MR.  R.  OWEN'S  REMARKS  ON  THE  ENTOZOA. 
389 
deed,  merged  into  the  class  Annelida,  as  Rudolphi  proposes,  but  are  made  a  distinct 
class  in  conjunction  with  the  Epizoaria  of  Lamarck,  which  Mr.  MacLeay  imagines  may 
be  entitled  to  a  place  between  the  Anoplura  of  Dr.  Leach  and  the  Chilognatha. 
The  simple  filamentous  disposition  of  the  nervous  system,  however,  which  opposes 
Rudolphi's  views  of  the  place  which  the  Nematoidea  ought  to  hold,  equally  forbids  their 
allocation  as  an  annectant  of  any  of  the  classes  of  the  Annulose  or  Homogangliate  divi- 
sion of  the  animal  kingdom. 
The  Acrita  of  MacLeay  are  thus  defined. 
"  Animalia  gelatinosa  polymorpha,  interaneis  nullis,  medullaque  indistincta. 
"  Os  interdum  indistinctum,  sed  nutritio  absorptione  externa  vel  interna  semper 
sistit.    Anus  nullus. 
"  Reproductio  fissipara  vel  gemmipara,  gemmis  modo  externis  modo  internis,  inter- 
dum acervatis. 
"  Pleraque  ex  individuis  semper  cohserentibus  animalia  composita  sistunt." 
This  definition,  able,  comprehensive,  and  accurate  as  it  undoubtedly  was,  agreeably 
to  the  state  of  anatomical  and  zoological  knowledge  at  the  time  when  it  was  penned, 
fails  now  to  convey  a  just  idea  of  the  prevailing  and  characteristic  organical  conditions 
of  the  Acrita.  Fourteen  years  of  subsequent  research,  crowned  by  discoveries  of  which 
the  most  brilliant  relate  to  the  structure  and  economy  of  the  lowest  classes  of  the 
animal  kingdom,  have  wrought  their  wonted  effect  on  what  was  intended  at  a  pre- 
vious period  to  be  the  most  general  expression  of  the  existing  knowledge  of  the  orga- 
nization of  a  natural  group. 
The  discoveries  of  Professor  Ehrenberg  with  reference  to  the  digestive  system  of  the 
Monads  not  only  obviate  the  necessity  of  ascribing  a  mode  of  nutrition  by  external  ab- 
sorption to  any,  even  the  lowest  of  the  animal  kingdom  ;  but,  while  they  prove  the  ex- 
istence of  a  complicated  internal  digestive  cavity  in  the  Agastria  of  M.  De  Blainville, 
show  also  tliat  in  many  of  the  genera  of  this  simple  class  the  alimentary  cavity  is  pro- 
vided with  a  distinct  anal  outlet.  The  larger  faecal  pores  of  the  Spongia  may  be  also 
considered  in  the  same  relation  to  the  digestive  system. 
With  respect  to  generation,  the  organs  of  which  function  afford  in  their  varieties  the 
least  certain  indications  of  the  relative  perfection  of  the  species,  it  may  be  observed  that 
in  the  TcenieE  the  race  is  propagated  neither  by  spontaneous  fission  nor  gemmation,  but 
by  true  ova,  frequently  formed  by,  and  contained  in,  distinct  ovaries,  of  which  one  is 
appropriated  to  each  joint.  The  ova  in  these  receptacles  are  commonly  advanced  in 
proportion  as  the  joints  recede  from  the  head ;  and  although  the  ovaries  have  distinct 
outlets  either  at  the  middle  part  or  margin  of  the  segments,  yet  these  are  commonly 
detached  as  the  ova  in  the  ovary  are  matured,  in  a  manner  analogous  to  the  bursting  of 
the  external  ovisacs  of  the  Lern(E(E  and  Monoculi.  In  another  order  of  Parenchymatous 
Worms,  the  Trematoda,  distinct  fecundating  glands  are  superadded  to  the  productive 
or  female  apparatus,  and  some  physiologists  suppose,  with  Cuvier,  that  generation  is 
VOL.  I.  3  F 
