Trachylinae (also Trachylina, Trachylinida, etc.) is a subclass of

hydrozoans 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 spe ...

. It is placed at

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

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 Trachymedusae belong to the phylum Cnidaria and the class Hydrozoa, among the 30 genera are 5 families containing around 50 species in all, the family Rhopalonematidae has the greatest diversity. Description Trachymedusae are identifiable by the ...

, the

Actinulidae Actinulida are an order of hydrozoans in the subclass Trachylinae 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 Narcomedusa ...

, then considered an independent order, and also the

Limnomedusae Limnomedusae is an order of hydrozoans. Taxonomy The taxon was erected by Danish marine biologist Paul Lassenius Kramp in 1938 to accommodate certain families of hydrozoans with biphasic life histories. It includes genera with medusae with ect ...

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 ''Craspedacusta sowerbii'' or peach blossom jellyfish is a species of freshwater hydrozoan jellyfish, or hydromedusa cnidarian. Hydromedusan jellyfish differ from scyphozoan jellyfish because they have a muscular, shelf-like structure called a ...

'' is a well-known member of the Limnomedusae and might thus belong here.

Development

Of the four orders:

, and

, 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