Eleutherozoa is a

subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used th ...

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

s. They are mobile animals with the mouth directed towards the substrate. They usually have a madreporite,

tube feet Tube or tubes may refer to: * Tube (2003 film), ''Tube'' (2003 film), a 2003 Korean film * "Tubes" (Peter Dale), performer on the Soccer AM#Tubes, Soccer AM television show * Tube (band), a Japanese rock band * Tube & Berger, the alias of dance/e ...

, and moveable spines of some sort. It includes all living echinoderms except for

crinoid Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...

s. The

monophyly 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 comm ...

of Eleutherozoa has been proven sufficiently well to be considered "uncontroversial."

Etymology

From Ancient Greek ἐλεύθερος (eleútheros, “free”) + ζῷον (zôion, “animal”), meaning "animal that moves freely."

Classification

History

Originally defined by F. J. Bell in a sense that excluded

Holothuroidea Sea cucumbers are echinoderms from the class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number being in the Asia ...

, Eleutherozoa was expanded by F. A. Bather in his 1900 taxonomy to include all free-living echinoderms. Bather considered the taxa within Eleutherozoa to have descended from the other subphylum in his two-subphylum system, the Pelmatozoa, either from different subgroups or at "widely different periods." In

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

terms, this would make Eleutherozoa a

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

group. In 1966–7, the ''

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

'' adopted a four-subphylum system to replace Bather's system, in part because of the belief that Eleutherozoa was polyphyletic, although it retained "pelmatozoic" and "eleutherozoic" as descriptions of attached and free-living modes of life, respectively. This classification introduced

Asterozoa The Asterozoa are a subphylum in the phylum Echinodermata, within the Eleutherozoa. Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the classes Asteroidea (the starfish or sea stars), ...

and Echinozoa as subphyla. However, this version of Echinozoa was much more expansive than its modern sense, and included all non-stalked classes that were assigned to neither Asterozoa nor

Homalozoa Homalozoa is an obsolete extinct subphylum of Paleozoic era echinoderms, prehistoric marine invertebrates. They are also referred to as carpoids. Description The Homalozoa lacked the typical pentamer body form of other echinoderms, but all were ...

. With the advent of

cladistics Cladistics ( ; from Ancient Greek 'branch') is an approach to Taxonomy (biology), biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesiz ...

, the taxonomy of echinoderms was re-evaluated, finding new support for both Pelmatozoa (in its original sense, per Leuckart, encompassing only stalked forms) and Eleutherozoa, with Echinozoa now having its modern contents of

Echinoidea Sea urchins or urchins () are echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body covered by a spiny ...

as the sister to a clade containing

and the extinct (and possibly paraphyletic) Ophiocistioidea. However, Edrioasteroidea was also included in this version of Eleutherozoa.

Asterozoa vs Cryptosyringida

While this initially restored Eleutherozoa as a monophyletic clade and defined the modern scope of Echinozoa, it followed embryological evidence grouping

Ophiuroidea Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

with Echinozoa. This clade, as sister to the

Asteroidea Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...

, was given the name Cryptosyringida, constructed from the Greek "kryptos" (hidden) and "syringos" (pipe or fistula), referring to the hiding of certain anatomical elements during development. More recent work has shown, through multiple lines of evidence, that

, consisting of the classes

and

, is the sister group of Echinozoa within Eleutherozoa, disproving the Cryptosyringida hypothesis. As of 2024, the emerging consensus regarding echinoderm phylogeny restricts Eleutherozoa to

plus Echinozoa, but has not fully resolved the nature of Eleutherozoa's relationship to Edrioasteroidea.

Phylogeny

The living clades of echinoderms are related to each other as follows:

Footnotes

Works cited

* * * * * * * * * * Echinoderm taxonomy Extant Cambrian first appearances Animal subphyla {{echinoderm-stub