There  are  cases,  however,  among  the  Echinodermata  in
which  the  eggs  are  kept  during  development  in  some  kind  of
external  brood-pouch,  and  this  structure,  when  confined  to
the  females,  constitutes  a  secondary  difference  between  the
sexes.  The  common  brittle-star  Amphiura  squamata  is  vivi-
parous, but  here  the  brood-pouch  is  a  portion  of  the  genital
apparatus  which  occurs  in  both  sexes.  In  some  Holothurians,
however  (Psolus  ephippiger),  there  is  a  brood-pouch  on  the
back  of  the  females  only,  a  condition  similar  to  that
which  occurs  in  certain  Amphibia.  The  receptacle  is  formed
by  calcareous  plates .  supported  by  rods  projecting  from  the
skin.  In  this  case  the  special  structure  is  a  modification  of
parts  originally  existing,  and  unmodified  in  the  male.  The
modification  is  evidently  dependent  on  the  stimulation  pro-
duced by  the  presence  of  the  eggs  and  embryos,  without  which
stimulation  the  "variations"  required  to  give  rise  to  the
modification  would  not  have  occurred.
Coelenterata. — In  this  sub- kingdom,  including  jelly-
fishes,  polyps,  corals,  and  sea-anemones,  secondary  sexual  differ-
ences are  generally  absent,  and  for  the  same  reasons  as  in  the
Echinodermata.  Here  also,  however,  occasional  cases  occur
in  which  some  slight  difference  in  the  female  are  developed
in  association  with  protection  of  the  eggs  or  larvae.  In  the
commonest  jelly-fish  of  our  coasts,  Aurelia  aurita,  the  female
differs  slightly  from  the  male.  The  difference  consists  in  the
form  and  length  of  the  arms  or  tentacles  surrounding  the


310


SEXUAL  DIMORPHISM


mouth.  In  the  female  the  ova  undergo  the  earliest  stages
of  development  in  small  pockets  formed  on  the  external
surface  of  the  arms,  so  that  here  again  the  modification  is
associated  with  the  special  stimulation  due  to  the  presence
of  the  ova  and  larvae,  and  therefore  not  applied  to  the  arms
of  the  male.
