Ambulacraria

Ambulacraria , or Coelomopora , is the clade of invertebrate phyla that includes echinoderms & hemichordates; a module of this house is called an ambulacrarian. Phylogenetic analysis suggests a echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a larger clade that also includes the Chordata, Vetulicolia and Saccorhytus.

The two alive clades with exemplification organisms are:

These together sometimes are called the lower deuterostomes.

Whether the Xenacoelomorpha clade is the sister corporation to the Ambulacraria maintains 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:

Ontogeny

As for numerous animals, the egg cell of any extant 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 grownup 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.