Trepaxonemata (from ''trepa''-, spiral + axoneme) is a subclass of the Platyhelminthes or flatworms.Ehlers, U. (1985). Das Phylogenetische System der Plathelminthes. Stuttgart: G. Fischer. It includes all parasitic flatworms (clade

Neodermata Neodermata is a clade of rhabditophoran flatworms containing the parasitic groups Trematoda, Monogenea and Cestoda. Description All neodermatans are parasites, in many groups having a free-swimming larval stage. The most striking feature uniti ...

) and several free-living species that were previously grouped in the now obsolete class

Turbellaria The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to large freshwater forms more ...

. Therefore, it contains the majority of species in the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Platyhelminthes, excluding the

Catenulida Catenulida is an order of flatworms in the classical classification, or a class of flatworms in a phylogenetic approach. They are relatively small free-living flatworms, inhabiting freshwater and marine environments. There are about 100 species d ...

, and the

Macrostomorpha Macrostomorpha is a clade of free living flatworms ranked either as class or subclass in the group Rhabditophora. There are about 260 described species in two orders - marine, freshwater and brackish Macrostomida, which group most of the species ...

Description

The Trepaxonemata are characterised by: * biflagellate spermatozoa * axoneme of the spermatozoa with a special type of dense core (9+“1” pattern). The axoneme in the spermatozoa of species of Trepaxonemata, also called "trepaxoneme" or "trepaxonematan axoneme" has nine peripheral doublets of microtubules as the usual 9+2 axoneme but the two central microtubules are replaced by a central core. This central core appears as a spiral when seen in longitudinal sections in transmission electron microscopy. This structure is found in all species of Trepaxonemata with very rare exceptions. The trepaxoneme is found only in the Platyhelminthes; no other

has this 9+“1” structure. However, the

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

in cells other than spermatozoa in species of Trepaxonemata have the classical 9+2 structure, for example in the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

of in protonephridia. The Trepaxonemata is one of the rare examples of major groups in zoology named after characteristics of spermatozoa, and especially after characteristics observed with transmission electron microscopy.

Systematics

Parasite170144-fig03 spermatozoon Chimaericola leptogaster

Current classification of Trepaxonemata based on several morphological and molecular studies: *Subclass Trepaxonemata **Clade Amplimatricata ***Order

Lecithoepitheliata The Lecithoepitheliata are an order of rhabditophoran flatworms. They are free-living worms, found in freshwater, soil, and marine environments. However, it is still poorly known their roles in the natural food web. Description Members of the or ...

***Order

Polycladida The Polycladida represents a highly diverse clade of free-living marine flatworms. They are known from the littoral to the Sublittoral zone, sublittoral zone (extending to the deep hot vents), and many species are common from coral reefs. Only a ...

**Clade Euneoophora ***Order

Rhabdocoela Rhabdocoela is an order of flatworms in the class Rhabditophora with about 1700 species described worldwide. The order was first described in 1831 by Christian Gottfried Ehrenberg. Most of rhabdocoels are free-living organisms, but some live symb ...

***Order

Proseriata Proseriata is an order of free-living flatworms in the class Rhabditophora with over 400 species described worldwide. Description Proseriate flatworms are minute aquatic organisms, usually with an elongate body. There are no strong synapomorph ...

***Clade Acentrosomata ****Clade Adiaphanida *****Order Prolecithophora *****Order

Fecampiida Fecampiida is an order of flatworms in the class Rhabditophora. It is a considerably recent clade, erected after molecular studies. Description The order Fecampiida, as currently defined, was erected based on molecular studies. They all are pa ...

*****Order Tricladida ****Clade

Bothrioneodermata Bothrioneodermata is a clade of flatworms containing the Bothrioplanida and the Neodermata Neodermata is a clade of rhabditophoran flatworms containing the parasitic groups Trematoda, Monogenea and Cestoda. Description All neodermatans are ...

*****Order

Bothrioplanida ''Bothrioplana'' is a genus of freshwater flatworms, the sole genus in the family Bothrioplanidae and order Bothrioplanida. Description Species of ''Bothrioplana'' are small organisms, usually measuring 2–3 mm in length, but reaching up to 7 ...

*****Clade

******Order Trematoda ******Order Monogenea ******Order Cestoda

Description

Systematics

References

Further reading