TRICHOCYSTS. 8 1

crowded  together  and  distributed  in  an  even  layer  immediately  beneath
the  cuticle  throughout  the  whole  extent  of  the  cortex,  their  disposition  with
respect  to  the  external  periphery  being  everywhere  perpendicular. Under
certain  conditions,  including  the  application  of  artificial  stimuli,  such  as
weak  acetic  acid,  these  trichocysts  become  suddenly  elongated,  and  their
distal  ends  piercing  the  overlying  cuticle  stand  out  like  fine,  stiff,  hair-like
sets,  beyond  the  cilia,  around  the  entire  circumference  of  the  animalcule,
frequently  becoming  entirely  separated  from  their  base  of  attachment
The  names  of  Ehrenberg  and  Oscar  Schmidt  are  most  usually  associated
with  the  earliest  discovery  of  these  special  structures,  the  first-named
authority  having  recorded  their  presence  in  Bursaria  {Panophrys)  vernalis
so  long  ago  as  the  year  1832,  and  in  which  connection  they  are  figured  and
described  as  minute  prismatic  rod-like  bodies  embedded  beneath  the  cilia
in  the  body-substance. By  Oscar  Schmidt,  in  the  year  1849,  similar  rod-like
bodies  were  reported  to  occur  in  Bursaria  (Panophrys)  leucaSy  and  Para-
mecium aureliOy  their  close  correspondence  with  the  rod-like  bodies  possessed
by  various  Turbellaria  being  indicated. It  has  been  recently  elicited,
however,  by  the  present  author  that  the  existence  of  these  trichocysts  in
Paramecium  aurelia  was  discovered,  and  their  characteristic  aspect  when
extended  figured  and  described,  by  our  distinguished  countryman  John  Ellis,
more  than  a  century  ago,  and  at  a  time  when  microscopical  science  was
quite  in  its  infancy. With  the  imperfect  magnifying  apparatus  employed
by  this  investigator  it  was  not  possible  to  recognize  the  fine  vibratile  cilia
by  which  the  ordinary  motions  of  the  animalcule  are  accomplished,  a
circumstance  which  led  him  to  attribute  such  a  special  function  to  the
bodies  in  question ;  this,  however,  in  no  way  detracts  from  his  merit  as
their  first  discoverer. Ellis's  account  of  these  trichocysts  being  of  high
intrinsic  as  well  as  classic  interest,  its  reproduction  in  extenso  is  herewith
appended. 