Traditional systematics divides the Hydrozoa into five orders:
Trachylinida Small medusae with no polyp generation;
medusae develop directly from a crawling larva known as an actinula.
Trachylines are very rare as fossils.
Hydroida Mostly colonial forms with alternating polyp and
medusa stages and a chitinous exoskeleton. A few, such as
Hydra, are solitary polyps that lack a
medusoid stage. The chondrophorines
are now usually classified in the Hydroida; formerly they were placed with the
Siphonophorida (see below).
Milleporina and Stylasterina Colonial forms with massive
skeletons of aragonite (calcium carbonate). These two orders differ in
details of skeletal construction and dactylozooid (prey-gathering polyp)
morphology. Sometimes grouped together as the Hydrocorallina, and known
as "fire corals" for their coral-like growth and their painful sting.
Siphonophorida Complex colonial forms, with individual polyps
specialized for feeding, swimming, prey capture, and reproduction. Some but
not all float by means of a large pneumatophore, or gas bag.
The best-known siphonophorid is Physalia, the stinging "Portuguese
man-o'-war." Siphonophores are unknown as fossils.