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  1  definition  found 
  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
  Hydroidea  \Hy*droi"de*a\,  n.  pl  [NL.  See  {Hydra},  and  {-oid}.] 
  An  extensive  order  of  Hydrozoa  or  Acaleph[ae].  [Written  also 
  Note:  This  order  includes  the  hydras  and  the  free-swimming 
  hydromedus[ae],  together  with  a  great  variety  of  marine 
  attached  hydroids,  many  of  which  grow  up  into  large 
  elegantly  branched  forms,  consisting  of  a  vast  number 
  of  zooids  (hydranths,  gonophores,  etc.),  united  by 
  hollow  stems.  All  the  zooids  of  a  colony  are  produced 
  from  one  primary  zooid,  by  successive  buddings.  The 
  Siphonophora  have  also  been  included  in  this  order  by 
  some  writers.  See  {Gymnoblastea},  {Hydromedusa}, 
  {Gonosome},  {Gonotheca}.