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 Blue_bottles.jpg  vallela.jpg porpita.jpg beroe.jpg
(Physalia utriculus)

By-the-wind sailor
(Velella sp)
Blue button
(Porpita pacifica)
Comb jelly
(Beroe sp)

Hydrozoan systematics is highly complex. Several approaches for expressing their interrelationships were proposed and heavily contested since the late 19th century, but in more recent times a consensus seems to be emerging.

For long, the hydrozoans were divided into a number of orders, according to their mode of growth and reproduction. Most famous among these was probably the assemblage called "Hydroida", but this group is apparently paraphyletic, united by plesiomorphic (ancestral) traits. Other such orders were the Anthoathecatae, Actinulidae, Laingiomedusae, Polypodiozoa, Siphonophora and Trachylina.

As far as can be told from the molecular and morphological data at hand, the Siphonophora for example were just highly specialized "hydroids," whereas the Limnomedusae - presumed to be a "hydroid" suborder - were simply very primitive hydrozoans and not closely related to the other "hydroids." Therefore, today the hydrozoans are at least tentatively divided into two subclasses, the Leptolinae (containing the bulk of the former "Hydroida" and the Siphonophora) and the Trachylinae, containing the others (including the Limnomedusae). The monophyly of several of the presumed orders in each subclass is still in need of verification.

In any case, according to this classification, the hydrozoans can be subdivided as follows, with taxon names emended to end in "-ae"

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