Ambulacraria, from the latinambulacrum, meaning "a walk planted with trees",[3] was named byÉlie Metchnikoff in reference to the larval similarities including a folded band lined with cillia that are used for both feeding and locomotion.[4] Metchnikoff also noted similarities among the coleomic systems of echinoderms and hemichordates and their larvae.[5]
Whether theXenacoelomorpha clade is the sister group to the Ambulacraria remains a contentious issue, with some authors arguing that the former should be placed more basally amongmetazoans,[8][9][1] and other authors asserting that the best choices of phylogenetic methods support the position of Xenacoelomorpha as the sister group to Ambulacraria.[10][11]
A third, extinct clade known as theCambroernida has recently been recognized as part of the ambulacrarianstem group. The morphology of this group suggests that the ancestral deuterostome did not possess a post-anal tail, which evolved convergently in chordates and hemichordates, and possessed gill slits but not additional structures such as the gill bars found in hemichordates.[12]
Two extinct taxa of uncertain placement, theVetulocystida andYanjiahella, have each been proposed as either stem-group echinoderms[13][14] or stem-group ambulacrarians.[15][16] Vetulocystids have also been tentatively proposed as the basal-most stem-groupchordates,[17] whileYanjiahella has also been proposed to be a stem-group hemichordate.[16]
The following cladogram is based on a simplification of Li et al. 2023,[12] with the possible placements of uncertain potential ambulacrarians shown with dashed lines and question marks:
As for many animals, theegg cell of anyextant ambulacrarian divides and develops into ablastula ("cell ball"), which develops into atriploblast ("three-layered")gastrula. The gastrula then develops into adipleurula larva form in the Asteroidea, Holothuroidea, Crinoidea, and Hemichordata, and into apluteus larva form in the Echinoidea and Ophiuroidea.[7][18] 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 thelast common ancestor of the ambulacrarians wasanatomically similar to the dipleurula larva; this hypothetic ancestor sometimes also is called dipleurula.[19]
^Nielsen, Claus (17 December 2013). "Ambulacraria".Animal Evolution: Interrelationships of the Living Phyla (3rd, online ed.). Oxford: Oxford Academic.doi:10.1093/acprof:oso/9780199606023.003.0057.
^Mitchell, C. E.; Melchin, M. J.; Cameron, C. B.; Maletz, J. (2013). "Phylogenetic analysis reveals thatRhabdopleura is an extant graptolite".Lethaia.46 (1):34–56.doi:10.1111/j.1502-3931.2012.00319.x.
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