Leptothecata

Leptothecata, or thecate hydroids, are an order of hydrozoans in the phylum Cnidaria. Their closest living relatives are the athecate hydroids which are similar enough to have always been considered closely related, and the very apomorphic Siphonophora which were placed outside the "Hydroida". Given that there are no firm rules for synonymy for high-ranked taxa, alternative names like Leptomedusa, Thecaphora or Thecata, with or without the ending emended to "-ae", are also often used for Leptothecata.[1]

The hydroid Halecium muricatum, Gulen Dive Resort, Norway

Thecate hydroids
Crystal Jelly (Aequorea victoria, Conica: Aequoreidae) with the parasitic amphipod Hyperia medusarum
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Hydrozoa
Subclass:	Hydroidolina
Order:	Leptothecata Cornelius, 1992
Synonyms
Leptomedusa Haeckel, 1879 Leptomedusae Haeckel, 1879 Leptothecatae Cornelius, 1992 Thecaphora Hincks, 1868 Thecaphorae Hincks, 1868 Thecata Fleming, 1828 Thecatae Fleming, 1828

The approximately 1,900 species of Leptothecata are characterized by a number of features: Their polyps are always living in colonies with the hydranths set in hydrotheca which are usually permanent and often long enough so the animal can fully retract into it; some have very reduced hydrothecae resembling Anthoathecata. There is a single whorl of tentacles.

The gonophores are borne on much reduced hydranths and usually protected in a peridermal gonotheca. Medusae forming on fully developed hydranths are extremely rare; usually the gonophores develop into medusae or into sessile sporosacs. The medusae have a shallow bell, bear the gonads on their radial canals, and usually have statocysts which are formed only from epidermal tissue and more than four tentacles and. The cnidome never has stenoteles.

Notable species of Leptothecata include:

• Air fern (Sertularia argentea), sold dried as novelty "plants" and aquarium ornaments
• Crystal jelly (Aequorea victoria), a bioluminescent hydrozoan
• Sea fur (Obelia spp.), a common coastal polyp and medusa