Hydrozoa

thumb|Spotted jelly|Spotted jellies swimming in a Tokyo aquarium Jellyfish, also known as sea jellies or simply jellies, are the medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals, although a few are anchored to the seabed by stalks rather than being motile. They are made of an umbrella-shaped main body made of mesoglea, known as the bell, and a collection of trailing tentacles on the underside.

Hydrozoa (hydrozoans; from Ancient Greek ('''') 'water' and ('''') 'animals') is a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in freshwater habitats. Hydrozoans are related to jellyfish and corals, which also belong to the phylum Cnidaria.

Hydroidolina is a subclass of Hydrozoa and makes up 90% of the class. Controversy surrounds who the sister groups of Hydroidolina are, but research has shown that three orders remain consistent as direct relatives: Siphonophorae, Anthoathecata, and Leptothecata.

Trachylinae (also Trachylina, Trachylinida, etc.) is a subclass of hydrozoans. It is placed at order rank in many older classifications, and limited to contain the Narcomedusae, Trachymedusae, the Actinulidae, then considered an independent order, and also the Limnomedusae which were traditionally placed in the paraphyletic "Hydroida". It is not entirely clear whether the Limnomedusae and the Trachymedusae as conventionally circumscribed are monophyletic

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