POSTLARVAL  DEVELOPMENT.  I07

Partial  studies  on  other  corals,  as  well  as  considerations  on  the  tentacular
development  in  actinians,  suggest  that  the  exosepta  may  arise  in  different  ways
in  different  species,  and  that  a  more  precise  significance  as  to  their  relation-
ships at  different  stages  ma}'"  be  forthcoming  than  is  possible  in  Siderastrea,
There  are  indications  that  in  some  forms  an  entoseptum  and  an  exoseptum
arise  together,  thus  more  closely  recalling  the  method  followed  by  the  tenta-
cles. Regarded  as  arising  anew  in  each  cycle,  the  two  exosepta  in  Siderastrea
appear  somewhat  in  advance  of  the  entoseptum  which  is  included  between  them.
The  relationships  proved  to  exist  between  entosepta  and  exosepta  involve
important  considerations  when  the  cyclic  hexameral  sequence  is  not  completed
in  the  mature  corallite,  as  almost  invariably  happens  in  S.  radians^  as  well
as  in  many  other  species  of  corals.  As  regards  both  the  septa  and  mesen-
teries it  is  found  that  the  last  cycle  is  rarely  a  multiple  of  6,  but  some
irregular  number  from  i  to  32,  resulting  from  the  fact  that  at  maturity  the
polyp  does  not  complete  the  last  cycle  begun.  Bxosepta  have  been  shown
to  appear  always  in  close  association  with  entosepta,  whatever  be  the  number
making  up  a  corallite ;  and,  as  often  remarked,  the  two  series  are  equal  in
number  and  the  exosepta  always  outermost  in  position.
If  we  regard  a  septal  cycle  as  made  up  only  of  entosepta,  or  of  exosepta,
then  in  mature  corallites  of  5.  radians  the  third  entoccelic  cycle  and  fourth
exoccelic  cycle  of  septa  will  vary  in  the  same  degree.  Whatever  number  of
entosepta  be  lacking  from  the  third  cycle  to  form  the  complete  cycle  of  12,
a  like  number  of  exosepta  will  be  wanting  from  the  fourth  cycle.
