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Leptothecata, or thecate hydroids, are an order of hydrozoans in the phylum Cnidaria. Their closest living relatives are the 
The approximately 1,900
athecate hydroids
Anthoathecata, or the athecate hydroids, are an order of hydrozoans 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 Gymno ...
, which are similar enough to have always been considered closely related, and the very apomorph
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
ic Siphonophorae
Siphonophorae (from Greek ''siphōn'' 'tube' + ''pherein'' 'to bear') is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 specie ...
, which were placed outside the "Hydroida". Given that there are no firm rules for synonymy
A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
for high-ranked taxa
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
, alternative names like Leptomedusa, Thecaphora or Thecata, with or without the ending emended to "-ae", are also often used for Leptothecata.
Leptothecata is surrounded by a chitinous outer layer as its exoskeleton, including gonophore
A gonophore is a reproductive organ in Hydrozoa that produces gametes. It is a sporosac, a medusa or any intermediate stage.
The name is derived from the Greek words (, that which produces seed) and (, -bearing).
Gonophores are borne on branch ...
s, their reproductive organ. Leptothecata obtain radial symmetry, in which their gonads can be found in their radial canals. Their morphological characters normally have ranged from benthic to planktonic stages. Characters associated with benthic are the polyps and colony forms, while planktonic is medusae. Leptothecata has a vast and complex variation among all species within the hydrozoan clade. Thecata colonies also have extensive specialization due to their polyps function and variation. Most Leptothecata possess statocysts, which are used for defense and protection. The classes that have lost their statocysts have been changed ancestrally over time rather than a direct loss.
The approximately 1,900 species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
of Leptothecata are characterized by a number of features: Their polyps are always living in colonies with the hydranth
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