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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 Narcomedusae is an order (biology), order of hydrozoans in the subclass Trachylinae. Members of this order do not normally have a polyp (zoology), polyp stage. The Medusa (biology), medusa has a dome-shaped bell with thin sides. The tentacles ar ...
, Trachymedusae, the Actinulidae, then considered an independent order, and also the Limnomedusae which were traditionally placed in the
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
" Hydroida". It is not entirely clear whether the Limnomedusae and the Trachymedusae as conventionally circumscribed are
monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...
The freshwater jelly '' Craspedacusta sowerbyi'' is a well-known member of the Limnomedusae and might thus belong here.


Development

Of the four orders:
Narcomedusae Narcomedusae is an order (biology), order of hydrozoans in the subclass Trachylinae. Members of this order do not normally have a polyp (zoology), polyp stage. The Medusa (biology), medusa has a dome-shaped bell with thin sides. The tentacles ar ...
, Trachymedusae, Actinulidae, and Limnomedusae, only Limnomedusae has any sort of polyp stage, and even then it is very tiny(less than 1mm) with no tentacles. This is uncommon for other jellyfish under the phylum cnidaria. These jellyfish also tend to have fewer cells during their embryonic and
larvae A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect developmental biology, development such as insects, some arachnids, amphibians, or cnidarians typical ...
stage.Osadchenko, Kraus (2018)


Footnotes


References

* (2005)
The Hydrozoa Directory


Retrieved 2008-JUL-08. *Osadchenko, B.V., Kraus, Y.A. Trachylina: The Group That Remains Enigmatic Despite 150 Years of Investigations. Russ J Dev Biol 49, 134–145 (2018). https://doi.org/10.1134/S1062360418030074 Hydrozoa Animal subclasses {{Hydrozoa-stub