Ambulacraria , or Coelomopora , is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the

deuterostome Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...

s, a larger clade that also includes the

Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...

, Vetulicolia. The two living clades with representative organisms are: * Echinodermata ( sea stars,

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

s, brittle stars,

sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...

feather star Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

s, sea lilies, etc.) * Hemichordata ( acorn worms,

Pterobranchia Pterobranchia is a class of small worm-shaped animals. They belong to the Hemichordata, and live in secreted tubes on the ocean floor. Pterobranchia feed by filtering plankton out of the water with the help of cilia attached to tentacles. The ...

, and possibly graptolites) (These together sometimes are called the ''lower

s''.) Whether the Xenacoelomorpha clade is the sister group to the Ambulacraria remains a contentious issue, with some authors arguing that the former should be placed more basally among metazoans, and other authors asserting that the best choices of phylogenetic methods support the position of Xenacoelomorpha as the sister group to Ambulacraria. Fossil taxa that may lie on the stem lineage: * Superphylum Ambulacraria ** † " Cambroernids" (informal unranked clade) *** '' Herpetogaster'' *** '' Phlogites'' *** " Eldoniids" / "Eldonioids"

Ontogeny

As for many animals, the

egg cell The egg cell, or ovum (plural ova), is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is ...

of any

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

ambulacrarian by cell division evolves to a blastula ("cell ball"), which evolves to a triploblast ("three-layered") gastrula. The gastrula then evolves to a dipleurula larva form, which is specific for the ambulacraria. This, in its turn, is developed in various different kinds of larvae for different taxa of ambulacrarians. It has been suggested that the adult form of the last common ancestor of the ambulacrarians was anatomically similar to the dipleurula larvae, whence this hypothetic ancestor sometimes also is called dipleurula.

References

{{Taxonbar, from=Q136956 Deuterostome taxonomy Ediacaran first appearances Superphyla