The Collaborative International Dictionary of English v.0.48:

Hydrozoa \Hy`dro*zo"a\, n. pl. [NL., fr. E. hydra + Gr. zo^,on
   an animal.] (Zool.)
   The Acaleph[ae]; one of the classes of c[oe]lenterates,
   including the Hydroidea, Discophora, and Siphonophora.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hydrozoon \Hy`dro*zo"["o]n\, n.; pl. L. Hydrozoa, E.
   Hydrozo["o]ns. [NL.] (Zool.)
   One of the Hydrozoa.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Molluscoidea, including Brachiopoda and Bryozoa. Mollusca,
including Cephalopoda, Gastropoda, Pteropoda, Scaphopoda,
Lamellibranchiata or Acephala. Echinodermata, including
Holothurioidea, Echinoidea, Asterioidea, Ophiuroidea, and
Crinoidea. C[oe]lenterata, including Anthozoa or Polyps,
Ctenophora, and Hydrozoa or Acalephs. Spongiozoa or
Porifera, including the sponges.
Protozoa, including Infusoria and Rhizopoda. For
definitions, see these names in the Vocabulary.
[1913 Webster] Animalcular

WordNet (r) 3.0 (2006):

Hydrozoa
    n 1: coelenterates typically having alternation of generations;
         hydroid phase is usually colonial giving rise to the
         medusoid phase by budding: hydras and jellyfishes [syn:
         Hydrozoa, class Hydrozoa]