| Velella, Fire coral, Turritopsis, Tubularia, Eudendrium|
Anthomedusae or Anthomedusa, the athecate hydroids, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. A profusion of alternate scientific names exists for this long-known, heavily discussed, and spectacular group. It has also been called Gymnoblastea and (with or without an emended ending -ae) Anthoathecata, Athecata, Hydromedusa, and Stylasterina. There are about 1,200 species worldwide.
These hydrozoans always have a polyp stage. Their hydranths grow either solitary or in colonies. There is no firm perisarc around the polyp body. The medusae, or jellyfish, are solitary animals, with tentacles arising from the bell margin, lacking statocysts but possessing radial canals. Their gonads are on the manubrium ("handle").
Except in Eudendriidae and Laingiidae, prey can be captured by discharging harpoon-like structures (desmonemes) from chambers (cnidae) in specialized cells (nematocysts) on the tentacles. In hydrozoans, these are nearly always adhesive and entrapping, rather than puncturing and poisoned as in other jellys.
Their close relationship to the Leptomedusae has been long known, but it was formerly believed that these two were also close to the Limnomedusae. However, their closest relatives are the highly advanced Siphonophorae, whereas the Limnomedusae are a rather primitive group, not very closely related to these three Leptolinae, and might belong to the Trachylinae subclass.
Some uncertainty existed regarding the internal taxonomy. The most simple scheme, used until recently by most authors since it was proposed in 1913, divided the order into a smaller suborder (Filifera) and a larger one (Capitata), but several unusual Anthomedusae did not fit into this arrangement, and a considerable number did so awkwardly. The Porpitidae, for example, are highly aberrant group, and were at one time even considered a separate order "Chondrophora". However, they are currently considered to be derived from Zancleida. In the early 21st century, the well-known Hydra and its relatives – and most of the supposed filiferan infraorders Tubulariida and Moerisiida – were determined to be a very ancient lineage, recognized as suborder Aplanulata. Although not all Anthomedusae have been firmly placed in the phylogeny, most are fairly certainly assigned at least to one of the major subdivisions. As a notable exception, a prehistoric family, the Heterastridiidae, is still highly disputed regarding its relationships to the extant taxa, as are a small number of very aberrant and/or little-known species such as the aptly named Saccohydra problematica.
The family Clathrozoellidae is placed with the Filifera here; in others it is placed in the Leptomedusae and sometimes even synonymized with their family Clathrozoidae. By contrast, the supposed filiferan genus Anthohydra is in fact the leptomedusan Eugymnanthea; similarly, "Gammaria" is also a leptomedusan and properly spelled Grammaria.
The supposed athecate family Monobrachiidae is apparently a close relative of the Olindiasidae, and belongs in the Limnomedusae. Halammohydridae and Otohydridae, sometimes placed here, appear to be trachyline hydrozoans of the order Actinulidae.