page_title: Leptothecata
hierarchical_section_title: Leptothecata
caption_attribution_description: English: The hydroid Halecium muricatum, Gulen Dive Resort, Norway.
context_page_description: 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.
  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.
context_section_description: 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.
  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
caption: The hydroid Halecium muricatum, Gulen Dive Resort, Norway
exif: {"Image Copyright": "Bernard Picton"}