Ambulacraria

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 deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.

The two living clades with representative organisms are: (These together sometimes are called the lower deuterostomes. )
 * Echinodermata (sea stars, sea urchins, brittle stars, sea cucumbers, feather stars, sea lilies, etc.)
 * Hemichordata (acorn worms, Pterobranchia, and possibly graptolites)

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 ambulacrarians
Fossil taxa that may lie on the stem lineage:


 * Superphylum Ambulacraria (total group)
 * Cambroernida (unranked clade)
 * Herpetogaster
 * Phlogites
 * Superfamily Eldonioidea
 * Family Eldoniidae
 * Family Rotadiscidae
 * Paropsonema
 * Yanjiahella

Ontogeny
As for many animals, the egg cell of any extant ambulacrarian divides and develops into a blastula ("cell ball"), which develops into a triploblast ("three-layered") gastrula. The gastrula then develops into a dipleurula larva form in the Asteroidea, Holothuroidea, Crinoidea, and Hemichordata, and into a pluteus larva form in the Echinoidea and Ophiuroidea. 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 larva; this hypothetic ancestor sometimes also is called dipleurula.