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 larger clade that also includes the Chordata, Vetulicolia and Saccorhytus.
The two living clades with representative organisms are:
The group Xenoturbellida had previously been considered to be in this clade but is now considered to be placed more basally among metazoans.
Fossil taxa that may lie on the stem lineage:
As for many animals, the egg cell of any extant ambulacrarian by cell division evolves to a blastula, which evolves to a triploblast 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.