the  ovary,  as  in  Trematoda ;  or  the  sexes  are  sepa- 
rate, as  in  the  Acanthocephala ;  so  as  already  to  typify  almost  all  the  modes  of  gene- 
ration by  which  the  higher  classes  of  animals  are  perpetuated. 
We  thus  perceive  in  the  Acrite  subkingdom  that,  with  the  exception  of  the  genera- 
tive and  digestive  organs,  all  the  other  systems  are  more  or  less  blended  together,  and 
the  corporeal  parenchyma  seems  to  possess  many  functions  in  common.  Where  a  distinct 
organ  is  eliminated,  it  is  often  repeated  almost  indefinitely  in  the  same  individual.  In 
the  Polypi  we  frequently  find  the  nutritious  canals  supplied  with  a  thousand  mouths ; 
and  the  Polygastrica  derive  their  name  from  an  analogous  multiplication  of  the  digestive 
organ  itself.  Among  the  Sterelmintha  the  generative  system  becomes  the  subject  of 
this  repetition,  each  joint  of  the  Tania  being  the  seat  of  a  separate  ovary,  though  all 
are  nourished  by  continuations  of  the  same  simple  tubes.  Again,  the  calcareous  and 
siliceous  Sponges,  which,  in  eliminating  the  first  sketch  of  an  internal  skeleton,  seem 
to  lose  the  few  characteristics  of  animal  life  which  they  before  possessed,  are  limited  to 
the  repetition  of  the  same  spiculum. 
The  formative  energies  being  thus  expended  on  a  few  simple  operations,  and  not  con- 
centrated on  the  perfect  development  of  any  single  system,  it  is  not  surprising  that  we 
should  find  in  the  Acrita  the  greatest  diversity  of  external  figure  ;  all  the  leading  types 
of  animal  organization  seem  to  have  their  origin  in  this  division ;  and  it  has  been  well 
observed,  that  "  Nature,  so  far  from  forgetting  order,  has,  at  the  commencement  of  her 
work  in  these  imperfect  animals,  given  us,  as  it  were,  a  sketch  of  the  different  forms 
which  she  intended  afterwards  to  adopt  for  the  whole  animal  kingdom  ^"  Thus  in  the 
soft  mucous  sluggish  Sterelmintha,  we  have  the  outline  of  the  Mollusca"^ ;  in  the  fleshy 
living  mass  which  surrounds  the  earthy  and  hollow  axis  of  the  Polypi  natantes,  she  has 
sketched  a  vertebrated  animal ;  and  in  the  crustaceous  covering  of  the  living  mass,  and 
the  structure  more  or  less  articulated  of  the  Polypi  vaginati,  we  trace  the  form  of  the 
Annulosa. 
'  Horae  EntomologicBe,  vol,  i.  part  ii.  p.  223. 
^  These,  however,  are  more  immediately  continuous  with,  the  composite  Polypes  by  means  of  the  genera  Bo- 
tryllus,  Eschara,  and  Cellaria ;  vi^hile  the  Trematoda  evidently  lead  to  the  Haustellate  Annelida,  as  the  Leech,  &c. 
