MR.  R.  OWEN'S  REMARKS  ON  THE  ENTOZOA. 
391 
system  obtains  is  so  great,  that  it  may  still  be  regarded,  with  little  inconvenience,  as  the 
chief  character  of  the  Acrita. 
I  have  already  observed  that  the  absence  of  an  egestive  outlet  to  the  alimentary  canal 
cannot  be  assigned  to  the  Acrita  as  a  character  of  that  division  ;  but  there  is  a  condition 
of  the  digestive  system  in  relation  to  the  parietes  or  substance  of  the  body,  which  is  as 
generally  apphcable  to  the  Acrita  as  the  molecular  distribution  of  the  nervous  sub- 
stance ;  in  this  division  the  intestine  is  not  separated  from  the  skin  by  an  abdominal 
cavity,  but,  whatever  be  its  form,  is  essentially  a  simple  excavation  of  the  parenchyma. 
Now  the  few  genera  which  recede  from  this  character  are  precisely  those  in  which  the 
existence  of  nervous  filaments  is  perhaps  least  ambiguous,  as,  for  example,  in  Actinia 
and  Beroe.  It  is  fortunate  for  the  systematic  naturalist  that  these  genera  form  so 
small  an  exception  to  the  rule  in  regard  to  the  conditions  of  the  nervous  and  digestive 
systems  in  the  Acrita,  since  the  filamentary  disposition  of  the  nervous  substance,  in  co- 
existence with  a  distinct  abdominal  cavity  and  muscular  parietes  of  the  alimentary  canal, 
constitute  the  distinctive  character  of  the  remaining  Radiated  classes  from  which  the 
Acrita  are  separated. 
In  all  the  Acrite  classes,  then,  we  find  that  there  is  an  internal  digestive  cavity,  one 
of  the  chief  characteristics  of  the  animal  kingdom.  In  the  Sterelmintha,  as  in  the  ma- 
jority of  the  Acrita,  there  is  but  a  single  communication  with  the  exterior  of  the  body, 
with  the  apparent  exception  in  the  genus  Coenurus,  in  which,  as  in  the  composite  Zoo- 
phytes, nutrition  is  effected  by  numerous  mouths,  but  without  an  anus. 
The  vascular  system,  where  traces  of  it  are  met  with  in  the  Acrita,  corresponds  with 
the  digestive  system,  being  equally  devoid  of  proper  parietes,  and  consisting  of  reticu- 
late canals  excavated  in  the  parenchymatous  substance  of  the  body,  generally  super- 
ficial, and  in  which  a  cyclosis  of  the  nutrient  fluids  is  observed  analogous  to  that  of 
plants ;  but  there  is  no  true  circulation.  This  condition  is  met  with  as  low  down  in 
the  scale  as  the  Polygastrica,  where  Professor  Ehrenberg  has  determined  the  existence 
of  a  superficial  network  of  hyaline  canals.  In  those  genera  of  the  Sterelmintha  which 
manifest  traces  of  a  sanguiferous  system,  the  fluids  undulate  in  canals  of  a  similar  struc- 
ture, form,  and  position,  as  in  the  Trematoda,  especially  the  Planarice,  and  in  Echino- 
rhynchi,  in  some  of  the  species  of  which  latter  the  cutaneous  vascular  network  is  ex- 
tremely rich^ 
In  the  Medusee,  among  the  Acalephce,  the  condition  of  the  vascular  system  is  equally 
simple  with  that  of  the  lowest  Acrita,  as  is  exemplified  in  the  marginal  vascular  reticu- 
is  connected  with  two  filaments  that  decussate  one  another  at  ahout  the  middle  of  their  course.  These  he  de- 
scribes as  forming  part  of  a  nervous  circle  placed,  throughout  the  greater  part  of  its  course,  immediately  along 
the  bases  of  the  row  of  tentacles  that  surround  the  disc,  so  as  to  form,  as  it  were,  the  outer  wall  of  the  circular 
vessel,  or  appendage  to  the  digestive  cavity  which  runs  round  the  margin  of  the  disc.  Ehrenberg  further  de- 
scribes another  nervous  circle,  composed  of  four  ganghon-like  masses,  disposed  around  the  mouth,  each  being 
in  connexion  with  a  corresponding  group  of  tentacles. — Miiller's  Archiv.,  1834,  p.  662. 
Echinorhynchus  vasculosus,  Rud.,  Syn.,  p.  581. 
3  F  2 
