Eleutherozoa

Taxobox
name = Eleutherozoa


image_caption = "Pisaster ochraceaus"
regnum = Animalia
phylum = Echinodermata
subphylum = Eleutherozoa
subphylum_authority = Bather, 1900
subdivision_ranks = Superclasses
subdivision =
* Superclass/Clade Asterozoa
** Order Asteroidea
** Order †Somasteroidea
* Superclass Echinozoa
** Order Echinoidea
** Order Holothuroidea
* "Incertae sedis"
** Order Ophiuroidea

Eleutherozoa is a subphylum of echinoderms. They are mobile animals with the mouth diceted towards the substrate. They usually have a madreporite, tube feet, and moveable spines of some sort, and some have Tiedemann's bodies on the ring canal. All living echinoderms except Crinoidea belong here.

ystematics

There are 2 main competing hypotheses about the internal subdivision, both about equally well supported by both molecular and morphological data. They differ in their placement of the Ophiuroidea (brittle stars), and are named accordingly.

The "Cryptosyringida" hypothesis posits that the "sea-star" morphology is plesiomorphic for Eleutherozoa as a whole, and that starfish (Asteroidea) and brittle stars are not very closely related, the latter forming the clade Cryptosyringida together with the Echinozoa. The "Asterozoa" hypothesis on the other hand implies that the "sea-star" arms of starfish and brittle stars, as well as the rounded shape of Echinozoa, both evolved independently from an ancestor of unknown morphology, but that each "armed" and "rounded" lineages are strictly monophyletic. Too little is known of the basal eleutherozoans and echinoderms to be able to firmly decide for or against any of these hypotheses at present. [Wray (1999)]

The Asterozoa would have to be ranked as a superclass or treated as an unranked clade between the Cryptosyringida and the Eleutherozoa, depending on whether the "Asterozoa" or "Cryptosyringida" hypothesis turns eventually out to be correct.

Emerging research favours the following classification:PalAss2007|author=Smith, A.|title=Echinoderms: Attachment, torsion and the origins of a radical new body plan] cite journal
author = Smith, A.B.
year = 2005
title = The pre-radial history of echinoderms
journal = Geological Journal
volume = 40
pages = 255–280
doi = 10.1002/gj.1018]

Footnotes

References

* (1999): Tree of Life Web Project: [http://tolweb.org/Echinodermata/2497/1999.12.14 Echinodermata] : Spiny-skinned animals: sea urchins, starfish, and their allies. Version of 1999-DEC-14. Retrieved 2008-FEB-02.