This  latter  point  more  especially  was  cited  as  indicative  of  a  still  more
intimate  relationship  with  the  sponges  than  that  presented  by  the  two  last-
named  genera. Professor  Clark  further  placed  on  record  in  the  same  serial
the  results  of  his  recent  investigations  into  the  ultimate  structure  of  the
American  Fresh-water  Sponge,  Spongilla  arachnoides  J.-Clk.,  the  results
confirming  substantially,  and  adding  considerably  to,  the  data  elicited  through
his  previous  examination  of  the  marine  calcareous  type  Leucosolenia  botry-
aides. In  this  fresh-water  form,  however,  the  characteristic  collared  flagellate
cells  were  found  occupying  definite  spherical  excavated  chambers,  corre-
sponding with  those  described  by  Mr. Carter  of  Spongilla  alba  under  the
name  of  ampullaceous-sacs,  which  received  from  Professor  Clark  the  equi-
valent title  of  the  "  monad  chambers."  Taken  in  its  entirety,  the  sponge-
body  of  this  species  was  declared  to  be  composed  of  the  three  following
distinct  and  well-marked  elements  :  Firstly,  of  a  common,  glairy,  gelatinous
basis,  within  which  all  the  remaining  constituents  were  embedded,  and
which  he  designated  the  cytoblastema. Although  presenting  the  same
characteristic  consistence  throughout,  this  glairy  cytoblastema  exhibited  a
separation  into  two  distinct  regions,  the  one  consisting  of  a  thin  superficial
stratum,  stretched  out  on  the  points  of  the  larger  externally  projecting
spicula,  after  the  manner  of  a  tent-canvas  extended  upon  the  ends  of  its
supporting  poles. Within  this  peripheral  cytoblastematous  layer,  designated
by  Professor  Clark  in  its  separate  form  the  "  investing  membrane,"  there  was
no  trace  to  be  found  of  the  monad-chambers. These  latter,  which  consti-
tuted the  second  and  most  important  structural  element,  were  confined
entirely  to  the  deeper  and  comparatively  solid  substratum  of  the  cytoblas-
tema, and  which  he  therefore  distinguished,  with  relation  to  its  contents,  as
the  "  monadigerous  layer."  The  third  and  remaining  essential  element  recog-
nized by  Professor  Clark  consisted  of  the  innumerable  amoebiform  cells  or
bodies  scattered  more  or  less  abundantly  throughout  the  substance  of  the
cytoblastema,  and  most  conspicuously  visible,  in  consequence  of  the  absence
of  the  monad-chambers,  in  the  thin  superficial  stratum  or  so-called  investing
membrane. 