Hydrozoan is a hypernym of hydractinian.
In biology|lang=en terms the difference between hydrozoan and hydractinian
is that
hydrozoan is (biology) any of many colonial coelenterates, of the class
hydrozoa , including the hydras, hydroids, hydrocorals, and siphonophores while
hydractinian is (biology) a marine hydroid of the genus (
taxlink) or a closely-related genus.
As nouns the difference between hydrozoan and hydractinian
is that
hydrozoan is (biology) any of many colonial coelenterates, of the class
hydrozoa , including the hydras, hydroids, hydrocorals, and siphonophores while
hydractinian is (biology) a marine hydroid of the genus (
taxlink) or a closely-related genus.