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}.