The  aggregation  of  large  nematoblasts  is  restricted  to  the  apical  knobs.
The  stems  are  hollow  and  brown  in  color,  the  latter  character  due  to  the
presence  of  Zooxanthellae  in  the  endodermal  tissues.  The  knobs,  on  the
other  hand,  are  solid  and  colorless.  The  organs  are  adhesive  and  able  to
take  particles  of  food  from  the  tips  of  forceps.  There  is  little  difference  in
size  among  the  tentacles  of  the  various  cycles,  but  the  bifurcated  members
are  larger  than  the  simple.
During  retraction,  and  even  sometimes  on  full  expansion  of  the  polyp,
the  tentacles  appear  as  unstalked,  spheroidal  tubercles,  sessile  on  the  disc.
In  the  case  of  the  bilobed  tentacles,  a  knob  is  situated  on  each  side  of  the
septum  with  which  the  tentacle  corresponds,  while  in  the  simple  tentacles  the
knob  is  directly  over  the  septum  (plate  6,  fig.  32).  No  stem  can  be  recog-
nized under  such  circumstances,  and  sections  show  that  it  is  not  invaginated,
but  has  become  part  of  the  discal  wall  (plate  7,  fig.  40).  Such  a  condition
of  the  tentacular  stems  is  often  found  in  corals,  but  rarely  among  the  larger
tentacles  of  actinians.  When  the  tentacles  are  fully  expanded  they  stand
obliquely  to  the  surface  of  the  disc,  and  are  bent  outwardly.  The  organs
have  never  been  found  invaginated,  a  phenomenon  which  frequently  occurs
in  coral  polyps  and  alcyonarians.
