Hydrozoa: Systematics

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.