AND  ON  THE  STRUCTURE  OF  ITS  CILIA. 
11 
animals,  and  in  the  embryo  condition  of  more  elevated  classes.  They  are  the  organs  of 
motion  and  of  respiration  in  the  Polygastrica  and  Rotifera.  They  are  the  instruments 
of  respiration,  and  produce  the  currents  to  the  mouths  of  the  polypi  of  Zoophytes. 
They  are  the  organs  of  locomotion  of  the  reproductive  gemmules  of  Poriferous  and 
Polypiferous  animals,  and  probably  produce  the  currents  through  the  pores  of  the  former 
class.  They  are  important  parts  of  the  respiratory  apparatus  of  adult  Conchifera,  and 
I  have  shown  them  to  be  the  organs  of  locomotion  in  the  embryos  of  naked  and  tes- 
taceous Gasteropods.  They  constitute  the  organs  of  locomotion  and  of  respiration  in 
several  genera  of  Acalepha  allied  to  the  Beroe,  of  which  M.  Blainville  has  formed  a 
family  thence  called  Ciliogrades. 
These  minute  hair-like  organs  are  variously  disposed  on  the  surface  of  animals  ac- 
cording to  the  object  of  their  motions,  whether  for  respiration,  progressive  motion,  or 
obtaining  food.    They  move  with  great  regularity  and  velocity,  and  they  occur  so  nu- 
merous on  a  single  animal,  that  I  have  calculated  about  four  hundred  millions  of  them 
on  a  single  Flustra  foliacea.    The  cilia  are  generally  organs  so  minute,  that  with  the 
aid  of  the  microscope  we  can  only  discover  their  outward  form,  their  position,  and  the 
direction  of  their  motions,  their  intimate  structure  entirely  escaping  observation.  They 
appear  like  flat  tapering  filaments  prolonged  from  the  homogeneous  cellular  tissue  of 
the  body  to  which  they  are  attached.    The  magnitude,  however,  of  these  organs  in  the 
Beroe,  and  the  transparency  of  the  parts  around  them,  enabled  me  to  perceive  that  in 
this  animal  they  are  not  single  fibres,  but  consist  of  several  straight  short  transparent 
filaments  placed  parallel  to  each  other  in  a  single  row,  and  connected  together  by  the 
skin  of  the  animal,  like  the  rays  supporting  the  fins  of  a  fish.    These  fins  are  of  the 
same  breadth  with  the  bands  to  which  they  are  attached,  and  they  extend  from  the 
mouth  to  the  anus,  there  being  about  forty  on  each  band.    Viewed  with  the  aid  of  a 
'^ns,  the  parallel  fibres  appear  like  transparent  tubes,  sometimes  a  little  detached  from 
'.ch  other  at  their  free  extremities  by  injury  done  to  the  connecting  membrane,  and  at 
these  parts  the  isolated  spines  projected  stiffly  outwards.    Where  the  tins  w^ere  quite 
entire,  the  membrane  connected  the  tubular  rays  to  their  extremity,  where  the  fin  pre- 
sented a  slightly  rounded  outline.   Dr.  Fleming  observed  in  Beroe  ovatus  water  moving 
in  vessels  along  the  middle  of  the  bands  to  which  the  cilia  are  attached  ;  and  M.  Audouin 
has  observed  that  in  the  closely  allied  genus  Idya,  the  water  is  sent  into  the  cilia,  which 
he  considers  as  respiratory  organs.    The  animals  can  change  the  direction  of  the  cur- 
rents of  water  in  the  vessels,  and  also  the  direction  of  the  motions  of  the  cilia.  When 
the  cilia  are  in  active  vibration,  the  motion  appears  like  the  continued  undulations  of 
a  fluid  along  the  surface  of  each  band.    This  structure  cannot  be  observed  in  the  mi- 
nuter forms  of  the  cilia  in  other  classes  ;  but  from  the  similarity  of  their  arrangement, 
and  their  mode  of  action,  it  is  probable  that  the  structure  is  similar.    The  cilia  of  Tri- 
choda  patula,  MiilL,  a  minute  animalcule,  are  disposed  in  longitudinal  series  from  the 
mouth  to  the  anus,  precisely  as  in  the  Beroes.    It  does  not  appear  probable  that  the 
c  2 
